# The Structure and Dynamics of the Upper Chromosphere and Lower Transition
Region as Revealed by the Subarcsecond VAULT Observations

A. Vourlidas1B. Sanchez Andrade-Nuño1,2E. Landi1S. Patsourakos2L. Teriaca3U.
Schühle3C.M. Korendyke1I. Nestoras4 1 Space Science Division, Naval Research
Laboratory, 4555 Overlook Ave, SW, Washington, D.C., USA 11email:
<EMAIL_ADDRESS>George Mason University, 4400 University Dr, Fairfax,
VA, USA 11email<EMAIL_ADDRESS>MPI for Solar System Research,
37191 Katlenburg-Lindau, Germany 11email<EMAIL_ADDRESS><EMAIL_ADDRESS>Max-Plank-Institut für Radioastronomie, Auf dem Hügel 69,
53121 Bonn, Germany

###### Abstract

The Very high Angular resolution ULtraviolet Telescope (VAULT) is a sounding
rocket payload built to study the crucial interface between the solar
chromosphere and the corona by observing the strongest line in the solar
spectrum, the Ly$\alpha$ line at 1216Å. In two flights, VAULT succeeded in
obtaining the first ever sub-arcsecond ($0.5\hbox{${}^{\prime\prime}$}$)
images of this region with high sensitivity and cadence. Detailed analyses of
those observations have contributed significantly to new ideas about the
nature of the transition region. Here, we present a broad overview of the
Ly$\alpha$ atmosphere as revealed by the VAULT observations, and bring
together past results and new analyses from the second VAULT flight to create
a synthesis of our current knowledge of the high-resolution Ly$\alpha$ Sun. We
hope that this work will serve as a good reference for the design of upcoming
Ly$\alpha$ telescopes and observing plans.

###### keywords:

line: Hydrogen Ly alpha —- atomic data —- Sun: corona —- Sun: UV radiation —-
Sun: transition region

##  1 Introduction s:Intro

The structure of the solar atmosphere as a function of temperature has been a
’thorny’ issue of solar physics research for decades. As the density
decreases, the temperature, instead of decreasing, abruptly increases from
$\sim 10^{4}$ K to a million K within a thousand km. It is known since the
first solar imaging space missions that this so-called temperature transition
region (TR) between the chromosphere and the corona, is also where the
morphology of the atmospheric structures changes strongly. At the base of the
atmosphere, the photosphere consists of small scale convective granules
interlaced with occasional smaller intergranular lanes concentrating strong
magnetic flux elements($|B|\leq 1kG$, e.g. 2004Natur.430..326T). The
chromosphere (T$\leq 10^{4}$ K for the discussion here) consists of a very
rugged, inhomogeneous, and very filamentary layer blanketing the photosphere.

Beginning at the chromosphere, the geometry of the individual structures is
increasingly dominated by the local magnetic field. At the lower transition
region (T$\leq 2\times 10^{5}$ K), the structures strongly reflect the
morphology of the underlying supergranular network. As the magnetic pressure
overtakes the gas pressure leading to the low beta corona, the percentage of
emission in filamentary loops steadily increases until the network completely
disappears at temperatures above $10^{6}$ K. It may seem that a
straightforward interplay between heating and morphology takes place in the
transition region but this is not the case.

The traditional picture of the transition region as the interface between the
footpoints of large-scale structures and their coronal tops has been
contradicted by the weakness of its observed emission [Landi and Feldman
(2004)]. While the emission in the upper TR ($T>2\times 10^{5}$K) can be
understood in terms of heat conduction from the corona along magnetic field
lines, the lower TR ($T<2\times 10^{5}$ K) cannot. Instead, this plasma forms
a completely separate component of the solar atmosphere [Feldman (1983),
Feldman (1987)]. This component could consists of small “cool” loops
[Antiochos and Noci (1986), Dowdy, Rabin, and Moore (1986), Feldman, Dammasch,
and Wilhelm (2000), Peter (2001)] that are best seen in the Quiet Sun and that
probably correspond to the upper reaches of the mixed polarity magnetic carpet
[Schrijver et al. (1997)]. 2009ApJ…693.1474F showed that the Differential
Emission Measure (DEM) of the TR has the same shape everywhere (coronal holes,
Quiet Sun, active regions) while coronal DEM of the very same regions are very
different. Why and how are transition region loops different from higher
arching coronal loops? Are they also comprised of unresolved strands? Are they
heated in a fundamentally different way? Recently, Judge (2008) proposed a
radically different view of the transition region emission, suggesting that it
might result from cross-field diffusion of plasma from very fine cool threads
extending into the corona (e.g. spicules), and its subsequent ionization. Cool
threads gradually expand in thickness as the ionizing front expands across the
field lines and emits at TR temperatures, and provide images of the transition
region similar to those observed by the SUMER [Wilhelm et al. (1995)]
spectrometer aboard SOHO.

Hinode observations revealed a dramatically new picture of the solar
chromosphere and demonstrated its potential importance for the dynamics,
energy and mass supply of the transition region and corona. High temporal
($\simeq$5s) and spatial ($\simeq$0.2′′) Hinode/SOT observations have shown
that the chromosphere is much more structured and dynamic than previously
believed. SOT has revealed a chromosphere hosting a wealth of wave and
oscillatory phenomena manifested as longitudinal and transverse motions within
structures at the resolution limit [de Pontieu et al. (2007a), Ofman and Wang
(2008), Okamoto et al. (2007)]. Even a fraction of the inferred wave energy
flux could account for the coronal energy losses if it reached the corona. SOT
also showed that a significant fraction of observed spicules (’type II’),
known for decades to dominate the chromospheric landscape, disappear very
rapidly (De Pointieu et al. 2007b). This was interpreted as a signature of the
plasma heating up to transition region and coronal temperatures; the mass
contained in these disappearing spicules is sufficient to account for the mass
present in the corona.

Capturing the fine spatial scales and rapid temporal evolution of the
chromosphere and transition region plasmas represents a considerable
observational and technical challenge. Nonetheless, recent significant
improvements on instrumentation and image processing has been achieved both
from ground (e.g., 2006A&A…454.1011P, 2008SoPh..251..533R, 2007ASPC..368…65D)
or spaceborne instruments (e.g. 2007PASJ…59S.655D), reaching in all cases
spatial resolution under 1′′ for plasmas at chromospheric regimes. Reaching
these resolution on the TR involves the use of strong UV lines, accessible
only above the Earth’s atmosphere.

The Very high Angular resolution ULtraviolet Telescope (VAULT,
2001SoPh..200…63K), a sounding rocket payload, is the only instrument that has
observed this critically important layer of the solar atmosphere at such high
resolution. VAULT is specifically designed to obtain high spectral purity,
zero dispersion spectroheliograms in the Lyman-$\alpha$ (1216 Å) resonance
line of hydrogen. This emission line emanates from plasmas at 8000 to 30000K
[Gouttebroze, Vial, and Tsiropoula (1986)]. The Ly$\alpha$ radiation directly
maps the dominant energy loss from plasmas at these temperatures which
correspond to the lower TR [Fontenla, Reichmann, and Tandberg-Hanssen (1988)].
This instrument is the latest in a long and distinguished line of solar
optical instruments obtaining observations in the Ly$\alpha$ emission line
[Purcell and Widing (1972), Prinz (1974), Bartoe and Brueckner (1975), Bonnet
et al. (1980)]. The VAULT observations are the highest quality UV observations
of the solar atmosphere ever obtained and are a considerable improvement over
previous instruments. Each rocket flight obtained observations with observable
structures of $<0.5\hbox{${}^{\prime\prime}$}$ spatial scale, exposure times
of 1 second with a 17 second cadence and a 355′′$\times$235′′ instantaneous
field of view (FoV).

