∎

11institutetext:  I. I. Baliukin, V. V. Izmodenov 22institutetext: Space
Research Institute of Russian Academy of Sciences, Profsoyuznaya Str. 84/32,
Moscow 117997, Russia
Lomonosov Moscow State University, Moscow Center for Fundamental and Applied
Mathematics, GSP-1, Leninskie Gory, Moscow 119991, Russia
22email<EMAIL_ADDRESS>33institutetext: J.-L. Bertaux, E. Quémerais,
44institutetext: LATMOS/IPSL, UVSQ, France 55institutetext: M. Bzowski
66institutetext: Space Research Centre, Polish Academy of Sciences, Warsaw,
Poland
66email<EMAIL_ADDRESS>77institutetext: R. Lallement 88institutetext:
GEPI, Observatoire de Paris, PSL University, CNRS, 5 Place Jules Janssen,
92190, Meudon, France 99institutetext: E. Provornikova 1010institutetext:
Johns Hopkins University Applied Physics Laboratory, USA

# Backscattered solar Lyman-$\alpha$ emission as a tool for the heliospheric
boundary exploration

Igor Baliukin  Jean-Loup Bertaux  Maciej Bzowski  Vladislav Izmodenov  Rosine
Lallement  Elena Provornikova  Eric Quémerais

(Received: date / Accepted: date)

###### Abstract

This review summarizes our current understanding of the outer heliosphere and
local interstellar medium (LISM) inferred from observations and modeling of
interplanetary Lyman-$\alpha$ emission. The emission is produced by solar
Lyman-$\alpha$ photons (121.567 nm) backscattered by interstellar H atoms
inflowing to the heliosphere from the LISM. Studies of Lyman-$\alpha$
radiation determined the parameters of interstellar hydrogen within a few
astronomical units from the Sun. The interstellar hydrogen atoms appeared to
be decelerated, heated, and shifted compared to the helium atoms. The detected
deceleration and heating proved the existence of secondary hydrogen atoms
created near the heliopause. This finding supports the discovery of a Hydrogen
Wall beyond the heliosphere consisting of heated hydrogen observed in HST/GHRS
Lyman-$\alpha$ absorption spectra toward nearby stars. The shift of the
interstellar hydrogen bulk velocity was the first observational evidence of
the global heliosphere asymmetry confirmed later by Voyager in situ
measurements. SOHO/SWAN all-sky maps of the backscattered Lyman-$\alpha$
intensity identified variations of the solar wind mass flux with heliolatitude
and time. In particular, two maxima at mid-latitudes were discovered during
solar activity maximum, which Ulysses missed due to its specific trajectory.
Finally, Voyager/UVS and New Horizons/Alice UV spectrographs discovered
extraheliospheric Lyman-$\alpha$ emission. We review these scientific
breakthroughs, outline open science questions, and discuss potential future
heliospheric Lyman-$\alpha$ experiments.

###### Keywords:

Heliosphere Lyman-alpha Interstellar Neutrals Interplanetary background

##  1 Introduction

The first measurements of ultraviolet radiation in the Lyman-$\alpha$ line
(with a wavelength at the center $\lambda_{0}$ = 121.567 nm) were made at the
very beginning of the space era using night rocket launches. In particular,
Kupperian et al. (1959) launched at night a rocket reaching 146 km of
altitude, which allowed to look both to the nadir and the upper hemisphere.
The experiment recorded a diffuse Lyman-$\alpha$ emission from all directions.
The emission from the nadir hemisphere was interpreted correctly as hydrogen
(H) atoms in the atmosphere below the rocket, being illuminated by the
(stronger) emission coming from the upper hemisphere. This sky emission
displayed a minimum right in the anti-solar direction, $\sim$50∘ away from the
zenith, which could not be explained by the expected radiation from nearby
stars. It was (incorrectly) assigned to H atoms in the interplanetary medium
scattering solar photons, with the density estimate 0.2 cm-3 (Kupperian et
al., 1959). Shklovsky (1959) and Brandt and Chamberlain (1959) considered two
hypotheses concerning a source of the observed emission: (1) H atoms in
interplanetary space and (2) H atoms in the Earth’s geocorona. The “geocorona”
term for extended terrestrial atmosphere of H atoms was proposed by Shklovsky
(1959). Both papers ruled out the second explanation and concluded that the
emission is a result of scattering of solar photons from H atoms in the
interplanetary space. Conversely, the following year, Johnson and Fish (1960)
favored the second explanation. Their model suggested an extended exosphere
with H atom densities reaching 10 cm-3 at the distance $7$ Earth Radii
($R_{\rm E}$).

Further progress was made by using a hydrogen absorption cell (H cell) in
Lyman-$\alpha$ radiation measurements during nighttime rocket launches (Morton
and Purcell, 1962). It was found that 85% of the measured radiation was
absorbed by the H cell. This implied a small Doppler shift relative to the
center of the line, and, therefore, proved its terrestrial origin (explanation
2). The remaining non-absorbed 15% of the radiation had a large Doppler shift,
meaning that such scattered photons were produced by the interaction of a
solar photon with a H atom having a high speed relative to the Earth. An atom
can have a high speed in two cases: (1) if the flow of atoms moves fast
relative to the Earth, and (2) if the scattering gas has a high temperature.
Morton and Purcell (1962) argued that if the radiation was coming from
interplanetary H atoms, its absorption by the H cell would be changing
depending on a look direction because of a variable Doppler shift with the
angle to the Earth’s orbital motion. Since the 15% residual intensity did not
change with a look direction on the sky, they favored the other explanation,
namely that the emission was produced by scattering of solar photons on “hot”
H atoms (7000 K) in the Earth’s upper atmosphere (exosphere). In retrospect,
we understand what happened. The H cell was launched on April 17. At this time
of year, the Earth’s orbital velocity vector is oriented opposite to the
interstellar H flow velocity vector. Hence, the emission from the
interplanetary H atoms has a large Doppler shift. A region in the sky where
the H cell could have absorbed the interplanetary emission (along the Zero
Doppler Shift Circle, see Section 4.3) is quite narrow and was out of lines of
sight of the H cell in these rocket experiments.

After having discarded an existence of the extended geocorona in 1959,
Chamberlain (1963) returned to this concept motivated by the observation of
85% absorption with the H cell. Extending the work of Johnson and Fish (1960),
he established a theory of H atom distribution in the exosphere. Now well-
known, the Chamberlain’s model introduced a concept of escaping, ballistic and
satellites particles (H atoms), populating an extended exosphere (geocorona).

Meanwhile, Patterson et al. (1963) interpreted the residual 15% of emission as
being due to H atoms in the interplanetary space. They put forward two
hypotheses that the H atoms were either a “cold” component of interstellar
atoms approaching the Sun or produced at the solar wind (SW) termination shock
by charge exchange with neutral galactic (interstellar) H atoms. They rejected
(wrongly) the “cold” component explanation for the same reason as Morton and
Purcell (1962), i.e. a lack of variation in the absorption with changing line
of sight (LOS). They favored the latter idea, extending a work of Axford et
al. (1963) about the termination of the solar wind. A population of atoms
produced at the TS is similar to so-called “hot” population, which is
discussed below in Section 4. From the non-absorbed Lyman-$\alpha$ intensity
(4.1 $\times$ 10-4 erg cm-2 s-1 sr-1), they derived a distance to the
termination shock of 20 AU and a magnitude of the interstellar magnetic field
of 3 nanoteslas (nT). Note, these early quantitative estimates resulted from
the understanding that the extraterrestrial Lyman-$\alpha$ emission carry
imprints of the interaction of the SW with the interstellar medium (ISM).

New experimental data obtained in the 1960s on the Soviet interplanetary
spacecraft Zond 1, Venera 2, 3 and 4 (Kurt and Germogenova, 1967; Kurt and
Syunyaev, 1967) have confirmed the conclusion that 15 % of the radiation
measured during night rocket launches has an extraterrestrial origin. However,
it was also found that the maximum of backscattered radiation is located in
the direction close to the center of our galaxy (Kurt and Syunyaev, 1967).
This prompted new theories about the Galactic sources of observed
Lyman-$\alpha$ radiation, which later turned out to be erroneous.

Another breakthrough came in the early 1970s with new measurements of
backscattered Lyman-$\alpha$ radiation on the OGO-5 (Orbiting Geophysical
Observatory number 5) spacecraft (Bertaux and Blamont, 1971; Thomas and
Krassa, 1971). These measurements made it possible to resolve the question on
the nature of the source of radiation. The apogee of the OGO-5 spacecraft was
outside the geocorona, and the spacecraft itself was purposely put in a
rotation mode, which allowed constructing complete maps of the emission
intensity. The so-called parallax effect (angular shift of the position of the
radiation maximum at different points of the spacecraft’s orbit) was
discovered, which would be negligible in the case of a Galactic source. Thus,
it was established that the source of the measured Lyman-$\alpha$ radiation is
indeed interstellar H atoms that flow to the solar system from the ISM.

From the late 1970s until the end of the 1980s, the main results on
backscattered Lyman-$\alpha$ radiation were obtained using measurements on
Soviet spacecraft Mars-7 (Bertaux et al., 1976) and Prognoz-5/6 (Bertaux et
al., 1977, 1985; Lallement et al., 1984, 1985b). These spacecraft were
equipped with photometers with an H absorption cell, which made it possible to
study not only the absolute value of the radiation (intensity) but also the
spectral properties of the line. Backscattered Lyman-$\alpha$ radiation has
been measured in the outer heliosphere by Mariner 10 (Ajello, 1978; Kumar and
Broadfoot, 1978, 1979), Pioneer 10 (Wu et al., 1988; Gangopadhyay and Judge,
1989), Pioneer Venus (Ajello et al., 1987; Ajello, 1990), Galileo/Ultraviolet
Spectrometer (UVS) (Ajello et al., 1993), and Voyager/UVS. Since 1990, it has
been measured on Solar and Heliospheric Observatory (SOHO)/Solar Wind
ANisotropy (SWAN), Hubble Space Telescope (HST)/Goddard High Resolution
Spectrograph (GHRS), Space Telescope Imaging Spectrograph (STIS), and New
Horizons/Alice. As we discuss in this review, in the last 30 years,
backscattered Lyman-$\alpha$ radiation was observed on several spacecraft and
actively studied. A large amount of experimental data has been accumulated,
providing us new important knowledge on the heliosphere.

This paper is structured as follows. Section 2 describes basic theoretical
ideas. Section 3 summarizes photometric (intensities) observations. Section 4
presents spectrometric and H cell (spectral line shape) observations.
Conclusions, open questions and future experiments are discussed in Section 5.

##  2 Basic theoretical ideas

###  2.1 Lyman-$\alpha$ emission

The Sun constantly emits photons at different frequencies, while the spectrum
of solar radiation consists of various selected lines. The solar spectrum in
the Lyman-$\alpha$ line has the shape $s(\lambda)$ with a dip in the middle
since photons from the center of the line are actively scattered in the
atmosphere of the Sun (see, e.g., Lemaire et al., 1998, 2005), so the flux of
photons at the Earth’s orbit with the wavelength $\lambda$ can be represented
as follows:

$F_{\rm E}(\lambda)=s(\lambda)\cdot F_{\rm E,0}.$ (1)

These quantities vary with solar activity and the statistical relationship
between total flux in the Lyman-$\alpha$ line $F_{\rm E,0}$ and flux at line
center $f$ established by Emerich et al. (2005) from Solar Ultraviolet
Measurements of Emitted Radiation (SUMER) on board Solar and Heliospheric
Observatory (SOHO) can be used to derive the flux at line center:

$\frac{f}{10^{12}\>\rm s^{-1}\>cm^{-2}\>nm^{-1}}=0.64\left(\frac{F_{\rm
E,0}}{10^{11}\>\rm s^{-1}\>cm^{-2}}\right)^{1.21}\pm 0.08$ (2)

The daily total solar Lyman-$\alpha$ flux measured at the Earth’s orbit
($F_{\rm E,0}\sim 3-7\times 10^{11}$ photons $\rm s^{-1}cm^{-2}$) is available
in the form of so-called solar composite Lyman-$\alpha$ flux, with the most
recent version given by Machol et al. (2019, see
http://lasp.colorado.edu/lisird/data/composite_lyman_alpha/), which is
based on observations from several spacecraft. The variation of the solar
Lyman-$\alpha$ line profile with the level of solar activity, represented by
the total solar Lyman-$\alpha$ flux, was studied by Tarnopolski and Bzowski
(2009); Kowalska-Leszczynska et al. (2018a, b); Kowalska-Leszczynska et al.
(2020); Kretzschmar et al. (2018).

On the way through the heliosphere, the Lyman-$\alpha$ photon can experience
scattering. Considering $n$ photons with the same direction $\mathbf{\Omega}$,
wavelength $\lambda$ within ${\rm d}\lambda$ and going through a medium with
local hydrogen density $n_{\rm H}$, the absorption probability after crossing
a distance ${\rm d}s$ is proportional to the density multiplied by ${\rm d}s$.
The absorbed fraction ${\rm d}\tau=-{\rm d}n/n$ is the following:

${\rm d}\tau=\sigma(\lambda)n_{\rm H}{\rm d}s,$ (3)

where $\sigma({\lambda})$ is the absorption cross-section at wavelength
$\lambda$, which has the dimension of an area. The cross-section
$\sigma(\lambda)$ is proportional to the projection of the velocity
distribution function of H atoms on the direction $\mathbf{\Omega}$
(Quémerais, 2006). To be more precise,

$\sigma(\lambda)=k_{0}\cdot\hat{f}_{\rm p}(u),$ (4)

where $\hat{f}_{\rm p}(u)$ – the normalized projection of H velocity
distribution function such as $\int_{-\infty}^{+\infty}\hat{f}_{\rm
p}(u)\,{\rm d}u=1$, and $u=c(\lambda/\lambda_{0}-1)$ is the value of projected
velocity. The total (integrated) cross-section $\sigma_{\lambda}$ can be
obtained from the harmonic oscillator theory, and it is given by Mihalas
(1978):

$\sigma_{\lambda}=\int_{0}^{+\infty}\sigma({\lambda})\,{\rm
d}\lambda=\left(\frac{\lambda_{0}^{2}}{c}\right)\frac{\pi\,e^{2}}{m_{e}\,c}f_{12}\\\
=5.445\times 10^{-15}\rm cm^{2}\>\mbox{\AA},$ (5)

where $m_{e}$ – mass of electron, and $f_{12}=0.416$ – oscillator strength.
The proportionality coefficient in Equation 4 is
$k_{0}=(c/\lambda_{0})\cdot\sigma_{\lambda}$, since ${\rm
d}u=(c/\lambda_{0})\cdot{\rm d}\lambda$.

In the interplanetary medium, all absorptions of Lyman-$\alpha$ photons by H
atoms are followed by the emission of new photons (resonance scattering).
During the process of absorption, the photon energy is transferred to the
electron, the electron transits to the second electron orbit (i.e., the atom
is promoted to the excited state) and then, since this state is not stable,
the electron returns to the ground state, as a result of which a new
Lyman-$\alpha$ photon is emitted. In general, the direction of motion and
frequency of the original and emitted photons are different. In principle, the
direction of movement of the emitted photon can be arbitrary, however, some
directions are more probable. A function that determines a probability that
the scattering occurs exactly at the angle $\omega$ between the directions of
motion of the incident and scattered photons is called the scattering phase
function, and it expresses the relation between the absorbed and the emitted
intensities. Brandt and Chamberlain (1959) obtained an expression for the
normalized scattering phase function in the Lyman-$\alpha$ line:

$\varphi(\omega)=\frac{11/12+1/4\,\cos^{2}\omega}{4\pi}.$ (6)

More recently, Brasken and Kyrola (1998) computed a new function based on the
general theory of resonance scattering with quantum mechanics. Their result is
numerically very similar to the expression 6.

In the process of scattering, an atom can lose part of its energy, while the
frequency of the emitted photon in the coordinate system of the atom will
differ from the frequency of the primary photon. This effect is called the
natural broadening of the line. However, in the case of heliospheric studies,
the natural broadening can be neglected (Quémerais, 2006). It can be explained
by the fact that the lifetime of an electron at the upper energy level (i.e.,
when the atom is in an excited state) is small in comparison with the
characteristic times of other considered processes, so the change in the
velocity of the atom during this time can be neglected. Thus, as a result of
the interaction of a solar photon with a H atom, a new scattered
Lyman-$\alpha$ photon is formed, and the characteristics of the Lyman-$\alpha$
emission are strongly dependent on the velocity distribution function of H
atoms. Such scattered emission can be measured by various spacecraft.

The interaction of Lyman-$\alpha$ photon with a H atom is twofold. On one
hand, the wavelength of the backscattered Lyman-$\alpha$ photon depends on the
velocity of the H atom. On the other hand, when an atom absorbs a photon, the
value of atom momentum increases (by photon momentum transfer) in the
antisolar direction. Due to subsequent photon emission, it also changes by
almost the same amount, but in a random direction. The average momentum change
of many emissions is zero, since the scattering phase function is symmetrical
along the photon motion direction (see Equation 6). Therefore, a net effect of
many scatterings can be described as a radiation pressure force that acts on
the atom in the antisolar direction.

The radiation pressure force is directly proportional to the solar
Lyman-$\alpha$ flux that in the optically thin medium approximation (no
absorption by atoms) falls off $\propto 1/r^{2}$ with distance to the Sun, as
the solar gravitational force. The ratio $\mu$ of solar radiation pressure and
solar gravitational force is commonly used in the literature. The dependence
of the radiation pressure on time, radial velocity, and heliolatitude of the H
atom was studied by Tarnopolski and Bzowski (2009); Kowalska-Leszczynska et
al. (2018a, b); Kowalska-Leszczynska et al. (2020). It is important to note
that, strictly speaking, the optically thin medium approximation is not
applicable in the whole heliosphere since the hydrogen number density
increases with distance from the Sun. The interplanetary medium can be
considered optically thin only until 10 AU (Quémerais, 2000). Therefore, the
solar radiation pressure and the ratio $\mu$ depend also on the distance to
the Sun. Due to the absorption of photons by H atoms, the solar pressure drops
more rapidly than $\propto 1/r^{2}$. While the importance of absorption
increases with distance from the Sun, the hydrogen distribution is essentially
affected by the solar pressure (and gravitation) only in the vicinity of the
Sun. A step towards the exploration of the influence of absorption of the
solar Lyman-$\alpha$ photons on the hydrogen distribution is the recent
analysis by Kowalska-Leszczynska et al. (2022) who showed that ignoring the
absorption in total results in underestimating the hydrogen number density (in
the worst case by $\sim$9% in the downwind direction).

Actually, the secondary (scattered) photons can also provide the momentum
transfer to the H atoms, so the regions of enhanced number density “push”
other atoms away, even though the influence should be minor. Since the
hydrogen distribution depends on Lyman-$\alpha$ emission and vice versa, to
investigate the correct influence, one should perform self-consistent
numerical calculations of the atom interaction with Lyman-$\alpha$ emission
(with the multiple scattering effect taken into account).

The main source of the heliospheric Lyman-$\alpha$ emission is the
backscattered solar photons by the interstellar H atoms, which penetrate the
heliosphere due to the relative motion of the Sun and LISM (Local InterStellar
Medium). Actually, astronomers are using the acronym “LISM” to designate the
interstellar medium, which is not far from the Sun. Later on, the acronym
“VLISM” (Very Local InterStellar Medium) was used to designate the particular
part of the LISM in which is presently located the Sun, and which interacts
with the heliosphere. For simplicity, in the following we use “LISM” instead
of “VLISM”. Numerous measurements of the backscattered solar Lyman-$\alpha$
radiation have motivated the development of theoretical models of interstellar
H atoms propagation from the LISM to the vicinity of the Sun. The next Section
2.2 briefly reviews existing models of hydrogen distribution in the
heliosphere.

To model the interplanetary (IP) Lyman-$\alpha$ background, one should (a)
calculate the hydrogen distribution in the heliosphere, and (b) solve the
radiative transfer equation. To obtain the correct solution of the radiative
transfer equation, the multiple scattering effects must be considered
everywhere in the heliosphere, which has been proven quantitatively by
Quémerais (2000). However, since the problem is computationally time-
consuming, some approximations, which take into account only single scattered
photons, have been developed: (1) the optically thin (OT) approximation
neglects the extinction of photons due to the absorption, and it generally
provides appropriate intensity estimates in the inner heliosphere since the
extinction and multiple scattering effects act to balance each other; (2) the
primary term (PT) approximation gives the exact single scattering intensity
when both extinctions (between Sun and the scattering point, and on the LOS
between the scattering point and the observer) are included; in the inner
heliosphere, the PT approximation gives $\sim$60% of the intensity seen from 1
AU (Quémerais, 2000); (3) the self-absorption (SA) approximation proposed by
Bertaux et al. (1985) is the trade-off between the OT and PT cases; in
contrast to the OT approximation, the extinction on the LOS between the
scattering point and the observer is included in this case. Numerical
estimates of the errors introduced by these approximations are provided by
Quémerais (2000). Details on the simulations of the interplanetary hydrogen
Lyman-$\alpha$ background can be found in Quémerais (2000); Quémerais and
Izmodenov (2002); Quémerais et al. (2008).

###  2.2 Hydrogen distribution in the heliosphere

Fahr (1968) and Blum and Fahr (1970) proposed a so-called “cold model”, which
assumes that all H atoms have the same velocity in the LISM. However,
measurements of the interplanetary Lyman-$\alpha$ background with the use of
the hydrogen absorption cell on board the Prognoz-5 spacecraft allowed to
estimate the LISM temperature, which appeared to be not negligible ($\sim$8000
K; see, e.g., Bertaux et al., 1977), so the thermal and bulk flow speeds of
atoms in the LISM are comparable. Further developments were associated with
consideration of the realistic temperature and the corresponding thermal
velocities of H atoms in the LISM, which resulted in construction of so-called
“hot models”. In the classical formulation of a “hot model”, the problem is
stationary and axisymmetric, and the velocity distribution function of H atoms
at infinity (i.e., in the LISM) is a uniform Maxwell-Boltzmann distribution.
Meier (1977) and Wu and Judge (1979) presented the analytical solution for the
interstellar hydrogen velocity distribution. The solution takes into account
the solar gravitational force, the solar radiation pressure, and the losses
due to photoionization and charge exchange with the solar wind protons.

Further improvements can be in practice divided into two groups:

  1. 1.

Development of models, which take into account temporal and heliolatitudinal
variations of the solar wind. Temporal variations are caused by the 11-year
cycle of solar activity and have been considered in many works (e.g., Bzowski
and Rucinski, 1995; Summanen, 1996; Bzowski et al., 1997; Pryor et al., 2003;
Bzowski, 2008). The heliolatitudinal variations are connected with the non-
isotropic SW structure. Joselyn and Holzer (1975) were the first to show that
the anisotropic SW would strongly affect the ISN (InterStellar Neutral) H
distribution in the heliosphere. Several authors (Lallement et al., 1985b;
Bzowski et al., 2002, 2003; Bzowski, 2003; Pryor et al., 2003; Nakagawa et
al., 2009) assumed some analytical relations for the heliolatitudinal
variations of the hydrogen ionization rate and took them into account in the
frame of the hot model. Additionally, some models (Tarnopolski and Bzowski,
2009; Kubiak et al., 2021a) take into account the dependence of radiation
pressure force on radial velocity of ISN H atoms and the influence of the
Lyman-$\alpha$ absorption on the radiation pressure force (Kowalska-
Leszczynska et al., 2022).

  2. 2.

Development of models, which take into account disturbances of the
interstellar hydrogen flow at the heliosheath. Charge exchange of protons
(Wallis, 1975) with primary H atoms in the heliospheric interface creates the
“secondary” population of H atoms, which have the individual velocities of
their original parent protons, so the velocity distribution function of newly
created atoms depends on the local plasma properties. The accumulation of the
heated and decelerated secondary component creates the so-called “hydrogen
wall” (the region of the pile-up of H atoms between the heliopause and the bow
shock) that was firstly reported by (Baranov et al., 1991). Such hydrogen
walls have been observed around other stars (Wood et al., 2005). Therefore, a
mixture of the primary and secondary interstellar H atoms penetrates the
heliosphere. The properties of the H atom component depend on both the LISM
parameters and the plasma distribution in the heliosheath. Disturbances of the
interstellar hydrogen flow in the heliosheath were included in the hot model
using the different approaches suggested by (Bzowski et al., 2008; Nakagawa et
al., 2008; Katushkina and Izmodenov, 2010; Izmodenov et al., 2013).

Modern versions of the hot model of the interstellar hydrogen distribution in
the heliosphere, such as the models by Katushkina et al. (2015b) and Kubiak et
al. (2021a, b), take into account both the solar cycle and heliolatitudinal
variations of the solar wind and the disturbances of the hydrogen flow by the
heliospheric interface.