The VAULT data and, more recently, the Hinode/SOT observations have
invigorated the debate about the nature of the solar Transition Region. Not
surpsingly, Ly$\alpha$ telescopes are planned for the upcoming Solar Orbiter
mission, and, possibly, the proposed Solar-C mission. It is therefore, an
appropriated time for a review of the VAULT observations. We believe that as a
trailblazer project in the exploration of the upper chromosphere-corona
interface, the VAULT experiences will be a useful reference for the instrument
design and science operations for those missions. We also take this
opportunity to present the final calibration of the data and introduce the
project website where all the data are publicly available. This paper presents
a detailed examination of the Ly$\alpha$ structures near the base of the solar
corona obtained during the second flight of the payload (hereafter, VAULT-II).
We are specifically concerned with those plasmas whose temperatures lie
between 8 000 and 30 000K, ranging roughly from $\sim$2 000 km to $\sim$60 000
km above the photosphere.

The paper is organized as follows. Section s:calib describes the latest
instrument calibration and the observations from the second VAULT flight.
Section s:ly summarizes the importance of the Ly$\alpha$ in the frame of
Coronal and TR models. Section s:inten discusses the sources of Ly$\alpha$
emission as determined in the VAULT images. Sections s:prom, s:qs, s:spicules
focus on, respectively, prominences, Quiet Sun, and spicules. We discuss our
findings and conclude in Sections s:dis-s:con.

Figure 1.: The total solar field of view observed during the second VAULT
flight. The image is a composite of all VAULT-II observations after dark
current subtraction and flatfielding. It covers a $\sim 600\times 450$ arcsecs
area, with $0.12\hbox{${}^{\prime\prime}$}$ pixel size. Solar North is to the
right and Solar East at the top of the image. The image is plotted with
histogram equalization of the intensities.fig:composite

##  2 Data Analysis and Observationss:calib

VAULT has been successfully launched twice (May, 7, 1999 and June, 14, 2002).
Using the experience from the first flight [Korendyke et al. (2001)], the
instrument performance during the second flight was improved by using a higher
transmission Ly$\alpha$ filter (higher throughput) and better filtering of the
power converter output (lower noise/higher quality data). So we concentrate on
the VAULT-II images for the remainder of the paper.

VAULT-II was flown on June 14, 2002 from White Sands Missile Range onboard a
Black Brant sounding rocket. The observations took place around the apogee of
the parabolic trajectory while the rocket was above 100 km. This minimum
altitude was chosen to minimize absorption effects from the geocorona [Prinz
and Brueckner (1977)]. The duration of science operations was $363$ sec and
the rocket peaked at an altitude of 182 miles (294 km). The entire flight,
from launch to recovery, lasted 15 minutes.

Figure 2.: _Left:_ Alignment of VAULT-TRACE Ly$\alpha$ images. We find the
best correlation by optimizing the position, rotation and scale in both x-y
directions. We derive a pixel size of $0.125\hbox{${}^{\prime\prime}$}\times
0.110\hbox{${}^{\prime\prime}$}$. The TRACE Ly$\alpha$ was taken at 18:18:54
UT (B&W figure and white contour), the VAULT was taken at 18:17:30 UT (three
red contours). _Right:_ Estimation of the VAULT resolution using the thin
spicule located at the center of the image. The median normalized cross
section (in arcseconds) of the spicule along its length (plot inset in figure)
is fitted with a gaussian, which leads to a FWHM $\approx
0.49\hbox{${}^{\prime\prime}$}$ as an upper limit. fig:align

fig:resolution

###  2.1 VAULT-II Observations s:obs

VAULT-II obtained 21 images from 18:12:01 to 18:17:47 UT with a cadence of 17
seconds. The integration time was 1 second for all frames except for a 5-sec
image (the 2nd in the series, not shown here). The target was an old active
region complex near the east limb which included NOAA regions 9997-9999, Quiet
Sun, filaments, plage and the limb.

Figure fig:composite is a composite image of all VAULT-II frames. The
composite field of view (FOV) covers nearly 10% of the total visible solar
disc area. To investigate possible center-to-limb variation [Miller, Mercure,
and Rense (1956)] we have calculated the radial median intensity of non-active
region areas (excluding plage region, prominences, flaring regions). We do not
find any significant center-to-limb gradient in agreement with Curdt et al.
(2008).

The VAULT flight was supported by several other instruments. All corresponding
data (see table tab:JOP) are available online or per request.

All VAULT data are publicly available online in FITS format and compatible
with _SolarSoft_ mapping routines. The images are interaligned, the dark level
is subtracted and an _ad-hoc_ synthetic flat-field is also created and
provided with the data, but not applied on the online set. The flat-field is
generated by retrieving the median (in time) pixel value as the solar image
moves during the observations. It therefore accounts for flatfield and
scattered light. Intensities are left in DN.

To improve the visibility of faint, small scale structures, we have applied a
wavelet enhancement technique Stenborg, Vourlidas, and Howard (2008). This
method decomposes the image into frequency components (scales). The frequency
decomposition is achieved by means of the so-called a-trous algorithm. With
this method we can then obtain an edge-enhanced version of the original image
by assigning different weights to the different scales upon reconstruction. We
note that the aforementioned decomposition does not create orthogonal
components, and therefore the reconstruction does not conserve the flux. It is
also possible to retain the low-scale information by adding a model background
image. Both processed data, with and without the model are freely available in
the VAULT website. The level 0.9 VAULT-II data, together with the IDL-
_SolarSoft_ routines, composite full field image, flat-field and wavelets
processed data are available under:
`http://wwwsolar.nrl.navy.mil/rockets/vault/`

Telescope |  Channel |  co-temporal time serie
---|---|---
SOHO |  MDI/EIT/CDS |  EIT-304 Å partial FoV
TRACE |  171, WL,1600 |  171 Å, partial FoV
BBSO |  H$\alpha$,Ca,BLOS,WL |  H$\alpha$, partial FoV
Kitt-Peak |  Mgram |  Photospheric magnetogram
Table 1.: Joint observing campaign supporting the VAULT-II launchtab:JOP

###  2.2 Spatial resolution s:spatial

The rocket pointing accuracy was $\sim 1$ arcmin with exceptional pointing
stability of 0.25′′peak-to-peak over 10 sec. To obtain the solar coordinates,
rotation relative to solar North, and pixel size for the VAULT-II images we
used TRACE Ly$\alpha$ images taken only minutes apart from the VAULT images.
Figure fig:align shows the alignment results. The resulting VAULT pixel size
is 0.125′′$\times$0.110′′ which is in excellent agreement with the optical
design expectations Korendyke et al. (2001). During the flight, a small
thermal expansion of the spectrograph structure relative to the primary mirror
resulted in an apparent pointing drift which was variable but less than
$\approx$3 pixels/second. If it was uniform during the flight, this drift
would place a conservative lower limit on the instrument resolution of $\sim
0.75\hbox{${}^{\prime\prime}$}$ (0.375′′/pixel). However, we could visually
identify smaller structures (always in absorption on disk) in several images.
To better estimate the actual image resolution we measured the FWHM of the
smallest structure we could find in the images. We used the median cross
section along the 110-pixel length of the thinnest spicule, located at the
center of Figure fig:resolution (right), and fitted it with a Gaussian. The
$FWHM\approx 2.35\cdot\sigma$, where $\sigma$ is the standard deviation of the
fitted gaussian profile. This leads to a VAULT-II resolution
$0.49\hbox{${}^{\prime\prime}$}$. However, the upper limit may be dictated by
opacity effects rather than instrumental ones.