Kleimann et al. (2022) presents an overview of the current state of research
in global modelling of the heliospheric interface. Galli et al. (2022)
summarizes all direct observations of interstellar H atoms (and other
neutrals), most of which were acquired with the Interstellar Boundary Explorer
(IBEX) launched in 2008.

##  3 What do we learn from maps and radial/angular profiles of backscattered
Lyman-$\alpha$ emission?

###  3.1 SOHO/SWAN sky maps from 1 AU

From 1996 to the present, the backscattered solar Lyman-$\alpha$ emission has
been measured by the SWAN instrument (Solar Wind ANisotropy) on board the SOHO
spacecraft placed in a halo orbit around Lagrange point $\rm L_{1}$. The SWAN
instrument was designed and built by the Service d’Aéronomie (France) and the
Finnish Meteorological Institute (Finland). Consisting from the two identical
Sensor Units, the instrument is equipped with a two-axis periscope, which
allows obtaining a complete map of the sky in Lyman-$\alpha$ intensity in
about 24 hours (Bertaux et al., 1995). Each Sensor Unit (SU) covers roughly
one half of the sky: SU+Z views the ecliptic Northern Hemisphere and SU+Z –
the Southern Hemisphere, with some overlap between the two hemispheres. A
recent comparison of the SWAN data (from 1996 to 2018) with model calculations
shows a sensitivity degradation of the SWAN Sensor Units with the rate of
$\sim$2% per year. In addition, SWAN is equipped with a hydrogen cell, which
makes it possible to study the Lyman-$\alpha$ line shape and derive spectral
properties. The results obtained with the usage of the SWAN H cell are
discussed in Section 4.3.

Figure 1: The full sky map (in ecliptic J2000 coordinates) of the
Lyman-$\alpha$ intensity observed by SWAN/SOHO on 17 March 1996. The white
areas represent regions of the sky which can not be observed by SWAN or which
are contaminated by hot stars of the Galactic plane. The color represents the
intensity in counts per second per pixel (one square degree). Credit:
https://sohowww.nascom.nasa.gov/gallery/Particle/swa005.html.

Figure 1 shows a typical full-sky map of Lyman-$\alpha$ intensity (in the
solar ecliptic coordinates) obtained by SWAN/SOHO on 17 March 1996. The color
represents the intensity in counts per second per pixel (one square degree),
which corresponds to 1.3 Rayleigh (1 Rayleigh = 1 R = 106/4$\pi$ photons cm-2
s-1 sr-1). A maximum of Lyman-$\alpha$ intensity is seen in the upwind
direction, while in the opposite direction the emission is much weaker since
in the downwind region the number density of H atoms is smaller (so-called
ionization cavity). The white areas represent regions of the sky that are
masked for safety reasons: the area around the Sun (with $\approx 0^{\circ}$
ecliptic longitude), the anti-solar region (near the center of the map), which
is contaminated by parts of SOHO spacecraft body (illuminated by the Sun).
There is also a shark fin-shaped piece of hardware above the anti-solar
direction, which is placed to protect the SU+Z from SOHO thrusters firings.
These three artificial features associated with the spacecraft (Sun shade,
anti-solar contaminated region, shark fin) are moving from right to left with
$\approx 1^{\circ}$ shift per day since the SOHO orbit follows the Earth on
its yearly orbital motion.

As the name suggests, the main purpose of the SWAN instrument is to study the
heliolatitude dependence (or anisotropy) of the solar wind parameters by
analyzing the intensity of the backscattered solar Lyman-$\alpha$ radiation.
The parameters of the solar wind determine the ionization rate of H atoms due
to the charge exchange with the solar wind protons in the heliosphere. In the
region of the supersonic solar wind the charge exchange ionization rate
$\beta_{\rm ex}(t,{\bf r})\approx n_{\rm sw}\cdot V_{\rm sw}\cdot\sigma_{\rm
ex}(V_{\rm sw})$, where $n_{\rm sw}(t,{\bf r})$ and $V_{\rm sw}(t,{\bf r})$
are the solar wind number density and velocity, and $\sigma_{\rm ex}$ is the
charge exchange cross-section (see, e.g. Lindsay and Stebbings, 2005). The
cross-section generally depends on the relative velocity between the H atom
($\sim$20 km/s) and the solar wind proton ($\sim$400-800 km/s). However, the
latter is much higher and for solar wind velocities the dependence of
$\sigma_{\rm ex}$ is weak. Therefore, the charge exchange ionization rate is
proportional to the solar wind mass flux. If, for example, the charge exchange
ionization rate in the plane of the solar equator is greater than at the
poles, then the number density of H atoms in the plane of the equator is less
than at the poles. This means that the intensity of the solar Lyman-$\alpha$
radiation backscattered by the H atoms will have a minimum in the equatorial
plane and increase monotonically towards the poles. Thus, measurements of the
backscattered Lyman-$\alpha$ emission can be used as a tool for an indirect
diagnostic of the heliolatitude dependence of the solar wind parameters.

Indeed, the distribution of Ly-$\alpha$ intensities recorded by Prognoz-5 and
6 along several great circle scans during 1976-77 (solar minimum) displayed a
conspicuous dip near the ecliptic plane in the upwind sky region. This was
interpreted (Bertaux et al., 1996) as the trace of a “groove” carved by a peak
of enhanced solar wind flux along the heliospheric current sheet, which was
quite flat at this time of the solar minimum. This confirmed the idea of
Joselyn and Holzer (1975), except that they had predicted the inverse effect:
higher solar flux outside the solar equatorial plane. The effect of the groove
is visible on the left side of Figure 1 (upwind region of the maximum
emission). The two distinct maxima of the emission are seen in the north and
south lobes, which is representative for the solar minimum conditions. A
corresponding heliolatitudinal profile of the charge exchange ionization rate
is shown in Figure 2B by a red curve.

SWAN has been mapping the sky in the Lyman-$\alpha$ intensity for more than 25
years. Using SWAN data, it is possible to reconstruct a charge exchange
ionization rate of H atoms as a function of time and heliolatitude. To do
this, it is necessary to simulate a distribution of H atoms and the intensity
of backscattered radiation for every moment of time, assuming different
dependencies of the charge exchange ionization rate on heliolatitude. Then
comparing simulation results with the SWAN data, an inverse problem can be
solved to obtain the dependence that fits the data best. An early attempt to
reconstruct the solar wind structure using the SOHO/SWAN data was made by
Kyrölä et al. (1998). This developed technique is described in detail by
Quémerais et al. (2006b), and the results are presented in Lallement et al.
(2010); Izmodenov et al. (2013); Katushkina et al. (2013).

Figure 2: (A) Temporal and heliolatitudinal variations of the charge exchange
ionization rate derived from SOHO/SWAN data. Panels B and C show averaged
latitudinal profiles in particular years of the solar minimum (1996 and 2009)
and solar maximum (2003 and 2015) periods, respectively. Adapted from
Katushkina et al. (2019).

Using the SOHO/SWAN Lyman-$\alpha$ maps during 1996–2018, Katushkina et al.
(2019) have studied a temporal and heliolatitudinal dependence of the solar
wind mass flux at 1 AU. Figure 2 shows the charge exchange ionization rate
retrieved from these SWAN observations. During the solar minima (in 1996 and
2009) the charge exchange ionization rate has a distinct maximum at zero
latitude (see also Figure 2B). During the solar maxima (2003 and 2015), there
are two separated maxima at mid-latitudes ($\pm$30 – 50∘) in 2003, which seem
to be merged in 2015 (see Figure 2C). Unfortunately, these maxima in 2003 were
not observed by Ulysses (operated in 1990 – 2010), the only spacecraft that
performed in situ measurements out of the ecliptic plane (McComas et al.,
2008), because its trajectory passed between them. In Figures 2A and 2C, it is
seen that the latitude distributions are somewhat different for the two
successive solar maximum periods. Note, however, that the Sun’s magnetic field
returns to the same polarity after two 11-year solar cycles (22 years).
Continuous sky mapping with the SOHO/SWAN mission will allow determining
whether the latitudinal pattern will follow the same trend in the next 22-year
cycle.

Katushkina et al. (2019) compared latitudinal variations of the solar wind
mass flux inferred from the SWAN results and obtained from the Wang-
Sheeley–Arge (WSA)-Enlil model of the inner heliosphere. The model simulates a
global 3D solar wind structure from the solar corona to 1 AU driven by the
output from the coronal WSA model (WSA-Enlil is available for runs on request
at the website of the Community Coordinated Modeling Center,
https://ccmc.gsfc.nasa.gov/). Despite some quantitative differences, the WSA-
Enlil results are consistent with the SW latitudinal structure seen in the
SWAN data at the solar maxima. However, physical reasons for formation of the
two maxima in middle latitudes remain unknown.

Koutroumpa et al. (2019) used the same dataset of Lyman-$\alpha$ observations
from SOHO/SWAN during 1996-2018 to calculate heliolatitudinal profiles of the
SW velocity. To derive the velocity profiles, they assumed that either the
dynamic pressure or the energy flux (both calculated using the OMNI data)
conserves over heliolatitude. Both methods showed similar results (up to at
least $\pm$60∘). The profiles are consistent with annual averages of the solar
wind velocity profiles from Interplanetary scintillation observations provided
by (Tokumaru, 2013), and also with in situ measurements by Ulysses (McComas et
al., 2013). Therefore, the SW latitudinal structure derived from SWAN is in
agreement with the assumptions of the dynamic pressure and energy flux
invariance. However, during solar maxima, the differences are more pronounced.
A possible explanation is that SWAN data indicate global heliospheric
conditions, while OMNI and Ulysses data provide in situ observations of the
local SW conditions.

Strumik et al. (2020) used a database of International Ultraviolet Explorer
(IUE) spectral ultraviolet (UV) observations of astrophysical point sources
and SWAN maps to estimate the distribution of non-Lyman-$\alpha$
extraheliospheric emission in the sky. This emission is a result of a
contamination by UV continuum of stars, resolved or unresolved. They developed
a method to estimate this emission given the spectral characteristics of
individual instruments and applied it to SWAN. They found that the
extraheliospheric signal is expected at a level of several dozens Rayleigh,
peaking at the Galactic disk. Away from the Galactic disk, it is estimated at
$\sim 20$ R, but the distribution of the intensity is quite patchy. These
results support earlier SWAN sky-maps of this type of stellar contamination,
obtained with a dedicated pixel (1∘ FOV) equipped with a $\rm BaF_{2}$ filter,
which cuts off photons below 140 nm (thus, excluding Lyman-$\alpha$). Such a
map was used as a screening tool when deriving the latitudinal and temporal
distribution of the total ionization rate from SWAN sky maps (Quémerais et
al., 2006b).

Strumik et al. (2021) used SWAN observations performed during 1996-2020 to
investigate an anisotropy of the solar Lyman-$\alpha$ emission. They used the
WawHelioGlow model of the heliospheric Lyman-$\alpha$ background (helioglow)
by Kubiak et al. (2021a), the model of the solar Lyman-$\alpha$ illumination
by Kowalska-Leszczynska et al. (2020), and a model of charge exchange and
photoionization by Sokół et al. (2020), which in the charge exchange part is
based on interplanetary scintillation observations. They assumed that the
solar Lyman-$\alpha$ emission is anisotropic in heliolatitude, similar to
Pryor et al. (1992). They obtained a good agreement between the model and
observations, assuming that the ratio of the solar polar to equatorial
emission varies with time from $\sim$0.85 in solar maximum to $\sim$1 during
solar minimum. Also, they found that these ratios are not identical for the
north and south poles.

In addition to determining the heliolatitudinal and temporal evolution of the
solar wind mass flux, SWAN enabled many other important investigations with
significant results. One of them is the ability to study evolution of comets
by observing Lyman-$\alpha$ radiation backscattered by cometary H atoms
(Bertaux et al., 1997). From 1996 to 2016, 61 comets were observed, and their
$\rm H_{2}O$ production rate monitored during their passage in the inner solar
system (Combi et al., 2019, and references therein).

Bertaux et al. (2000) and Quémerais and Bertaux (2002) showed that the sky
maps of the Lyman-$\alpha$ intensity obtained by SWAN can be used to detect
solar flares, including those which occur on the far side of the Sun.
Interplanetary H atoms are like a screen on which solar Lyman-$\alpha$
radiation is scattered. Therefore, if the solar flux from some region on the
Sun increases sharply, which usually happens during flares, a noticeable
intensity increase of the backscattered radiation will be seen by SWAN in a
certain region of the sky. A solar flare, which occurs on the opposite side of
the Sun, as seen from the Earth, will face the Earth in about 14 days (half
the period of the Sun’s rotation around its axis). Thus, SWAN data allow
remote diagnostics of the flare location and intensity already up to two weeks
before it becomes available for direct viewing at the solar limb from Earth’s
orbit. These studies are directly relevant to space weather research and of
practical interest since strong solar flares are sources of solar energetic
particles which can damage spacecraft electronics.

###  3.2 Radial and angular profiles of the Lyman-$\alpha$ intensity

####  3.2.1 Voyager/UVS

Of great interest are measurements of backscattered emission by Voyager 1 and
2 spacecraft launched in 1977. The Voyagers move radially from the Sun in two
different directions in the nose part of the heliosphere ($\sim 35^{\circ}$
above (V1) and below (V2) the ecliptic plane). Voyager 1 & 2 crossed the
termination shock in 2004 and 2007 at distances of $\sim$94 AU (Decker et al.,
2005; Stone et al., 2005) and $\sim$84 AU (Decker et al., 2008; Stone et al.,
2008), respectively. The heliopause crossings from Voyager 1 & 2 occurred in
2012 and 2018 at $\sim$122 AU (Krimigis et al., 2013; Stone et al., 2013;
Burlaga et al., 2013; Gurnett et al., 2013) and $\sim$119 AU (Krimigis et al.,
2019; Stone et al., 2019; Richardson et al., 2019; Gurnett and Kurth, 2019;
Burlaga et al., 2019), respectively.

Both spacecraft have the Ultraviolet Spectrometer (UVS) on board, which is a
grating spectrograph with photons collected in 128 different channels
according to their wavelength in the range 54 – 170 nm. The Lyman-$\alpha$
signal is obtained by summing the nine spectral channels (from 70 to 78). A
detailed description of the instrument can be found in Broadfoot et al.
(1977). Note that the emission due to unresolved sources mentioned in the
previous section applies to wide-band instruments, and not to Voyager
measurements. As a matter of fact, the UVS on board Voyager is a spectrometer,
and the Lyman-$\alpha$ emission line is measured above the stellar continuum
(see all details in the Supplementary Online Material of Lallement et al.,
2011).

While the UVS instrument on Voyager 2 was turned off in 1998, the UVS on
Voyager 1 had been working until 2016. However, an abrupt decrease in the
signal was noticed in October 2014 (possibly due to issues with the
instrument), and since then, the data are more difficult to interpret. The
Voyager 1/UVS observations have been performed in three phases: (1) 1979–1993
($\sim$7–53 AU), a variety of LOS’s is used with some rolls over the sky
(Lallement et al., 1991; Hall et al., 1993; Quémerais et al., 2013a; Fayock et
al., 2015); (2) 1993–2003 ($\sim$54–89 AU), regular scans over the sky from
the upwind to downwind directions (Quémerais et al., 2009, 2010; Lallement et
al., 2011; Katushkina et al., 2016); (3) 2003–2014 ($\sim$90–130 AU), all
measurements were performed for nearly the same LOS close to the upwind
direction (Quémerais et al., 2009; Katushkina et al., 2017).

The dependence of the Voyager UVS upwind intensity with distance to the Sun
was studied by many authors based on a power-law description ($I\propto
r^{\alpha}$) to estimate how fast it falls off with heliocentric distance.
Assuming a constant density with distance, the power-law coefficient $\alpha$
is expected to vary between $-1$ and $-2$. In the limiting case of the
optically thin medium $\alpha=-1$, since the volume emission (emissivity) is
proportional to the constant density and the solar flux that is decreasing
$\propto 1/r^{2}$, and the intensity is the integral of the emissivity over
the LOS. However, the optical thickness increases with distance and modifies
the power-law coefficient, so the other limit $\alpha=-2$ of the optically
thick case (when multiply scattered photons dominate) was estimated by Hall
(1992).

The first extensive study of the radial variation of the Lyman-$\alpha$
intensity observed by Voyager UVS in the upwind direction was performed by
Hall et al. (1993), based on the hot model of hydrogen distribution. However,
the numerical values derived by Hall et al. (1993) were not accurate since the
data should be corrected for variations of the solar Lyman-$\alpha$ flux, and
their correction was based on the old solar flux measurements. Using a new
correction for the illuminating Lyman-$\alpha$ flux and a more realistic model
of the hydrogen distribution in the heliosphere including effects of the
heliospheric interface (Baranov and Malama, 1993; Izmodenov et al., 2001),
Quémerais et al. (2003) have confirmed the qualitative result obtained by Hall
et al. (1993). The analysis performed by Quémerais et al. (2003) based on the
Voyager 1/UVS data allowed estimating the intensity decrease with power-law
coefficient $\alpha=-1.58\pm 0.02$ for distances between 50 and 65 AU equal,
and, more surprising, $\alpha=-0.22\pm 0.07$ for observations at distances
further than 70 AU from the Sun. Therefore, this result indicates that the
upwind Lyman-$\alpha$ intensity is decreasing substantially slower than any
model predicts (whether it includes the effect of the heliospheric interface
or not).

The measurements of the backscattered solar Lyman-$\alpha$ emission on Voyager
1 and 2 in the period since 1993 were analyzed by Quemerais et al. (1995);
Quémerais et al. (2003, 2010). In these works, for the analysis the
distributions of H atoms were obtained from the results of the self-consistent
model of the heliospheric interface (Baranov and Malama, 1993). The Voyager
measurements are of particular interest, since during the period under
consideration they were already quite far from the Sun (at a distance of more
than 40 AU). At such distances, the influence of local effects associated with
the Sun on the distribution of H atoms becomes small, and therefore the
intensity of the backscattered Lyman-$\alpha$ emission is mainly determined by
the parameters of the atoms at the heliospheric boundary. In particular,
Quémerais et al. (2010) suggested that a hydrogen wall is “visible” in the
Voyager data. Namely, in the direction close to the LISM inflow, a region of
increased intensity (an emission “bump”) is observed, presumably associated
with the solar Lyman-$\alpha$ photons backscattered by secondary interstellar
atoms that form a hydrogen wall at the heliospheric boundary. However, later
it turned out that the conclusions of Quémerais et al. (2010) are not correct,
since the authors found an error in the radiation transfer code (Lallement et
al., 2011). Theoretical calculations have shown that the maximum intensity
present in the measurement data cannot be explained by the effect of the
hydrogen wall.

A detailed re-analysis by Lallement et al. (2011) showed that the position of
the excess intensity found in the Voyager data, correlates with strong $\rm
H_{\alpha}$ emission from so-called HII regions, mostly located along the
Galactic plane. This made it possible to conclude the Galactic nature of the
excess emission observed by Voyager. The model-dependent estimation of the
intensity of the Galactic emission was $\sim$3–4 R (toward nearby star-forming
regions). Thus, in 2011 it was first established that the Galactic
Lyman-$\alpha$ radiation can be measured from inside the heliosphere. The
measurements of the Galactic Lyman-$\alpha$ emission on the Voyager 1
spacecraft open a way for future cartographic and spectral experiments
studying the Galactic Lyman-$\alpha$ radiation, and also make it possible to
verify the radiation transfer models that are currently used in the study of
the properties and the structure of the interstellar medium. Note that this
amount of emission is restricted to the “bump” of emission seen above the
otherwise smoothly varying signal. This means that any uniformly distributed
extraheliospheric (Galactic or extragalactic) Lyman-$\alpha$ emission could
not be detected in this study and the existence of such an additional signal
(see the next paragraph), in addition to the detected signal, is not precluded
at all.

Katushkina et al. (2016) performed the analysis of the Lyman-$\alpha$ data
obtained by Voyager 1 during the spatial scans in 1993–2003 while Voyager 1
was at 53–88 AU from the Sun based on the state-of-the-art self-consistent
kinetic-magnetohydrodynamic (kinetic-MHD) model of the heliospheric interface
(Izmodenov and Alexashov, 2015) and radiative transfer model (Quémerais,
2000). The authors have shown for the first time that the ratio of the
Lyman-$\alpha$ intensities in the downwind and upwind directions in the outer
heliosphere is sensitive to the configuration (peak value and location) of the
hydrogen wall. Since the H-wall is a source of Doppler-shifted backscattered
Lyman-$\alpha$ photons, it can be seen from inside the heliosphere, so the
Voyager 1/UVS Lyman-$\alpha$ data can be used for remote sensing of the
H-wall. The authors show that the heliospheric model by Izmodenov and
Alexashov (2015), which is consistent with many other measurements including
Ly-$\alpha$ data from both Voyager 1 and 2 in 1980–1993, provides a
systematically larger downwind to upwind intensity ratio compared with the UVS
data in 1993–2003, and to decrease the ratio, a higher and/or closer H-wall is
needed.

Katushkina et al. (2017) presented the Lyman-$\alpha$ intensities measured by
Voyager 1/UVS in 2003–2014 (at 90–130 AU from the Sun). The data show an
unexpected behavior in 2003–2009: the ratio of observed intensity to the solar
Lyman-$\alpha$ flux is almost constant. The authors performed numerical
modeling of these data using the heliospheric model (Izmodenov and Alexashov,
2015), which predicted (for various interstellar parameters) a monotonic
decrease of intensity not seen in the data. Two possible scenarios that
explain the data qualitatively were proposed: (1) the formation of a dense
($\sim$10 cm-3) layer of H atoms near the heliopause that provides an
additional strongly Doppler-shifted (with velocities +50 km/s) backscattered
Lyman-$\alpha$ emission, which is not absorbed inside the heliosphere and may
be observed by Voyager 1 ($\sim$35 R of intensity from the layer is needed);
(2) the existence of an external nonheliospheric Lyman-$\alpha$ component (of
Galactic or extragalactic origin), with $\sim$25 R of additional emission (as
their parametric study showed). Due to the quite unusual parameters of H atoms
in the first scenario, the second scenario appears to be more probable.
Katushkina et al. (2017) have demonstrated that the additional constant
emission of $\sim$15-20 R leads to a good agreement with the Voyager 1 data in
1993-2003 as well, even without a higher/closer H wall suggested by Katushkina
et al. (2016). Therefore, the additional constant emission allows explaining
the Voyager 1 data not only after 2003, but also for the previous period of
observations (1993–2003).

####  3.2.2 New Horizons/Alice

After exploring the Pluto system and the Kuiper Belt object Arrokoth, now the
New Horizons mission is on the escape trajectory from the solar system. It
provides excellent opportunity for observations of interplanetary
Lyman-$\alpha$ emission in the outer heliosphere, and, in particular, to study
the Lyman-$\alpha$ background as a function of distance from the Sun. The
Alice ultraviolet spectrograph on the New Horizons spacecraft is equipped with
a telescope, a Rowland-circle spectrograph, a double-delay-line microchannel
plate (MCP) detector at the focal plane, and associated electronics and
mechanisms (Stern et al., 2008). The wavelength range of the instrument is
52–187 nm with a filled-slit spectral resolution of 0.9 nm.

During the cruise, the Alice ultraviolet spectrograph has performed multiple
observations of the interplanetary Lyman-$\alpha$ emission. In late 2007,
2008, and 2010 single-great-circle scan observations were performed during the
annual checkouts (ACOs 1, 2, and 4, at 7.6, 11.3, and 17.0 AU from the Sun,
respectively). Before the Pluto flyby (at distance $\sim$33 AU in 2015) the
observational strategy was changed to provide a denser sky using six great
circles spaced 30∘ apart. The great circles were chosen to avoid directions
towards the Sun and, as much as possible, bright UV stars. Two six-great-
circle Lyman-$\alpha$ scans of the sky were made just before and after the
encounter of Pluto, and the other five scans – during the Kuiper Belt Extended
Mission (KEM) at a cadence of roughly six months in 2017–2020 at distances
38–47 AU from the Sun. The plan for future observations is to continue this
periodicity until the crossing of the solar wind termination shock. A detailed
description of New Horizons/Alice observations can be found in the recent
papers by Gladstone et al. (2013, 2021).

Besides the comparison with the Voyager 1/UVS data, Katushkina et al. (2017)
performed a comparison of the model calculations with the New Horizons/Alice
data from ACO-2 and ACO-4 rolls (see Section 6 of their paper). To calculate
hydrogen distribution in the heliosphere, the global 3D stationary kinetic-MHD
model of the SW/LISM interaction by (Izmodenov and Alexashov, 2015) was
employed, and to calculate the solar Lyman-$\alpha$ emission backscattered by
H atoms, the radiative transfer code developed by Quémerais (2000) was used.
The comparison has shown that the model reproduces the data very well
everywhere except the upwind region, where the excess brightness is
approximately 20 R. Moreover, the value of the additional emission needed to
fit the data is very similar for both rolls, which indicates the invariance of
this excess with the distance to the Sun. Such an excess over the model
supports the findings based on the analysis of the Voyager 1/UVS data, but
does not bring the information on the origin of the additional emission.