The photometric calibration of the instrument was originally determined from
the observations during its first flight in May 1999\. The calibration factor
from digital units (DNs) to intensity (ergs s-1 cm2 sr-1) was deduced by
comparing the average emission (in DNs) of an area of the Quiet Sun to the
Quiet Sun intensity obtained by Prinz (1974). This was a reasonable assumption
since both observations were made at similar phases of the cycle; the majority
of the VAULT I field of view contained Quiet Sun and the 1974ApJ…187..369P
measurements are well calibrated ($\sim 20\%$).

A comparison to SUMER Ly-$\alpha$ observations was made to improve on the
radiometric accuracy of our measurements. The first issue was the spectral
purity of the signal. The VAULT gratings transmit solar light in the range of
$1140-1290$Å. In this range there are only few relatively bright lines, the
brightest of which is Si iii at 1206.51Å. SUMER Quiet Sun spectra show that
almost all ($\sim 95$%) of the emission in this range comes from the
Ly-$\alpha$ (assuming a rectangular filter). Concerning the spectral purity on
the full range above 120 nm, our calculations show that the signal should be
70% pure.

Figure 3.: Normalized H i Ly$\alpha$ distributions as obtained from SUMER data
(dashed line) and the VAULT data (solid line) rebinned and convolved to match
the SUMER spatial resolution after subtracting a background signal level of 80
DN s-1 (see section s:spatial). fig:sumer

Since the SUMER instrument has a photon counting detector with no dark signal,
there is no background to be removed from the SUMER data. To establish the
comparison with SUMER we assumed that the normalized radiance frequency
distributions over quiet-Sun areas produced with data from the two instruments
should be equal or very similar to each other. To account for the different
spatial resolution we have also computed the radiance frequency distributions
after convolving the VAULT data with a 2-D Gaussian function (of 12
pixel=1.5′′ FWHM, equal to the SUMER spatial resolution) and binning over
$8\times 8$ pixels to yield the SUMER pixel size of $\approx
1^{\prime\prime}$. The comparison revealed that a low level signal of about
$\sim 80$ DN s-1 needs to be removed from the VAULT images to bring them in
accordance to the SUMER measurements (Figure fig:sumer. After a careful
examination of the VAULT-II images, we found a noise pattern of $83\pm 20$ DN
s-1 which is variable from image to image and cannot therefore be removed with
the dark current subtraction. We have traced the source of the noise to
interference from a faulty ground when the payload is switched to battery
power.

The final step is a comparison of the Quiet Sun level in our images with an
average Quiet Sun radiance measured at Earth as we did for the first flight.
For the VAULT Quiet Sun level we used the peak of the histogram of the image
intensities (in DN s-1) minus the 83 DNs of the background signal. The Quiet
Sun level was $217\pm 20$ DN s-1. The SUMER average Ly$\alpha$ radiance on the
Quiet Sun in 2008 was $73\pm 16$ W m-2 sr-1. This is well within uncertainties
with the 1974ApJ…187..369P measurement of $78\pm 16$ W m-2 sr-1. We adopt the
latter value for consistency with our VAULT-I results and because it was
obtained about two years after maximum and may better compare with our 2002
data. In this case, we derive a calibration factor of 1 DN s-1 = $0.359\pm
0.081$ W m2 sr-1.

Figure 4.: VAULT-II, TRACE Ly$\alpha$, and BBSO H$\alpha$ comparison of the
solar limb. All intensities are individually trimmed and scaled to emphasize
the fainter details. _Left:_ VAULT-II after alignment and calibration. The
black contour follows the visible limb, as aligned with the TRACE White light
channel. _Center:_ TRACE Ly$\alpha$. The black grid denotes the photosphere.
The prominence is much fainter than in the VAULT images but it is still
visible. The white contour close to noise level helps to correlate with VAULT.
_Right:_ H$\alpha$ channel from ground-based BBSO. The BBSO FOV is slightly
smaller than the other instruments. The dotted contour denotes the VAULT edge.
Note that the H$\alpha$ spicule heights are up to $\sim
2\hbox{${}^{\prime\prime}$}$ shorter than in Ly$\alpha$. fig:offlimb

Figure 5.: Prominence as seen in almost simultaneous observations with various
instruments. The contours mark the outer envelope of the Ly$\alpha$
prominence. The field of view is the same. _Top left_ : VAULT Ly$\alpha$._Top
right_ : TRACE 171Å. _Bottom right_ : SOHO/MDI photospheric magnetogram.
_Bottom left_ : BBSO H$\alpha$ center.fig:4promin

##  3 The interpretation of the Ly$\alpha$ emission s:ly

The hydrogen Lyman-$\alpha$ line, the strongest line of the solar spectrum, is
a $1s~{}^{2}S_{1/2}$ \- $2p~{}^{2}P_{1/2,3/2}$ doublet resonant line at
1215.67Å. The FWHM of the line core is very broad ($\sim 1$Å ) due to Stark
and Doppler broadening and the high optical thickness. The line center
probably forms in the lower TR ($\sim 40000$ K; 1981ApJS…45..635V) while the
wings form in the chromosphere ($\sim 6000$ K) by partial redistribution of
the core emission. Thus, the Ly$\alpha$ line plays a critical role in the
radiation transport in the chromosphere/TR interface. Below 8000 K, model
calculations show that the line is very close to detailed balance. For
temperatures between approximately 8000 and 30000 K, the dominant energy loss
is through Ly$\alpha$ emission. For temperatures higher than about 30000 K,
Ly$\alpha$ is transparent Gouttebroze (2004). The physics of this line have
been explored in a number of papers Vernazza, Avrett, and Loeser (1981);
Gouttebroze, Vial, and Tsiropoula (1986); Woods et al. (1995); Fontenla,
Avrett, and Loeser (2002); Gouttebroze (2004) , and the average full-Sun line
profile and its variation over the solar cycle has been measured by the SUMER
instrument Lemaire et al. (2004) but most deal with the spectral
characteristics and are of more interest to spectroscopic analysis. On the
contrary, VAULT data consist of the integrated line intensity over a wide
bandpass which includes contributions from other lines such as Si iii, N i, N
v, and C iii. Because of the complexity of the line, model calculations are
the easiest way to interpret imaging observations. Past analysis was based on
plane parallel radiative transfer models using the Ly$\alpha$ contrast (the
ratio of the Ly$\alpha$ emission of a structure relative to the average Quiet
Sun) to derive estimates of pressure and temperature within the observed
structures Bonnet and Tsiropoula (1982); Tsiropoula et al. (1986). Recent
computational and theoretical improvements have enabled the calculation of the
emission from models with more realistic cylindrical geometries Gouttebroze
(2004) and therefore direct comparison with observed Ly$\alpha$ intensities
Gunár et al. (2006); Patsourakos, Gouttebroze, and Vourlidas (2007). However,
calculations from the latter models remain time-consuming and difficult to
apply over the wide range of structures seen in the VAULT images. Since the
scope of our paper is to present a broad overview of the Ly$\alpha$
atmosphere, we return to the plane parallel assumption and adopt the approach
of 1986A&A…167..351T to estimate physical paramaters for the structures in our
images. More careful analyses of specific features will be undertaken in the
future.