Gladstone et al. (2018) analyzed New Horizons/Alice observations from 2007 to
2017 at distances 7.6–39.5 AU from the Sun and found that falloff of
interplanetary Lyman-$\alpha$ emission in the direction close to the upwind
can be well approximated by a $1/r$ dependence (where $r$ is the distance from
the Sun to the spacecraft) with an additional constant emission of $\sim$40 R.
Gladstone et al. (2018) concluded that it is a possible signature of the
hydrogen wall or Galactic/extragalactic background. This study also showed
that New Horizons/Alice data agrees with the earlier Voyager/UVS observations.

Gladstone et al. (2021) have analyzed further data, which include six great
circles spread over the sky at 30∘ intervals, from distances up to 47 AU from
the Sun. Based on the two independent approaches (fitting of the
Lyman-$\alpha$ brightness $1/r$ falloff with distance from the Sun and
estimation of the residuals between the observed brightness and model
calculations), the distant Lyman-$\alpha$ background of 43 $\pm$ 3 R was
evaluated. Gladstone et al. (2021) concluded that this excess emission is
constant and distributed almost uniformly over the sky, in contrast to
reported earlier (Gladstone et al., 2018) excess only in the upstream
direction. Additionally, this analysis showed a weak correlation of the
background distribution with the local cloud structure and the absence of
signatures of emission, which can be associated with the hydrogen wall.

New Horizons mission continues its journey beyond 50 AU and maps the sky in
Lyman-$\alpha$ emission intensity with Alice instrument.

##  4 What do we learn from the spectral properties of backscattered
Lyman-$\alpha$ emission?

The interplanetary Lyman-$\alpha$ background seen from 1 AU is mainly affected
by the variability of the Sun, since in its vicinity the H atoms are strongly
influenced by ionization processes (charge exchange with protons and
photoionization), solar radiation pressure and gravitational forces.
Therefore, it is challenging to study the effects of the heliospheric
interface using photometric observations in the inner heliosphere. Voyager and
New Horizons missions obtained unique observations of backscattered
Lyman-$\alpha$ emission intensity at large distances from the Sun (see
previous Sections 3.2.1 and 3.2.2). However, to distinguish contributions into
the emission of different hydrogen populations and determine properties of the
heliospheric interface, measurements of Lyman-$\alpha$ spectra are required.

A spectrum of backscattered Lyman-$\alpha$ radiation is determined by the
Doppler effect and depends on the projection of the distribution function of H
atoms on the LOS. Thus, the analysis of the spectral properties of
backscattered Lyman-$\alpha$ radiation makes it possible to obtain strong
constraints on the hydrogen distribution function inside the heliosphere and
derive information on the processes occurring in the vicinity of the
heliopause.

Figure 3:  Modeled interplanetary Lyman-$\alpha$ line profile in the upwind
direction. The simulation was performed in the self-absorption approximation
for an observer at 1 AU in the upwind direction from the Sun. The Doppler
shift velocity in the solar rest frame is presented on the x-axis in km/s. The
emission due to the primary H population is shown by the dash-dotted line, the
secondary hydrogen – by the dashed line, the hot component – by the dotted
line. The solid line presents the total emission due to the three hydrogen
populations. Adapted from Quémerais and Izmodenov (2002).

The influence of the heliospheric boundary on the backscattered Lyman-$\alpha$
emission profiles was studied by Quémerais and Izmodenov (2002) based on a
self-consistent model of the SW/LISM interaction (Izmodenov et al., 2001).
Figure 3 presents a modeled Lyman-$\alpha$ line profile in the upwind
direction, with the superposition of three populations of H atoms in the
heliosphere (see also Izmodenov et al., 2001):

  1. 1.

The primary population of the H atoms penetrates the heliosphere directly from
the LISM. The contribution of this population to the total line is shown by
the dash-dotted line. Note that this population is accelerated and cooled due
to the selection effect (the slower the atom, the higher probability to be
ionized).

  2. 2.

The secondary H atoms are slower and hotter than primary because they are
created by the charge exchange of protons in the outer heliosheath, where the
interstellar plasma is decelerated and heated, with primary H atoms. The
emission produced by the secondary population is presented by the dashed line
in Figure 3, and it has a smaller Doppler shift compared to the LISM
population line.

  3. 3.

The third population, which we refer to as the hot component, is created by
charge exchange of the core solar wind protons and pickup protons in the inner
heliosheath (so-called energetic neutral atoms, ENAs), where the solar wind is
decelerated to subsonic speed and the local plasma temperature of the order
105 – 106 K (see also Sokol et al., 2022; Galli et al., 2022). It is seen in
Figure 3, that the hot component produces a very broad spectrum, and its peak
emission is two orders of magnitude lower than the peak value of the total
line. Quémerais and Izmodenov (2002) estimated that the contribution of the
hot component to the backscattered emission intensity in the upwind direction
is less than 5% of the total intensity, and for the downwind direction it
reaches 15%. The spectrum of the hot component might be even broader (than it
is shown in Figure 3) since the ENA population was simulated under the
assumption of a Maxwellian distribution of protons in the inner heliosheath
(which are parental to ENAs), while the pickup protons should be treated
kinetically and separately from the core SW protons like it was performed by
Malama et al. (2006); Chalov et al. (2016); Baliukin et al. (2020, 2022). The
observations of the Lyman-$\alpha$ emission from the hot component can set
strong constraints on the properties of the inner heliosheath. Nevertheless,
it requires a very high signal-to-noise ratio, which is not achievable by
existing UV spectrographs.

Also, a fourth component of hydrogen exists in the heliosphere, which is
formed in the process of charge exchange of SW protons with H atoms in the
supersonic solar wind region. However, this component is minor by its
abundance, and the velocity of such atoms is $\sim$400 km/s (SW speed), which
is out of the exciting solar Lyman-$\alpha$ line.

To distinguish contributions to the Lyman-$\alpha$ emission line of different
hydrogen populations (see Figure 3), created in the heliosheath, from the
hydrogen wall, or coming from the pristine LISM, as well as the Galactic
emission component, a Doppler velocity resolution on the order of $\sim$5–10
km/s is required. This corresponds to a wavelength resolution of 0.002 – 0.004
nm or resolving power R $\sim$ 30,000 – 60,000. The highest resolution
observations of the interplanetary hydrogen were performed by STIS on board
HST (see Section 4.2) and Imaging Ultraviolet Spectrograph (IUVS; McClintock
et al., 2015; Mayyasi et al., 2017) on board Mars Atmosphere and Volatile
EvolutioN (MAVEN). However, the resolving power of these observations was only
R $\sim$ 15,000 – 20,000, which corresponds to the velocity resolution of
$\sim$15–20 km/s.

The other possibility to study the line profile is to use a hydrogen
absorption cell, which contains $\rm H_{2}$ gas that is transparent to the
Lyman-$\alpha$ photons. In the active state of the cell, the diatomic hydrogen
gas is dissociated into monoatomic (utilizing the current passing through the
filament), which is already not transparent and absorbs Lyman-$\alpha$
photons. Therefore, the cell can be used as a negative filter since it absorbs
some fraction of the Lyman-$\alpha$ photons (depending on the Doppler shift
between the cell and the incoming photon). The ratio of the intensity
observations made with the hydrogen cell in the active and inactive states
characterize the properties of the measured Lyman-$\alpha$ line profile. The
hydrogen cell was first used in the 1980s on the soviet Prognoz-5/6
spacecraft, and the SWAN/SOHO instrument is the latest instrument to use this
technique (see Section 4.3).

###  4.1 Early results: Mars-7 and Prognoz-5/6

Bertaux et al. (1976) analyzed the observations of Lyman-$\alpha$ emission on
the Mars-7 spacecraft collected in 1973–1974. The spacecraft was equipped with
a photometer and a hydrogen cell. As a result of the analysis, the first
estimates for the average velocity and temperature of hydrogen gas in the
heliosphere have been obtained: $T_{\rm H}=12000\pm 1000$ K, $V_{\rm
H}=19.5\pm 1.5$ km/s. Note that further measurements showed that these
estimates describe the parameters of interstellar H atoms after they pass
through the termination shock (i.e. at distances of about 90 AU from the Sun)
quite well.

The Prognoz-5 and 6 spacecraft performed measurements in 1976 – 1978 on the
high Earth orbit, which made it possible to minimize the influence of the
Earth’s geocorona. The spacecraft were oriented to the Sun and scanned the sky
in a plane perpendicular to the direction of the Sun along a great circle.
Bertaux et al. (1977) provided the estimate for the LISM temperature ($T_{\rm
LISM}=8800\pm 1000$ K) based on the first results of measurements by the
Prognoz-5 spacecraft. Later, a series of papers by Bertaux and Lallement
(1984); Lallement et al. (1984, 1985a, 1985b), and Bertaux et al. (1985), was
devoted to the analysis of the observations by the Prognoz-5 and 6 spacecraft.
These studies were performed based on the classical hot model for the
distribution of interstellar hydrogen in the heliosphere. The measurements
using a hydrogen cell made it possible to determine the line width of
backscattered Lyman-$\alpha$ radiation and the Doppler shift relative to the
line center. The operational principle of the hydrogen cell and the method of
data analysis are described in Bertaux and Lallement (1984). As a result of
the analysis, the velocity vector of interstellar H atoms in the heliosphere
was determined ($V_{\rm H}=20\pm 1$ km/s, $\lambda_{\rm H}=71^{\circ}\pm
2^{\circ}$ – ecliptic longitude of the vector $\textbf{V}_{\rm H}$,
$\beta_{\rm H}=-7.5^{\circ}\pm 3^{\circ}$ – ecliptic latitude of
$\textbf{V}_{\rm H}$), the temperature of the LISM is $T_{\rm LISM}=8000\pm
1000$ K, and the number density of H atoms in the LISM is $n_{\rm
H,LISM}=0.03-0.06$ cm-3. In addition, it was shown that the intensity of the
Lyman-$\alpha$ emission of a Galactic origin does not exceed 15 R, which is
less than 5% of the total intensity of backscattered emission measured in the
heliosphere. Based on the hot model of the distribution of H atoms, Lallement
et al. (1985b) showed that Lyman-$\alpha$ emission data obtained in 1976 –
1977 by Prognoz-5/6 can be explained by a 40% decrease in the charge exchange
ionization rate at the poles compared to the plane of the solar equator.

###  4.2 HST/GHRS and STIS

Figure 4:  The Lyman-$\alpha$ line profile in the upwind direction observed by
HST/STIS in March 2001. The dotted line presents the original profile. The
geocoronal emission (dashed line) was removed from the data, and the rest is
associated with the IP background emission (solid line). The x-axis shows the
wavelength in the Earth rest frame in units of Angstrom, so the geocoronal
line is centered at 1215.67 Å. Adapted from Quémerais et al. (2009).

The interplanetary Lyman-$\alpha$ line profile have been observed by the
Goddard High Resolution Spectrograph (GHRS) and Space Telescope Imaging
Spectrograph (STIS), which replaced the GHRS in 1997, on board the Hubble
Space Telescope (Clarke et al., 1995, 1998).

Observations of HST are complicated by the Earth geocorona emission at
Lyman-$\alpha$ (see also Section 4.3.3) seen from the low Earth orbit
($\sim$540 km). The emission is very bright and tends to mask the
interplanetary Lyman-$\alpha$ line. The best time for measurements is when the
Doppler shift between the Earth emission and the interplanetary line is
largest, which occurs in March each year. Figure 4 shows an example
measurement of the upwind IP line profile obtained by STIS on board Hubble
Space Telescope in early March 2001. As can be seen from these data, it is
impossible to identify the hot component of H atoms (ENAs) that originated in
the inner heliosheath because the signal-to-noise ratio is insufficient.

Linsky and Wood (1996) have analyzed the Lyman-$\alpha$ absorption spectra
recorded by HST/GHRS toward $\alpha$ Centauri and discovered the solar system
hydrogen wall, a pile-up of the H number density in front of the heliopause.
In addition to the ISM and heliospheric absorptions, the Lyman-$\alpha$
absorption spectra towards nearby stars illustrate absorption from the
analogous ”astrosphere” surrounding the star (Wood et al., 2005). Since the H
wall neutrals are heated and decelerated, for an observer inside the
heliosphere the heliospheric absorption is redshifted (relative to the ISM
absorption), while the analogous astrospheric absorption is blueshifted, so
these absorptions avoid complete obscuration by the ISM.

The interplanetary hydrogen (IPH) line shift (or apparent velocity) has been
estimated from GHRS and STIS data by several authors (Clarke et al., 1998;
Scherer et al., 1999; Ben-Jaffel et al., 2000; Quémerais et al., 2006a). One
of the most recent studies by Vincent et al. (2011) presented an updated
analysis of IPH velocity measurements from GHRS and STIS during solar cycle 23
and summarized all previous estimations of the line shift (compiled in tables
2 and 3 of Vincent et al., 2011).

To observe the IPH along a line of sight, STIS has been used in cross-
dispersed echelle mode with the long slit
($$52\text{\,}\mathrm{\SIUnitSymbolArcsecond}$\times$0.5\text{\,}\mathrm{\SIUnitSymbolArcsecond}$$).
Using a long slit allows the collection of more photons and increases the
signal-to-noise from an extended source (such as IPH). However, this
configuration results in the superposition of different orders of the echelle
spectrum and contamination of the Lyman-$\alpha$ line (121.6 nm) by
overlapping light from the 130.4 nm triplet line of geocoronal oxygen ($\rm
OI$) in lower orders. Vincent et al. (2011) showed a significant change in
apparent velocity due to this contamination, not accounted for in earlier
reports. Their reanalysis of the observations of HST/GHRS and HST/STIS
provided IPH line shifts of 22.2 $\pm$ 1.5, 24.0 $\pm$ 0.9, and 22.4 $\pm$ 0.4
km/s in 1994, 1995, and 2001, respectively, consistent (within 1$\sigma$) with
predictions of the two different numerical models by Quémerais et al. (2008)
and Scherer et al. (1999). However, some discrepancy has been found in
comparison with SOHO/SWAN observations in 1997 and 1998.

Vincent et al. (2014) studied the IPH measurements provided by the HST/STIS
during solar cycle 24. The results of the analysis have been compared with
SWAN observations and simulations of the global heliospheric model by
Izmodenov et al. (2013) using two sets of LISM physical parameters based on
Ulysses (Witte, 2004) and IBEX (McComas et al., 2012) measurements, which
predict different LISM velocity vectors. The authors showed that the IPH data
(both from STIS and SWAN) fit Ulysses-based model results better than IBEX-
based model results. Their analysis of HST/STIS observations also provided IPH
line shifts of 23.5 $\pm$ 0.5 km/s and 23.3 $\pm$ 0.5 km/s in 2012 and 2013,
respectively.

###  4.3 SOHO/SWAN hydrogen cell observations

The absorption hydrogen cell of the SWAN instrument is placed in the optical
path and filled with $\rm H_{2}$ gas, which is transparent to the
Lyman-$\alpha$ emission. When the cell is activated, two tungsten filaments
become heated. They dissociate the molecules into atoms, which results in a
cloud of H atoms that scatters the Lyman-$\alpha$ photons near the line
center. The characteristics of the hydrogen cell are the optical thickness at
line center $\tau_{\rm c}$ and temperature $T_{\rm c}$, and for the SWAN
instrument, these parameters are $\tau_{\rm c}\approx 3$ and $T_{\rm c}\approx
300$ K (see Quémerais et al., 1999). For each LOS, the photons are counted
first with the cell switched off, giving the intensity $I_{\rm off}$, and then
with the H cell switched on, which gives the intensity $I_{\rm on}$.

Figure 5: Full sky sinusoidal projections (in ecliptic J2000 coordinates) of
the Lyman-$\alpha$ intensities $I_{\rm off}$ (panel A), $I_{\rm on}$ (panel
B), $I_{\rm off}-I_{\rm on}$ (panel C), and reduction factor $R=I_{\rm
on}/I_{\rm off}$ (panel D) observed by SWAN/SOHO on 26 January 1997. Adapted
from Baliukin et al. (2019).

Figure 5 presents the distribution (in ecliptic coordinates) of Lyman-$\alpha$
intensities, which were measured by SWAN on 26 January 1997: panel A shows the
intensity $I_{\rm off}$, panel B represents the intensity $I_{\rm on}$, panel
C shows the difference of intensities observed at two hydrogen cell states
($I_{\rm off}-I_{\rm on}$), and panel D is the ratio $R=I_{\rm on}/I_{\rm
off}$ (a so-called reduction factor). The great circle that passes through the
ecliptic poles is seen in panels B and C, and it is called a Zero Doppler
Shift Circle (ZDSC). This circle consists of directions perpendicular to the
relative motion between the interstellar hydrogen flow and SOHO, so for these
directions, the interplanetary hydrogen is absorbed by the H cell the most
(Bertaux and Lallement, 1984). The motion of the ZDSC along the year includes
the composition of the orbiting Earth’s velocity vector ($\sim$30 km/s) with
the fixed vector of the interstellar flow velocity of about 26 km/s. At $\sim
100^{\circ}$ ecliptic longitude and zero latitude, there is a brighter region,
which is produced by the Lyman-$\alpha$ emission from the geocorona, even more
conspicuous on panel C (see Section 4.3.3).

Costa et al. (1999) performed a parametric study based on a single-component
hot model and determined the parameters of H atoms at 50 AU from the Sun. The
results of Costa et al. (1999) confirmed the existence of secondary
interstellar H atoms, which leads to a decrease in the velocity and an
increase in the temperature of the total flow of atoms at 50 AU from the Sun
in comparison with the parameters of the primary interstellar atoms in the
LISM. Using the hydrogen cell absorption map, Costa et al. (1999) derived the
LOS temperature (or linewidth) variation with the angle from the upwind
direction directly from the data and found a temperature minimum between the
upwind and crosswind directions, while classical models predict a monotonic
increase of the linewidth from upwind to downwind. The existence of two
distinct populations of H atoms (primary and secondary) with different
velocities was suggested to explain this behavior.

####  4.3.1 Line profile reconstruction

Quémerais et al. (1999) have developed a technique to derive the
interplanetary Lyman-$\alpha$ line profiles by accumulating one year of
hydrogen absorption cell measurements performed by the SWAN instrument on
board SOHO. For a given fixed LOS, the spectral Doppler scan is provided by
the orbital velocity of the Earth ($\sim$ 30 km/s) over one full year. The
achieved Doppler scan is $\pm$ 30 $\times\cos\beta$ km/s, where $\beta$ is the
ecliptic latitude of the LOS. The spectral scan across the Lyman-$\alpha$ line
allowed deriving the line profiles and, consequently, the velocity
distribution. This technique is independent of both the hydrogen distribution
model and the multiple scattering effects. Thus, the results represent the
ground truth, which puts strong constraints on the global heliospheric models
of SW/LISM interaction.

Quémerais et al. (2006a) continued the work on the line profile reconstruction
technique. While Quémerais et al. (1999) were focused only on the first year
of measurements, Quémerais et al. (2006a) have reconstructed the
interplanetary Ly-$\alpha$ line profiles using the SWAN hydrogen cell
observations from the solar minimum in 1996 to the solar maximum in 2001 and
found that the line shift (LOS apparent velocity) of the interplanetary
Lyman-$\alpha$ line in the upwind direction varies significantly (from 25.7
km/s to 21.4 km/s from solar minimum to solar maximum, respectively). The
analysis of the line width (LOS temperature) suggested that the interplanetary
line consists of two components, which are produced by the backscattered
photons by two distinct populations of hydrogen that have different parameters
(bulk velocities and temperatures). This circumstance is an obvious sign of
the heliospheric interface influence on the line profiles observed at 1 AU
from the Sun. The SWAN data comparison with an interplanetary background
upwind spectrum obtained by HST/STIS in March 2001 yielded a good agreement
(within the uncertainties and possible biases involved in the analysis).

####  4.3.2 Estimation of the hydrogen flow direction

Another extremely important result obtained using the SWAN data is the proof
of the effect of the interstellar magnetic field (IsMF) on the heliosphere. A
model-independent method of Lyman-$\alpha$ line profile reconstruction
developed by Quémerais et al. (1999) allowed also to estimate the H flow
direction ($\lambda$, $\beta$)H = ($252.3^{\circ}\pm 0.73^{\circ}$,
$8.7^{\circ}\pm 0.9^{\circ}$) in J2000 ecliptic coordinates.

Later, Lallement et al. (2005, 2010) analyzed SWAN data obtained with the
hydrogen cell using a single-component hot model of the hydrogen distribution
in the heliosphere. Temperature and bulk velocity of atoms at 80 AU from the
Sun in the model were set based on previous results from Costa et al. (1999),
and the bulk velocity direction was varied. As a result, the direction of the
velocity in the heliosphere, which leads to the best agreement between theory
and SWAN measurements, was determined. The values (ecliptic J2000 coordinates)
were found with two independent analyses ($\lambda$, $\beta$)H = (252.5∘ $\pm$
0.7∘, 8.9∘ $\pm$ 0.5∘), which differs by $\sim$4∘ from the direction of motion
of interstellar helium atoms in the LISM, i.e. ($\lambda$, $\beta$)He =
(255.4∘ $\pm$ 0.5∘, 5.2∘ $\pm$ 0.2∘) (Witte, 2004).

The plane formed by velocity vectors of H and He atoms is commonly called the
Hydrogen Deflection Plane (HDP). It should be clarified here that, unlike H
atoms, interstellar helium atoms penetrate the heliosphere almost freely since
they do not interact with protons (due to the small cross-section for charge
exchange and large ionization potential). Therefore, it is believed that the
direction of motion of helium atoms in the heliosphere, which is known from
both direct measurements of helium by the GAS instrument on board the Ulysses
spacecraft and indirect measurements of the UV radiation of the Sun
backscattered by them, coincides with the direction of the incident flow of
the LISM. Thus, Lallement et al. (2005, 2010) showed that during the passage
of the region of the heliospheric interface, the direction of the average
velocity of H atoms deviates by several degrees compared to the direction of
their velocity in the LISM. This effect was explained by the influence of the
interstellar magnetic field. The point is that if the interstellar magnetic
field is inclined with respect to the LISM velocity vector relative to the
Sun, then the LISM plasma flow in the vicinity of the heliopause will become
asymmetric (Izmodenov et al., 2005). Therefore, secondary interstellar H atoms
produced as a result of charge exchange on protons near the heliopause will,
on average, be deviated from the initial direction of the LISM motion. This is
exactly what is observed in the SWAN data on the backscattered solar
Lyman-$\alpha$ radiation. The effect of hydrogen deflection due to the
influence of the interstellar magnetic field was soon confirmed theoretically
by Izmodenov et al. (2005) based on the analysis of the simulation results of
the global self-consistent kinetic-MHD model of the heliospheric interface,
which take into account the interstellar magnetic field.

However, the direction of interstellar hydrogen flow in the vicinity of the
Sun may be influenced by other effects such as the solar radiation pressure
and gravitation, ionization, and kinetic non-Maxwellian properties of the
hydrogen distribution and also may depend on other LISM parameters besides
IsMF. Katushkina et al. (2015a) have performed theoretical modelling of the
backscattered solar Lyman-$\alpha$ radiation seen at 1 AU from the Sun and
analyzed the direction of H flow in the heliosphere, which can be obtained
from the spectral properties of the backscattered radiation, and investigated
the influence of different effects mentioned above. The results of simulations
in the frame of the state-of-art 3D time-dependent kinetic model of the H
distribution, which is based on the global heliospheric model by Izmodenov and
Alexashov (2015), were compared with the SWAN data of 1996. Katushkina et al.
(2015a) concluded that deflection of the interstellar hydrogen flow in the
heliosphere is potentially a very powerful tool to estimate the IsMF, but in
doing this the considered effects need to be taken into account.

Koutroumpa et al. (2017) suggested a method to determine the longitude of the
interstellar H flow, which is model-independent and based on the parallax
effect provided by the SWAN motion around the Sun. Even without the use of the
H cell data, Koutroumpa et al. (2017) have shown that for 20 years covered by
the SWAN dataset, the longitude of the interstellar hydrogen flow vector
varies insignificantly from its average value of $252.9^{\circ}\pm
1.4^{\circ}$, which reinforces the assumption of the interstellar gas flow
stability.