The calculations in 1986A&A…154..154G require the calculation of the ratio of
the intensity of a given structure over the average intensity over the solar
disk or “Ly$\alpha$ relative intensity” (LRI). Since we do not have full disk
images in Ly$\alpha$ we cannot compute directly a solar disk average. However,
the disk emission is dominated by the Quiet Sun (Figure 5 in
1974ApJ…187..369P) and we therefore need only to calculate the Quiet Sun
level. Thanks to the large FOV, the VAULT images contain large Quiet Sun
areas. So, we use the median of the lower part of the FoV
$\sim(x\in[-550,-400],\forall y)$ in Figure fig:composite as the ”Quiet Sun”
level. We then calculate the LRI range for several representative features.

The results are shown in Table tbl:meas. The corresponding pressure,
temperature and optical thicknesses derived from 1986A&A…154..154G are also
included. The numbers suggest that most solar structures are optically thick
in Ly$\alpha$ even at temperatures departing significantly from chromospheric
ones ($\geq 10^{4}$ K). Quiet Sun emission seems to arise at the chromosphere
while plage, prominence and offlimb structures have lower TR temperatures and
are presumably located at larger heights. These results are in agreement with
the earlier measurements of 1986A&A…167..351T except of the minimum LRI
values. 1986A&A…167..351T reported values as low as 0.05 but do not observe
LRI below about 0.2 anywhere but at the edges of offlimb loops. The difference
is most likely due to higher sensitivity and spectra purity of the VAULT
instrument which should increase the detected counts of the fainter structures
and minimize the continuum contribution to the Quiet Sun levels relative to
past instruments. The faintest structures (LRI $\sim 0.2$) seen in the VAULT
images are long, thin strands seen in absorption against the network. These
strands are also the smallest resolved structures with the lowest temperatures
(Table tbl:meas). They are very similar to chromospheric filaments but they do
not seem to be associated with any large scale structure. Their origin is
currently a mystery but they could be cooling loops. The best candidates for
optically thin emission are the offlimb loops seen in the northeastern edge of
the VAULT FOV. The observed LRI range of 0.4–0.5 could be consistent with
either chromospheric ($<10^{4}$ K) or TR emission. These loops were not
detected in the BBSO H$\alpha$ images and thus we selected the higher
temperature solutions (T$\sim 3-4\times 10^{4}$K) for them.

Structure | Intensity | Radiance | Opt. Depth | T | Pressure
---|---|---|---|---|---
| [LRI]† | $[10^{12}{ergs\over cm^{2}s\ sr}]$ | $Log$ | [$10^{3}$K] | [dyn/cm2]
Quiet Sun | 0.5 — 5 | 3.3 — 32.5 | 4 — 5 | 8 — 10 | 0.1 — 1
Quiet Sun Prom. | 0.2 — 1.4 | 1.8 — 9.5 | 6 — 3 | 7 — 9 (20)* | 1 (0.1)*
Plage | 5.7 — 12 | 37.5—75.0 | 4 | 10—13 | 1
Plage Prom. | 1—5 | 6.7—32.5 | 3—0 | 8—40 | 0.1—1
Offlimb Prom. | 0.8—1.1 | 5.8—7.8 | 3—0 | 15—80 | 0.1—1
Offlimb Loops | 0.4—0.5 | 2.8—3.8 | 0 | 30—40 | 0.1
Table 2.: Qualitative plasma diagnostics for several types of structures. See
Section s:ly.
* Likely to have reduced optical thickness. High values reflect underlying plage.
$\dagger$ Ly$\alpha$ relative intensity (LRI). LRI=1 represents median of
Quiet Sun region.tbl:meas

Table tbl:meas serves as a concise description of the physical parameters of
Ly$\alpha$ structures and we will refer to it in our subsequent discussions of
individual features starting the contribution of each of these features to the
overall Ly$\alpha$ intensity.

##  4 Sources of the Ly$\alpha$ Intensity s:inten

Figure 6.: Ly$\alpha$ emission histogram (black line). Different colors
represent partial histograms from the labeled subregions. In particular we
note that the ”Quiet Sun” emissions spans one order of magnitude. Intensities
are scaled to the median Quiet Sun level. Plot ordinates scaled to total
number of data pixels. Cover area for each type (integral over the histogram
curve) is: Total (black line): 100%, Quiet Sun: 61%, Plage:13%, Filament: 2%,
Flaring region: 1%, Offlimb: 1%, Rest: 23%. See text on Section s:dis for more
details.histogram

Ly$\alpha$ is a very optically thick line and results in both emission and
absorption depending on the properties of the surrounding plasma. This
interplay is at the region where the plasma starts to be dominated by the
magnetic fields, creating a wide range of intensities. On the other hand, the
strength and variability of the Ly$\alpha$ irrandiance has important effects
on Earth because it affects the chemistry of the mesosphere (e.g., ozon layer)
as well as the climate on longer time scales. Only the central part of the
broad spectral profile of the solar Ly$\alpha$ emission is effective for the
geo-environment. But there is a clear relationship between the central
radiance of the solar Lya line and the total irradiance of the line Emerich et
al. (2005).To understand changes in Ly$\alpha$ irradiance we first need to
identify the contributions of the various solar sources of this emission to
the total Ly$\alpha$ irradiance.

We attempt a first cut at this problem using our spatially resolved,
calibrated images. As we discussed before, we are able to differentiate among
Quiet Sun, Plage, Prominence over Plage, Offlimb and Flaring regions. Figure
histogram shows the corresponding intensity histograms for each domains (color
coded), relative to the overall histogram (black line). The values are
constructed from the pixels inside each region, and considering the median
value for each pixel in time (from Figure fig:composite):

_Quiet Sun (blue line):_ We select a region around the lower right corner in
Figure fig:composite as typical Quiet Sun. Based on this selection, the Quiet
Sun covers 61% of the pixels. We use the median value of the Quiet Sun as a
normalizing factor. Normalized values inside this region, however, span from
0.5 to 5. The Quiet Sun exhibits a wide range of intensities, as it can be
expected by the high optical thickness and strong structuring of the plasma.
The low end of the histogram reaches the edge detection of offlimb
prominences, while the high end reaches the plage levels. Scattered around
this Quiet Sun we find several cases of localized brightenings which may be
related to explosive events, which we discuss in Section s:qs.