####  4.3.3 Geocorona

Baliukin et al. (2019) have studied the Lyman-$\alpha$ observations of the
geocorona performed by the SOHO/SWAN in January 1996, 1997, and 1998 (low
solar activity). The use of the H cell allowed assigning almost the entire
difference of intensities $I_{\rm off}-I_{\rm on}$ to the geocorona emission
since, in this region of the sky, the interplanetary line is Doppler shifted
with respect to the H cell. Still, the small contribution from IP background
absorption was simulated and subtracted from the original data. To calculate
the IP background, the model by Katushkina et al. (2015b) with the boundary
condition at 70 AU from the global heliospheric simulations by Izmodenov and
Alexashov (2015) was used. The major result is that the emission from the
geocorona was found to extend up to $\sim$100 $R_{\rm E}$ from Earth
($6.4\times 10^{5}$ km), which is far beyond the Moon’s orbit ($\sim$ 60
$R_{\rm E}$) and exceeding the earlier estimate ($\sim$ 50 $R_{\rm E}$)
obtained with the LAICA imager (Kameda et al., 2017). For a distant observer
outside the exosphere, the geocoronal intensity is of the order of 5 R at
$\sim$100 $R_{\rm E}$ and about 20 R at the Moon’s orbit. The geocorona may be
an unwanted source of an excess Lyman-$\alpha$ emission for an observatory
dedicated to the universe exploration in UV.

##  5 Conclusions

In this paper, we summarized a knowledge about the heliosphere deduced from
numerous measurements (both photometric and spectrometric) of the
backscattered solar Lyman-$\alpha$ emission collected since the beginning of
the space era. We illustrated that analysis of Lyman-$\alpha$ observations
using physics-based global models has proven itself as an effective tool for
indirect diagnostics of the parameters of H atoms in the heliosphere,
properties of the interstellar medium, and the physical processes at the
heliospheric boundary. The following summarizes major discoveries enabled by
the studies and analysis of the backscattered solar Lyman-$\alpha$ emission:

  * (i)

Backscattered Lyman-$\alpha$ observations proved that parameters of
interstellar hydrogen in the solar system are different from those of
interstellar helium. Indeed, the interstellar hydrogen is decelerated by $\sim
3-4$ km/s and heated by $\sim$5000 K compared to helium. This provides
observational evidence for the effective charge exchange process between
interstellar hydrogen and slowed down and heated interstellar plasma around
the heliosphere. Similarly, such interaction would create slower and warmer
interstellar hydrogen around other astrospheres. Hence, the Lyman-$\alpha$
observations confirm an existence of hydrogen wall.

  * (ii)

SWAN/SOHO Lyman-$\alpha$ observations with the H cell discovered the
$4^{\circ}$ deflection of the hydrogen flow in the heliosphere relative to the
pristine flow in the LISM. This is a signature of an asymmetry of the global
heliosphere caused by the interstellar magnetic field.

  * (iii)

Analysis of the SOHO/SWAN intensity maps established variations of the solar
wind mass flux with heliolatitude and time over more than two solar cycles. In
particular, two maxima at mid-latitudes were discovered during the solar
activity maximum (of the solar cycle 23), which Ulysses missed due to its
specific trajectory.

  * (iv)

Lyman-$\alpha$ observations in the outer heliosphere on Voyager/UVS and New
Horizons/Alice revealed a presence of the extraheliospheric emission with a
magnitude in the order of a few tens Rayleigh.

These discoveries were key to advance our understanding of the ion-neutral
coupling processes determining the dynamic interaction of the heliosphere with
the LISM. However, a number of open questions exist that have to be addressed
with the future Lyman-$\alpha$ experiments and studies.

###  5.1 Open questions and future developments

We outline a list of investigations which can be done in the nearest (or not
in so nearest) future to advance the field and obtain more valuable
information from the existing and future Lyman-$\alpha$ data. Investigations
can be divided into two categories depending on a type of measurements,
photometric or spectrometric.

The following lists investigations with photometric Lyman-$\alpha$
observations (intensity):

  1. 1.

Various studies of Lyman-$\alpha$ emission (e.g., Quemerais et al., 1996;
Pryor et al., 2008) determined the reference number density of interstellar
hydrogen in the range of $\sim 0.08-0.15$ cm-3. These values refer to the
density at large distances (e.g., at 70–80 AU) in the upwind direction. Two
factors contribute to a large uncertainty: (1) the uncertainties and issues in
the calibration of Lyman-$\alpha$ instruments (the major effect), and (2) the
differences in methodology to determine the number density from the
Lyman-$\alpha$ data obtained mostly at a small heliocentric distance (1 AU).

The issues and details on calibration and inter-calibration have been reported
by Quémerais et al. (2013b). There are ways to improve and verify existing
calibration using numerical models. For instance, a comparison of long-term
observations with results of model calculations of Lyman-$\alpha$ intensities
allows estimation of the sensitivity degradation of a particular instrument.
This approach was applied to the SOHO/SWAN 1996–2018 dataset. A preliminary
result shows that the sensitivity of the SWAN sensor units degrades at a rate
of $\sim$2% per year. This should be taken into account in the future data
analysis.

The second factor is connected with the fact that in most studies, a hot model
and its modifications are applied to obtain the reference number density.
These models assume that parameters of interstellar hydrogen are uniform at
large heliocentric distances ($\sim$70-80 AU) and velocity distribution is
Maxwellian or bi-Maxwellian. These assumptions are not confirmed by self-
consistent models of the global heliosphere that show both radial and angular
gradients in the number density. A straightforward way is to use a global
model of the heliosphere and to use the H atom number density in the
undisturbed LISM as the reference number density. Global models of the
heliosphere require values of interstellar proton number density and magnitude
and direction of the undisturbed interstellar magnetic field. These values can
be derived using different observations (for instance, from Voyager) and
multi-parametric fitting of the model results with various data. Izmodenov et
al. (2003) performed such study and estimated number densities of H atoms and
protons in the undisturbed LISM: $n_{\rm H,LISM}=$ 0.2 cm-3 and $n_{\rm
p,LISM}=$ 0.04 cm-3, respectively. Recently, Bzowski et al. (2019) performed a
parametric analysis to determine the ionization state of the LISM and
concluded that $n_{\rm H,LISM}=$ 0.154 cm-3 and $n_{\rm p,LISM}=$ 0.054 cm-3.
Both Izmodenov et al. (2003) and Bzowski et al. (2019) used the reference
number density of interstellar hydrogen in the outer heliosphere obtained by
Bzowski et al. (2008) in the analysis of pickup ion observations from Ulysses
(Gloeckler et al., 1992; Gloeckler and Geiss, 2001), and the slowdown of solar
wind due to mass-loading by pickup ions, measured by Voyager (Richardson et
al., 2008).

However, a recent analysis of the pickup ion data from New Horizons/SWAP
(Swaczyna et al., 2020) suggested that the density of ISN H at the termination
shock is larger by almost 40% than previous estimates. Additionally, Swaczyna
et al. (2021) showed that elastic collisions in the outer heliosheath heat ISN
H and He by $\sim$1000 K. Thus, LISM temperature determined from in-situ
measurements on board Ulysses and from IBEX is overestimated. The actual
temperature of the LISM could be about 6500 K (from IBEX) or about 5300 K
(from GAS). However, existing models of the heliosphere, to our knowledge, do
not take into account these heating effects.

An updated analysis of newly calibrated Lyman-$\alpha$ intensity data must be
performed to derive independently both the reference number density of
interstellar hydrogen in the outer heliosphere and the LISM parameters. The
latter can be done using new state-of-the-art models which include recently
recognized physical processes such as effects of elastic collisions or the
electron thermal conduction in the inner heliosheath (Izmodenov et al., 2014).

  2. 2.

Another direction of future research is related to deriving the solar wind
flux as a function of the heliolatitude and time. For the first time, a
possibility of using measurements of the backscattered solar Lyman-$\alpha$
emission for diagnostics of the inhomogeneous structure of the solar wind was
noted by Joselyn and Holzer (1975). Quémerais et al. (2006b); Lallement et al.
(2010) applied inversion methods to the SOHO/SWAN Lyman-$\alpha$ maps. On the
one hand, recent analysis of SOHO/SWAN sky mapping data (Katushkina et al.,
2013, 2019; Koutroumpa et al., 2019) suggest that solar wind flux maxima at
mid-latitudes occur at maxima of solar activity. WSA-Enlil model of the solar
wind propagation show a qualitative agreement with the derived structure. On
the other hand, analysis of interplanetary scintillation (IPS) data (Sokół et
al., 2013; Bzowski et al., 2013a; Sokół et al., 2020; Porowski et al., 2022)
shows that these maxima may occur from time to time but do not seem to be a
regular solar wind feature. However, the IPS observations (Tokumaru, 2013;
Tokumaru et al., 2021) provide only heliolatitudinal profiles of solar wind
speed on a yearly timescale. To compute the flux, the SW density profile is
required. To calculate the density, Sokół et al. (2013); Bzowski et al.
(2013a) used the SW velocity-density relation derived from observations by
Ulysses. For the same purpose, Sokół et al. (2020); Porowski et al. (2022)
used the empirically derived (based on HELIOS, Wind, and Ulysses data)
latitudinal invariance of the SW energy flux (Le Chat et al., 2012). However,
the assumption about the invariance is not physically justified.

Will the solar flux structure have maxima at mid-latitudes in the upcoming
solar maximum (solar cycle 25)? What are the physical processes responsible
for their formation? These questions remain to be answered.

  3. 3.

Interstellar H atoms surrounding the Sun behaves like a screen on which solar
Lyman-$\alpha$ photons are projected. Active regions on the Sun (solar flares)
produce higher intensities of the backscattered Lyman-$\alpha$ emission, and
they are visible in the sky maps. As shown by Bertaux et al. (2000) and
Quémerais and Bertaux (2002), Lyman-$\alpha$ intensity sky maps can be
utilized to forecast the solar activity in advance up to two weeks (a half the
solar rotation period). Additionally, by applying an inversion technique to
the maps, location and strength of the active regions on the far side of the
Sun can be estimated. Future developments and automation of inversion
algorithms will provide an important contribution to a wide range of tools
used for space weather prediction.

The following lists investigations with spectrometric Lyman-$\alpha$
observations (line profiles):

  1. 1.

A measurement of the deflection of the interstellar hydrogen flow in the
heliosphere is potentially a powerful tool to constrain a direction of the
magnetic field. Lallement et al. (2005, 2010) performed analysis of the
SOHO/SWAN data with H cell using a hot model of interstellar hydrogen with a
single Maxwellian boundary condition. They determined a deflection of the
hydrogen flow by $\sim 4^{\circ}$. However, later Katushkina et al. (2015a)
demonstrated that a two-component model is required to estimate the
deflection. Additionally, one needs to take into account the solar radiation
pressure, gravitation, ionization, and kinetic non-Maxwellian properties of
the hydrogen distribution. Therefore, the SWAN reduction factor data should be
re-analyzed using the two-component hydrogen model to determine the deflection
and compare with previous finding.

From the observational point of view, to differentiate contributions of
primary and secondary populations of interstellar H atoms to the observed
Lyman-$\alpha$ emission spectra, a very high spectral resolution ($\sim$5
km/s) is required. To investigate the structure and properties of the hydrogen
wall, spectral observations from observing points far away from the Sun are
required. At large distances from the Sun, the emission from the H wall
becomes significant compared to the rapidly decreasing heliospheric emission.

  2. 2.

The hot component of H atoms which originates in the inner heliosheath (ENAs)
has not been detected yet in Lyman-$\alpha$ observations. To do this,
measurements need to have an extremely high signal-to-noise ratio, not
achieved by the existing Lyman-$\alpha$ instruments. Analysis of the ENA
distribution using Lyman-$\alpha$ spectra can serve as a new tool for studying
a nature of the heliosheath. A feasibility study of using Lyman-$\alpha$
spectra to constrain ENA distribution will be valuable in light of the planned
future space missions (such as Interstellar Probe, see Section 5.2.2), which
carries a UV spectrometer in an example payload. From the modelling
perspective, to make a correct estimate of the hot component contribution to
the observed intensities of the interplanetary Lyman-$\alpha$ emission, the
ENAs and their parental component (pickup protons) should be simulated with a
kinetic approach.

  3. 3.

SOHO/SWAN data show a minimum of the Lyman-$\alpha$ linewidth near the
crosswind direction, while all numerical models with a single component of H
atoms predict its monotonic increase from upwind to downwind. Costa et al.
(1999) and Quémerais et al. (2006a) suggested that the existence of primary
and secondary hydrogen atoms can explain this behavior. Katushkina and
Izmodenov (2011) have explored a nature of the linewidth minimum theoretically
and proven this assumption using model simulations. However, the minimum of
the Lyman-$\alpha$ linewidth appears only for the two-component hot model, as
shown by Katushkina and Izmodenov (2011), while there is no distinct minimum
in the simulations with the model that takes into account an influence of the
heliospheric interface. It means that the non-Maxwellian features of the
velocity distribution of H atoms (such as strong anisotropy of the kinetic
temperatures) caused by the heliospheric interface compensate for the effect
of two populations. This explanation is also supported by the recent
calculations based on the state-of-the-art global heliospheric model by
Izmodenov and Alexashov (2020).