_Plage (green line):_ The central part of the VAULT FOV shows a bright plage.
Following a similar method as for the Quiet Sun we find that the plage covers
13% of the pixels, without considering the central overlying filament. Typical
normalized intensities range from 5 to 15\. The only other contribution at
these levels comes from the flaring region at the north edge of the image.
This means that one approximation to the total solar Ly$\alpha$ irradiance can
be obtained using the Quiet Sun level adding a multiplying factor $\sim$7 for
the percentage of the disc corresponding to plages (which could be obtained
from other lines like Ca).

_Filaments over plage (red line):_ Our results show that the plasmas in the
filaments over the plage are sufficiently opaque to reduce the observed
intensity to Quiet Sun values. This particular filament blocks the central 22%
of the plage area.

_Offlimb (purple line):_ The VAULT images contain several examples of limb
structures, including spicules. As discussed later, we find higher heights for
the spicules compared to H$\alpha$. Large overlying loops reaching projected
heights of 60′′ can also be observed. The emission from these structures
indeed shows Quiet Sun levels, down to our detection threshold for the
histogram (0.5). It is likely that these structures are nearly optically thin,
implying temperature $\gtrsim$30,000K. The large heights imply a dynamic state
for these loops and they are probably associated with catastrophic cooling
episodes studied previously with TRACE Schrijver (2001).

##  5 Prominence and Filament Observations s:prom

The images contain a large number of filaments, filamentary structures and a
prominence and it is the first time that the fine scale structure of the
filaments is resolved in this wavelength. Figure fig:threads reveals a highly
organized filament comprised of parallel threads with little, if any, twist.
No obvious twist is evident in any of the other filaments as well. The threads
have a typical width of around 0.5′′or less, and are seen as intensity
enhancement profiles of about 5%. Figure fig:threads also shows a stable and
detached thread with a width reaching the instrument resolution and 30%
absorption over the underlying plage. The filament is further analyzed in
Millard:2009jk where the comparison with the H$\alpha$ observations suggests
that Ly$\alpha$ traces the cool outer plasma while H$\alpha$ originates from
the coolest part of the filament. There is also evidence for uneven absorption
across the filament axis. The northern side shows evidence of Ly$\alpha$
absorption while the southern side shows absorption only in the coronal lines
(171Å ) consistent with the presence of a void or cavity around the filament.
The northern absorption could be understood as a line of sight effect from
low-lying absorbing plasma at the filament flanks.

Figure 7.: Prominence shows reduced absorption threads along its axis below
0.5′′. This supports a non-axisymetric prominence. _Top_ : Single VAULT frame,
with labeled segments for the perpendicular cuts (labeled as ’A’ and ’B’).
Intensities are not scaled. _Bottom_ : Intensity profile perpendicular to the
prominence axis for ’A’ segment and ’B’ detached thread (plot inset).
Intensities are scaled to surrounding median plage value.fig:threads

The last panel in Figure fig:offlimb shows the size discrepancy between
Ly$\alpha$ and H$\alpha$ observations. Only a small knot, $\sim
5\hbox{${}^{\prime\prime}$}$ width, of H$\alpha$ emission is visible whereas
the Ly$\alpha$ prominence extends for almost 50′′.

##  6 Quiet Sun observations s:qs

Figure 8.: Detail of a supergranular cell in the Quiet Sun in Ly$\alpha$. The
horizontal extent is 121′′ and the vertical is 117′′ and the field of view is
centered at around (-500, 250) in Figure fig:composite. These are the first
spatially resolved images of the Ly$\alpha$ emission of a cell interior.fig:qs

The Quiet Sun has been the testing ground for the various theories and
concepts of the structure of the solar atmosphere. It is not surprising then,
that it is also the area where VAULT observations have generated the most
interesting results Patsourakos, Gouttebroze, and Vourlidas (2007); Judge
(2008). Earlier observations showed that Ly$\alpha$ emission is concentrated
along the supergranular lanes in clumps with small loop-like extentions
towards the cell interiors. Faint emission without spatial structures was
detected at the cell centers. VAULT images, especially VAULT-I which covered a
much larger Quiet Sun area, resolved the spatial stucture in the clumps along
the supergranular boundaries (Figure fig:qs). The Quiet Ly$\alpha$ Sun area
shows groupings of filamentary plasma, similar to the H$\alpha$ rosettes, with
a typical diameter of $\sim 23\hbox{${}^{\prime\prime}$}$. These rosettes show
filamentary structure up to resolution limit of the instrument, of about
$0.4\hbox{${}^{\prime\prime}$}$. This grouping in rosettes is stable through
the observations ($\sim 6$ min) but shows the presence of localized
brightening events with a timescale variation $60-120$ sec and sizes of a
couple of arcseconds. The network structures rise above the chromosphere about
7100 km or 10′′ as seen in Figure fig:offlimb. This measured value is
consistent with previously measured values of the height of the transition
region above the limb. Their location at the supergranular cell boundary
uniquely identifies these loops as being the byproduct of convective motion
driving together magnetic fields at the edges of the supergranular cell.

The outer areas consist of short loop-like structures while the centers of the
clumps have a more point-like nature. This morphology is consistent with loops
of progressively higher inclination towards the center of the boundary. The
obvious question is whether the more extended Ly$\alpha$ loops are full loops
or just the lower part of a larger structure, possibly extending to higher
temperatures. 2007ApJ…664.1214P applied an analysis method used for coronal
loops to a detailed Ly$\alpha$ emission model and found that the short loops
at the edges of the boundary channel were consistent with full Ly$\alpha$
loops and therefore could account for the “cool” loops predicted by models of
the transition region Dowdy, Rabin, and Moore (1986). However, the magnetic
footpoints of these loops could not be identified in photospheric magnetograms
due to the lower spatial resolution and reduced sensitivity of the MDI data.
Although these problems should not affect the larger loops, their footpoints
remain ambiguous. To address these problems, 2008ApJ…687.1388J decided to
investigate the magnetic origin of the extended Ly$\alpha$ loops using
magnetic field extrapolations. They found that the longer Ly$\alpha$ loops
originate near the boundary center and are more likely the lower extensions of
large scale loops that connect areas much more distant than the neighboring
cells. The extrapolations showed that the smaller loops at the edge of the
network lanes are indeed small scale loops supporting the interpretations of
2007ApJ…664.1214P.

###  6.1 Cell Interior

Another new observation from VAULT is the imaging of Ly$\alpha$ emission from
the cell interiors for the first time. As can be seen in the example of Figure
fig:qs, the emission extends over the full interior area and is structured in
various spatial scales. The emission is filamentary, optically thick with some
apparent dependence on the local radiation field. The associated time series
(movies available in the online VAULT archive) reveal significant evolution in
these structures, like flows and jets. The material within the filamentary
structures shows an overall motion towards the network boundary similar to the
motions of emerging magnetic field elements in photospheric magnetograms and
white light images. As magnetic field of opposing direction accumulates in the
boundary, it is expected that some cancellation is taking place. Indeed, there
are a few cases where Ly$\alpha$ material appears to jet out from smaller
emission clumps creating a bright point. These events are never seen in the
cell center and could originate from magnetic reconnection closer to the
photosphere. Some examples can be seen along the column at
$-500\hbox{${}^{\prime\prime}$}$ in Figure fig:composite. The limited
resolution of available magnetograms has not allowed us to locate the origins
of these jets.

Figure 9.: Microflaring event in the Quiet Sun detected in Ly$\alpha$ and He
ii. Top panels: EIT He ii images of the event. Middle panel: Comparison of the
energy curves from VAULT to the EIT light curve. The total counts within the
black outline (VAULT) and within the white boxes (EIT) were used to calculate
the curves. Bottom panels: Ly$\alpha$ images of the event at its initiation
(left) and peak (right). fig:blob

###  6.2 Mircoflaring in the Quiet Sun

Although the VAULT time series show continuous motions and brightness
evolution thoughout the full field of view, there are very few strong
enhancements that could qualify as flaring emission. The short duration of the
flight may be a reason for this but we were able to isolate only $2-3$ events.
Figure fig:blob shows an example from a Quiet Sun feature which gives rise to
a plasma jet rising from the cell center. The brightening was detected by
SOHO/EIT which classifies it as a regular bright point. The event lasts for
$\sim 500$ s.

Since we have calibrated images, we could estimate the thermal energy of the
Ly$\alpha$ flaring under some assumptions. We adopted equation (5) in
2002ApJ…568..413B

$E_{th}=3k_{B}T\sqrt{EMV}$ (1)

where the energy $E_{th}$ corresponds to Ly$\alpha$ plasma at temperature $T$
and emission measure, $EM$ integrated over volume $V$. We assumed $T=2\times
10^{4}K$, $V=~{}{\mathrm{d}}A~{}{\mathrm{d}}l$,
$dA=21\hbox{${}^{\prime\prime}$}\times 21\hbox{${}^{\prime\prime}$}$ area, and
$dl=0.5\hbox{${}^{\prime\prime}$}$ equal to the mean free path of a Ly$\alpha$
photon for optically thick emission. For the estimation of $EM$ we adopted the
calculations in 2001ApJ…563..374V but used the updated photometric calibration
reported here. The new $EM$ calibration for VAULT is 1 DN s-1 pix-1 =
$3.74\times 10^{26}$ cm-5. To account for integrating the energies over an
area which may contain both flaring and background (likely optically thin)
emission, we have subtracted the emission from the first, pre-event image from
the plots. The resulting energy levels are very similar to those for coronal
bright points Krucker and Benz (1998) as the EIT observation of plasma at
$T\geq 8\times 10^{4}$ K suggests. Unfortunately, we cannot tell whether there
is any coronal emission from this bright point because it lies outside the
TRACE field of view and EIT was observing solely in He i during the VAULT
flight. We only report counts for the EIT light curves because there is only
one wavelength available and the emission measure cannot be calculated (right
axis in Figure fig:blob). The VAULT and EIT curves are aligned at the pre-
event emission level along the intensity axis to allow a comparison. The main
conclusions from Figure fig:blob are that the Ly$\alpha$ and He ii have a
similar impulsive phase and the He ii emission seems to be the extention of
the cooler Ly$\alpha$ emission. This is also in agreement with the earlier
results showing a delay from cooler to hotter coronal lines and extends the
detection of heating events to a much lower layer of the atmosphere.

Figure 10.: Microflaring event in the Ly$\alpha$ plage. Left: The symbol ’x’
marks the location of the brightening. Right: The energy estimate for this
event.fig:blob2

The energy estimates in Figure fig:blob are in the range of microflares which
seems reasonable for the lower TR. An inspection of the plage area around the
filament shows fainter brightenings that could still be classified as
impulsive based on their light curves. Energy estimates for those brightenings
are around $<5\times 10^{23}$ ergs, lower than a microflare. Figure fig:blob2
shows an example of such a brightening. The energy was estimated over an area
of $\sim 1.8\hbox{${}^{\prime\prime}$}\times 1.8\hbox{${}^{\prime\prime}$}$;
all other assumptions are the same as above. Because these brightness changes
are very close to the overall brightness variability of the plage, it is
difficult to say with certainty that these are flaring events. A more
sophisticated analysis is required but it is beyond the scope of the paper.

##  7 Plage and Spicules in $Ly\alpha$s:spicules

The active plage has been studied in some detail using the first VAULT
observations Vourlidas et al. (2001). The large degree of spatial structuring
and the variability of these structures combined with the complex radiative
character of Ly$\alpha$ emission complicate the detailed analysis of the
plage. The plage has clearly a different morphology than the Quiet Sun. It
lacks extended loop-like structures, but contains many point-like brightenings
reminiscent of the 171 Å moss. Actually the TRACE 171 Å images show moss over
the majority of plage with large scale loops located only in the periphery
(Figure fig:4promin). As expected, the moss underlies hotter loops seen in the
EIT Fe XV 284 Å images but the Ly$\alpha$ brightness is not correlated with
the degree of coronal heating above. A quick inspection of the Ly$\alpha$ and
171 Å images in Figure fig:4promin shows that despite the largely similar
mossy appearance, there are several areas without a detailed correlation
between corona and lower TR as noted before (e.g., region R2 in Figure 2 of
2001ApJ…563..374V). Neutral hydrogen diffusion across field lines as proposed
by 2008ApJ…683L..87J maybe an explanation of the uniform brightness of the
plage in Ly$\alpha$ but better calculations are needed before we can establish
the viability of this mechanism.

###  7.1 Detection of Proper Motions

A significant part of the variability seems quite random. For a given pixel,
the brightness change could be due to the weakening of the emission, the
lateral motion of the bright point or the appearance of dark (likely
absorbing) features. We believe that these changes can be understood as the
buffeting of the Ly$\alpha$ moss by chromospheric H$\alpha$ jets similarly to
the picture proposed by 1999SoPh..190..419D for the 171 Å moss but extending
it to much smaller spatial scales.

On the other hand, we can identify coherent motions in several places. The
most obvious ones can be found at or near the filament footpoints and along
their backbone structure. Blobs of weakly emitting Ly$\alpha$ seem to flow
towards the lower atmosphere. At the same time, apparently upward moving blobs
can be seen also at the filament footpoints as well as along the boundaries of
the small network cell within the plage and basically in most locations where
there is high contrast with the background.