Therefore, the nature of the Lyman-$\alpha$ linewidth minimum in the data
remains unknown.

One can assume that the minimum can be explained by the presence of the hot
population of H atoms (ENAs) in the heliosphere, but numerical calculations to
justify/reject this assumption have not been performed yet. More detailed
spectroscopic measurements and/or numerical simulations are needed to resolve
this question.

  4. 4.

None of the existing models of hydrogen distribution in the heliosphere is
capable to quantitatively reproduce the observed by SWAN/SOHO Lyman-$\alpha$
intensities from the upwind and downwind directions simultaneously. What is
the reason for the discrepancy between models and data, and what physical
processes are missing in current models?

A possible answer is the inclusion of the absorption of solar Lyman-$\alpha$
photons by H atoms in the model of the solar radiation pressure, which may
explain the differences at least partially. Kowalska-Leszczynska et al. (2022)
showed that the effect of absorption modifies the hydrogen distribution
through the reduction of the radiation pressure force and, as a consequence,
it generally provides higher (by a few percent) number density, especially in
the downwind direction. However, an impact of the modified hydrogen
distribution on the simulated Lyman-$\alpha$ intensities was not studied yet.

The other aspect missing in most models is the hot component of H atoms.
Quémerais and Izmodenov (2002) performed calculations of the backscattered
Lyman-$\alpha$ emission produced by the hot component. They found that its
contribution to the total intensity seen from 1 AU can reach 15% in the
downwind direction. However, this simulation was based on the assumption of a
Maxwellian distribution of protons, but the population of pickup protons
should be treated kinetically to estimate the actual contribution of the hot
component.

  5. 5. Table 1: Estimates of the extraheliospheric Lyman-$\alpha$ component emission.

Reference | Data source | Observation | Distance to | Estimation
---|---|---|---|---
|  | years | the Sun (AU) | (Rayleigh)
Lallement et al. (1984) | Prognoz-5 and 6 | 1976-1977 | 1 | $\leq 15$
Lallement et al. (2011) | Voyager 1(2)/UVS | 1993–2003 | 53–88 (38–70) | 3–4
Katushkina et al. (2017) | Voyager 1/UVS | 2003–2014 | 90–130 | 25
Gladstone et al. (2018) | New Horizons/Alice | 2007–2017 | 8–40 | 40
Strumik et al. (2020) | SOHO/SWAN, IUE | 1997 | 1 | 28.9a
Gladstone et al. (2021) | New Horizons/Alice | 2015–2020 | 32–47 | 43 $\pm$ 3

Note. aBackground from unresolved point sources that was evaluated
specifically for observations of the SWAN instrument.

Several authors, mentioned earlier in this chapter, reported evidence of an
excess emission on top of the helioglow (see Table 1, which summarizes
existing estimations of the extraheliospheric background). The question of the
origin (Galactic, extragalactic, contamination by UV continuum of stars,
resolved or unresolved) and spatial distribution of the extraheliospheric
emission is connected with the exploration of the interstellar matter in the
Galaxy and beyond and can be resolved using spectrometric measurements. The
extraheliospheric component of Lyman-$\alpha$ emission can be studied directly
far away from the Sun, where it is not obscured by the interplanetary glow. It
is important to note that the extraheliospheric background is neglected in
most analyses of the helioglow, but its consideration could probably affect
the results.

These questions will hopefully be addressed by the future missions, such as
GLObal solar Wind Structure (GLOWS) experiment on board Interstellar Mapping
and Acceleration Probe (IMAP; see the following Section 5.2.1) and a UV
spectrometer on an Interstellar Probe (Section 5.2.2).

###  5.2 Future missions

####  5.2.1 IMAP/GLOWS

The forthcoming NASA mission IMAP is scheduled for launch in 2025 into a
Lissajous orbit around the Lagrange point L1 (McComas et al., 2018). GLOWS is
one of ten instruments on IMAP, and it will measure the heliospheric resonant
backscattered Lyman-$\alpha$ emission. The science objective of GLOWS is to
investigate a global heliolatitudinal structure of the solar wind and its
evolution during a solar cycle. Additionally, GLOWS investigates the
distribution of ISN hydrogen and the solar radiation pressure acting on ISN H.
The GLOWS detector is a non-imaging single-pixel Lyman-$\alpha$ photometer,
effectively a photon counting instrument. It is conceptually based on the Two
Wide-angle Imaging Neutral-atom Spectrometers (TWINS)/LaD photometer (Nass et
al., 2006). The instrument includes a collimator with a baffle, a spectral
filter, and a channeltron (CEM) detector. They are connected to an electronics
block, responsible for collecting the event pulses and binning them for
downlinking to the ground. The instrument is designed and assembled in the
Space Research Centre PAS (CBK PAN) in Warsaw, Poland.

The idea behind the observation strategy of GLOWS goes back to analysis of the
evolution of the solar wind structure by Bzowski (2003); Bzowski et al.
(2003). These authors took SWAN observations performed from the downwind and
upwind locations of the SOHO spacecraft and limited to Sun-centered great
circles in the sky. They assumed that the solar wind is bi-modal, slow and
dense at the ecliptic plane and fast and rarefied in the polar regions. Also,
they assumed that transitions from slow to fast wind occur rapidly as a
function of heliolatitude, and the transition latitudes vary during a cycle of
solar activity, independently in the north and south hemispheres. They allowed
for hypothetical differences between the solar wind flux at the north and
south poles and simulated the expected signal on a grid of the fast/slow
boundaries and pole/ecliptic flux ratios. With this, they simulated the
expected helioglow profiles along the selected great circles in the sky on a
grid of contrast magnitudes and boundary latitudes. They determined the best-
matching boundaries and contrast magnitudes between the north and south-polar
flux and that in the ecliptic plane between the solar minimum in 1996 and
solar maximum in 2002. As verified much later (Bzowski et al., 2013b), these
results turned out to be similar to those obtained from analysis of
interplanetary scintillations.

Based on this insight, GLOWS will scan a 75∘ circle in the sky, centered at a
point offset by 4∘ from the Sun towards lower ecliptic longitudes. With this,
the lines of sight will intersect the solar polar axis approximately within
the maximum emissivity region, and therefore, the observations are expected to
be the most sensitive to the solar wind imprints in the density distribution
of ISN H gas (Kubiak et al., 2021b).

The diameter of the field of view of the instrument is $\sim$4∘ full width at
half maximum (FWHM). Such a narrow field of view, provided by a specially
designed entrance system with a collimator, a baffle, and a sunshield,
eliminates solar glint and any diffuse radiation from the outside of the
desired region of the sky. After a day of observations, the spin axis of IMAP
is redirected to maintain the 4∘ offset from Sun’s center, and the strip
observed by GLOWS accordingly shifts in the sky.

Since the extraheliospheric sources, both those bright and spatially resolved,
and those dim and unresolved were found to provide a significant contribution
to the SWAN signal (Strumik et al., 2020), which is quite challenging to
mitigate, GLOWS takes measures to eliminate them almost entirely. This is
accomplished by using a narrow-band interference filter, which lets in only a
waveband of $\sim$5 nm. While the helioglow emission is practically
monochromatic and close to the Lyman-$\alpha$ wavelength, the stellar and
galactic emissions are not. Applying a narrow-band filter suppresses the light
from the outside of the filter transmission band. As a result, with the
filter, the relative contribution of starlight in the total signal is less
than without the filter because a large portion of starlight is cut out. With
this, the expected magnitude of the stellar contribution is lower from that
found for SWAN by a factor of 20.

Since GLOWS has only one detector, challenging issues of intercalibration
between two SWAN sensors observing the northern and southern hemispheres will
be absent. The planned GLOWS observations will be binned at a high resolution
(3600 bins for 360∘ of the scanning circle), so that contributions from bright
stars can be isolated and used for establishing the absolute calibration of
the instrument on one hand, and for tracking the inevitable changes in this
calibration during the mission on the other hand. This observation strategy is
expected to mitigate the challenges currently faced by SWAN.

####  5.2.2 Interstellar Probe

Interstellar Probe is a heliophysics mission concept to fly into the nearby
interstellar space through the heliosphere boundary with the science goal to
“Understand our habitable astrosphere and its home in the galaxy”.
Interstellar Probe will investigate (1) the fundamental physical processes
which form the heliosphere boundary and uphold the vast heliosphere, (2) the
global dynamics of the heliosphere driven by the Sun’s activity and possible
LISM structures, (3) characteristics of the LISM. Understanding the properties
of interstellar H atoms and ion-neutral interaction processes from the Sun to
the LISM span all the science objectives of the mission.

An Interstellar Probe mission with a UV spectrograph on board will answer the
compelling science questions by making high-spectral-resolution ($\sim 5-10$
km/s) measurements of scattered Lyman-$\alpha$ emission in different lines of
sight on outward trajectory from the Sun reaching distances up to 400 AU. The
spectral measurements will enable (1) constraining the hydrogen velocity
distribution function throughout the heliosphere and beyond in the LISM, (2)
discovering a position of the hydrogen wall and inferring the 3D structure of
this unique unexplored global feature, (3) determining the properties of hot H
atoms created in the heliosheath and their spatial variations (enabling an
independent diagnostic of the global heliosheath structure), (4) determining a
deflection of interstellar hydrogen flow in the heliosphere compared to
pristine interstellar flow and discovering any deviations from the previously
reported deflection of 4∘ (Lallement et al., 2005, 2010), and (5) identifying
Galactic and extragalactic components of Lyman-$\alpha$ (Lallement et al.,
2011; Katushkina et al., 2017; Gladstone et al., 2018, 2021).

Spectra taken at different distances from the Sun and in different LOS will
for the first time enable global diagnostics of the non-Maxwellian velocity
distribution function of the interstellar hydrogen and therefore understanding
of plasma–hydrogen coupling processes in the context of the global
heliosphere–LISM interaction. To infer spatial variations of LOS hydrogen
velocity distributions, multiple look directions are required, in particular
toward the nose, toward the tail of the heliosphere, and sidewise, covering at
least half of the sky. A measurement cadence on the order of a few months
would be sufficient to investigate possible variations of Lyman-$\alpha$
spectra within a year due to solar effects (Quémerais et al., 2006b).

UV instruments that are capable of resolving a line profile of backscattered
Lyman-$\alpha$ emission include a high-resolution spectrograph and a spatial
heterodyne spectrometer (SHS). MAVEN/IUVS is an example of a spectrograph that
includes a far-UV spectral channel that uses an echelle grating to resolve H
and D Lyman-$\alpha$ lines (echelle channels were also implemented in the
HST/GHRS and HST/STIS). These instrument packages are typically large. A
reduced-mass compact spectrograph with a high spectral resolution and
sensitivity is a subject of the future technology development for a UV
instrument on the Interstellar Probe mission.

An alternative to grating spectrographs would be an SHS with a high resolving
power and a compact design which recently has been under development for
laboratory tests and sounding rocket flights (Harris and Corliss, 2018;
Hosseini, 2019). At the time of this publication, SHS has not yet demonstrated
a capability to measure a Lyman-$\alpha$ line profile in space.

###### Acknowledgements.

This work was made possible by the International Space Science Institute and
its interdisciplinary workshop “The Heliosphere in the Local Interstellar
Medium”, URL: www.issibern.ch/workshops/heliosphere.
M.B. was supported by Polish National Science Centre grant
2019/35/B/ST9/01241. E.P. was supported by NASA Task Order NNN06AA01C.

## Conflict of interest

The authors declare that they have no conflict of interest.

## Acronyms

AU – astronomical unit, ENA – energetic neutral atom, GHRS – Goddard High
Resolution Spectrograph,
GLOWS – GLObal solar Wind Structure, HST – Hubble Space Telescope, IBEX –
Interstellar Boundary Explorer, IMAP – Interstellar Mapping and Acceleration
Probe, IP – interplanetary, IPH – interplanetary hydrogen, IPS –
interplanetary scintillation, IsMF – interstellar magnetic field, ISM –
interstellar medium, ISN – interstellar neutral, IUE – International
Ultraviolet Explorer, IUVS – Imaging Ultraviolet Spectrograph, LISM – local
interstellar medium, LOS – line of sight, MAVEN – Mars Atmosphere and Volatile
EvolutioN, MHD – magnetohydrodynamic, OGO-5 – Orbiting Geophysical Observatory
number 5, SHS – spatial heterodyne spectrometer, SOHO – Solar and Heliospheric
Observatory, STIS – Space Telescope Imaging Spectrograph, SW – solar wind,
SWAN – Solar Wind ANisotropy, UV – ultraviolet, UVS – Ultraviolet
Spectrometer.

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