Figure 11.: Detection of proper motions in Ly$\alpha$ plage. The lengths of
the displacement vectors are proportional to the esimated speed. Only pixels
with correlation coefficients $\geq 0.3$ and intensity changes $\geq 3\sigma$
above the background are considered. The units are VAULT pixels
($0.112\hbox{${}^{\prime\prime}$}$/pixel). Left: Filament and neaby plage.
Upward motions can be seen along the western footpoint. Right: Plage detail.
Note the counterstreaming motions along the filament boundary and diverging
(explosive?) motions at certain bright points.fig:motions

In an attempt to quantify these motions we used a local correlation method to
track the blobs in time. To suppress the influence of the background buffeting
motions we calculated the standard deviation, $\sigma$, of the intensity
variability for each pixel at the peak of the emission and then considered
only pixels with $\geq 3\sigma$ as inputs to the cross correlation algorithm.
The large degree of variability and spatial structuring results in many
correlations. So we kept only the pixel with correlation coefficients higher
than 0.3 and estimated their speeds and velocity vectors. We derive speeds in
the range of 5-20 km s-1 which are similar to the speeds of chromospheric
fibrils and spicules (e.g., 2000A&A…360..351W; 2008ApJ…673.1194L). In general,
the cross correlation results showed motions in all directions reinforcing the
visual impressions of the large degree of randomness in the Ly$\alpha$
structures. However, a closer inspection of the displacement vector revealed
several instances of coherent motions. In the example of Figure fig:motions,
we can see upward motions along the western filament footpoints and the
filament boundaries. There was clear evidence of counterstreaming motions
along the filament. Some of those were in the upper range of our estimated
speeds ($\sim 20$ km s-1) and are very close to H$\alpha$ measurements in
filaments Engvold (1998); Lin, Engvold, and Wiik (2003). The nearby plage
showed motions that followed the curvature of the filament (Figure
fig:motions, right panel). They may lie along thin, dark strands that are part
of the filament rather than the plage. Coherent apparently upward motions were
also detected at network boundaries along spicular-like structures. The most
interesting results were at locations of diverging motions as can be seen
towards the upper end of the field (Figure fig:motions). Some were associated
with moderate flare-like brightenings (right panel in Figure fig:motions and
Figure fig:blob2) and may suggest an explosive nature for these intensity
changes. It is possible that some of the TR variability seen in TR lines with
coarser resolution and attributed to stationary brightenings could actually be
an effect of spatial smoothing of the above mentioned flows. In other places,
we found diverging vectors suggesting rotation. In the wavelet-processed
movies, we see unwinding features at those areas. They are very suggestive of
the so-called mini-CMEs detected recently by STEREO and associated with vortex
flows at supergranular boundaries Innes et al. (2009).

A big advantage of VAULT’s large FOV is the observation of similar structures
both on disk and at the limb. An obvious candidate are the spicules.
2009A&A…499..917K measured the dynamics of several Ly$\alpha$ spicules and
found many similarities to the H$\alpha$ dynamic fibrils despite the short
VAULT time series. Based on the TRACE co-alignement we measured Ly$\alpha$
spicules to be $8\hbox{${}^{\prime\prime}$}-12\hbox{${}^{\prime\prime}$}$ in
height, from VAULT spicule edge to the TRACE limb position. When we consider
the co-aligned cotemporal BBSO H$\alpha$ channel, Ly$\alpha$ spicules can be
up to $\sim 2$′′ higher than in the comparatively optically thinner H$\alpha$.
Although scattered light may play a role in the ground-based observations, the
height difference between Ly$\alpha$ and H$\alpha$ appears to be significant.
These results imply that Ly$\alpha$ spicules could be the outer sheaths of the
H$\alpha$ fibrils Koza, Rutten, and Vourlidas (2009). When we take into
account similar results between H$\alpha$ and C iv de Wijn and De Pontieu
(2006) it becomes obvious that chromospheric mass is propelled to the corona
via the fibrils and undergoes heating appearing in successively higher
temperatures (Ly$\alpha$ to C iv, for example). This scenario seems to
corroborate the very recent results of 2009ApJ…701L…1D where it is proposed
that type-II spicules may be the means of chromospheric plasma transport to
coronal levels and temperatures and may play an important role in the coronal
heating problem. Further observations of both fibrils and type-II spicules
are, therefore, highly desirable in Ly$\alpha$ (in addition to chromospheric
and coronal lines) to provide a more robust connection between the evolution
of the chromospheric and coronal structures. For the moment, the above
discussion suggests that spicules/fibrils may provide the mass heated to
coronal temperatures (e.g., 2007PASJ…59S.655D).

Overall, our initial attempt to characterize the variability seen in the VAULT
images seems to provide reasonable results. The most serious problem is the
large amount of variability in all intensity and spatial levels. We plan to
revisit the analysis of proper motions using our newly available wavelet-
processed images which supress the background “noise” and may enhance the
effectiveness of cross-correlation techinques.

##  8 Discussion s:dis

The VAULT data, being taken from a sounding rocket platform, do not permit
long time series investigations of the Ly$\alpha$ atmosphere. However, they do
provide several tantalizing clues about the dynamics and morphology of the
crucial interface of the upper chromosphere/lower TR at least for long-lived
structures and for variability at a time scale of a few minutes.

The improved photometric analysis of the VAULT data combined with better
Ly$\alpha$ models show that, for wideband imaging at least, most of the
emission originates from the lower TR ($\geq 10^{4}$ K) and only the darker
areas contain much chromospheric material (Table tbl:meas). Therefore,
Ly$\alpha$ imaging observations are a great probe for the structure of the
transition region Teriaca et al. (2005). It seems that the Ly$\alpha$ Quiet
Sun is dominated by longer thread-like structures reminiscent of H$\alpha$
fibrils. The VAULT observations have provoked new ideas about the nature of
the TR as the region where neutral hydrogen atoms from these threads diffuse
across magnetic field lines, interact with nearby electrons and subsequently
excite, ionize, and/or radiate to provide the emission we see in TR lines
Judge (2008). These ideas remain to be tested in detail but they demonstrate
the value of sounding rocket observations.

The high spatial resolution of the VAULT data resolves a great deal of
variability, mostly associated with lateral motions, in the plage. We believe
that the majority of this variability can be explained as buffeting of the
Ly$\alpha$ structures by cooler material, such as H$\alpha$ jets. In addition,
the VAULT observation of spicules show that they extend higher and have larger
widths but otherwise similar dynamics Koza, Rutten, and Vourlidas (2009) with
their H$\alpha$ counterparts. These observations verify past SUMER results
Budnik et al. (1998) and provide significant support for an interesting idea
put forth recently by 2009ApJ…702.1016D to explain the large emission measure
discrepancies between coronal and lower TR structures Vourlidas et al. (2001)
as a result of EUV absorption from chromospheric material injected in the
corona. When we consider these observations/ideas together; namely, the long
network loops and neutral cross-field diffusion, the continuous buffeting, and
the Ly$\alpha$ jets as extension of H$\alpha$ dynamic fibrils, we come to the
conclusion that the transition region may be nothing more than the transient,
evaporating part of the chromosphere rather than the stable layer in the
simple 1D models, such as Vernazza, Avrett, and Loeser (1981), long favoured
in our discipline. The VAULT and more recently Hinode/SOT observations are
making us reassess our views on the structure of the lower solar atmosphere.

The large field of view of the instrument led to observations of basically
every solar structure, with the exception of coronal holes, which enabled us
to estimate the contribution of various Ly$\alpha$ sources to the observed
intensity and thereby introducing the first empirical segmentation of
Ly$\alpha$ irradiance to its sources (Sec. s:inten). We found that Quiet Sun
features can have intensities several times the intensity of the average Quiet
Sun and that filaments exhibit both absorption and emission in Ly$\alpha$. The
latter can be as bright as weak bright points. Optically thin structures, up
to 50% fainter than the average Quiet Sun may exist in the center of cell
interiors and as off limb loops. We did find that high temperatures are likely
in off-limb Ly$\alpha$ loops which may explain their large heights ($\sim
60^{\prime\prime}$, corresponding to $\approx 45,000$ km) in the VAULT images.
We also found that active region filament partially absorbs plage emission, by
around 20% to 30%, and this effect may need to be considered carefully in
irradiance studies. These segmentation results may be useful to irradiance
studies until a full disk Ly$\alpha$ imaging becomes available.

The VAULT images provide the first ever unambiguous Ly$\alpha$ imaging of the
fine structure of filaments/prominences and show that both emission and
absorption takes places along the prominence backbone. It is interesting to
note, that the underlying plage is visible through several locations along the
prominence suggesting that Ly$\alpha$ is optically thin and that the
distribution of hydrogen is highly anisotropic through these structures. It is
also clear that the Ly$\alpha$ filament is larger than the H$\alpha$ one
Millard, Vial, and Vourlidas (2009) and is likely to reach a higher altitude.
The high LRI measurements in the filaments (up to 5, Table tbl:meas) are again
consistent with a decreased optical thickness, even to the point of being
optically thin. According to 1986A&A…154..154G and their Figure 5, a very hot
temperature of $\sim 5\times 10^{4}$K is also possible. For this study we have
chosen the cooler more plausible solution of the curve. Nevertheless, with the
lack of other observational constraints, it remains unsolved whether the hot
solution is possible. One approach would be a point-to-point correlation with
other chromospheric-TR lines. This calls for a high-resolution spectrograph
which is not currently available in space.

VAULT images also reveal a wealth of activity in both the plage and the Quiet
Sun regions. In the latter, we see evidence of braiding in the loop structures
outlining the cell boundaries. However, we do not see any direct unambiguous
evidence of reconnection as would be expected from such activity. It may be
that longer time series are needed to evidence such events. Alternatively, a
mixture of cool absorbing structures propagating alongside these loops may
create the appearance of braiding. Those structures may be the same absorbing
structures that create the buffeting motions in the plage. On the other hand,
we see frequent brightenings and even jets in the interior of the cells. This
is the first time that the Ly$\alpha$ emission from these areas has been
imaged and the amount of observed activity was unexpected. The brightenings
seem to be associated with the emergence of magnetic field elements and their
subsequent movement towards the cell boundary. These motions are regularly
seen with sub-arcsecond resolution magnetographs (e.g., SOUP instrument,
1989ApJ…336..475T) but we did not have any available during the flight. The
relation between the emerging flux and the Ly$\alpha$ brigtenings remains to
be confirmed in a future flight but if it is true it suggests that the effects
of even such small magnetic elements reach substantial heights in the solar
atmosphere. We wonder whether some of those jets are the Ly$\alpha$
counterparts of the Type II spicules seen in the SOT observations de Pontieu
et al. (2007a).

Another rather surprising observation is the relative scarcity of microflaring
events. We have been able to identify a handful in the five minutes of
observation. These have been previously identified by 1998A&A…336.1039B. They
suggest a possible link with atmospheric turbulence. In our observations we
have observed them in both active regions and Quiet Sun regions. The largest
of them had a light curve and energy consistent with a microflare and was
detected in He ii as well (Figure fig:blob). Others had energies in the range
of $10^{24}$ ergs. Although the short duration of the observation does not
allow proper statistics for the occurrence of these events, our field of view
covers a substantial part of the solar disk. Therefore, it seems unlikely that
microflares are a common occurrence in this temperature range.

##  9 Conclusions s:con

We conclude our overview of Ly$\alpha$ imaging observations with a set of
“lessons learned” that may be useful in the design of future Ly$\alpha$
instruments or observation campaigns.

  * •

Ly$\alpha$ is formed at the critical interface between the upper chormosphere
and the low TR. Thus, imaging is very useful and the well-known difficulties
surrounding the interpretation of Ly$\alpha$ emission are not insurmountable
anymore. We can rely on models to derive reasonable physical parameters for
the observed structures.

  * •

We see few Ly$\alpha$ structures close to the instrument resolution limit of
$0.5\hbox{${}^{\prime\prime}$}$. Only absorbing (dark) features and off-limb
structures (in emission) can be identified at that resolution. Most of the on-
disk structures are much larger. This could be due to the high optical
thickness of the line throughout these structures. In any case, this
observation should be considered in the design of future Ly$\alpha$
telescopes. Extreme resolutions may not be useful unless the instruments can
spectrally resolve the line or their science objectives include absorption or
off-limb features.

  * •

There is evidence of optically thin emission in many locations besides the
obvious limb structures. Areas around filaments are especially interesting.
This would require observations of the spectral profiles to be confirmed.

  * •

There is considerable structure and variability within the cell interiors
which is probably linked to photospheric flux emergence. This is a new area
for Ly$\alpha$ studies and to understand it will require a future telescope
sensitivity equal or better than VAULT.

  * •

Even if flaring activity is relatively unimportant, there is variability.
Future instruments should achieve both high signal-to-noise ratio and cadence
to allow the study of both.

  * •

Both types of spicules are observable and given the significant temperature
range of Ly$\alpha$ formation, observations in Ly$\alpha$ are excellent
tracers of the injection of material from the chomosphere to the oorona.

For the near future, the advent of Hinode/SOT has created a new and unique
opportunity to address the nature of the transition region by combining VAULT
and SOT observations of Quiet Sun structures and spicules. We plan to seek
funding for refurbishment of the VAULT payload, which was damaged during its
last flight, and for an underflight with SOT with the specific objectives of
addressing the nature of the long Ly$\alpha$ fibrils over the quiet network
and investigate type-II spicule dynamics particularly at coronal holes.
However the dynamics of the type-I spicules and macro-spicules may need longer
time series due to their longer lifetimes Xia et al. (2005).

To summarize, we presented a broad overview of the morphology and dynamics of
the Sun’s Ly$\alpha$ atmosphere; an important but rarely imaged region. These
were the first sub-arcsecond, high sensitivity observations of this line and,
at that time, the highest resolution observations of any solar structure from
space. The VAULT observations showed that Ly$\alpha$ emission arises from
every location and in every solar feature, and generated new ideas about the
nature of the transition region and coronal heating. These results demonstrate
the wideranging value of sounding rocket experiments despite their short
observing windows.

#### Acknowledgements

This work is dedicated to the memories of D. Prinz, G. Bruecker, and D. Lilley
whose efforts have contributed enormously to the success of the NRL sounding
rocket programs. We are grateful to V. Yurchyshyn for providing calibrated
BBSO H$\alpha$ images, and to J. Cook, J. Koza, S. Martin, R. Rutten, J.C.
Vial, for useful discussions and constant encouragement. The achievements
presented in this paper are the product of many years of development work at
the Naval Research Laboratory Solar Physics Branch and the NASA sounding
rocket program. The VAULT instrument borrows heavily from the High Resolution
Telescope and Spectrograph. The NRL rocket team of J. Smith, R. Moye, R.
Hagood, R. Feldman, J. Moser, D. Roberts, T. Spears and R. Waymire did a
superb job in preparing and launching the instrument. We would like to
acknowledge the efforts of the sounding rocket support team that made the
VAULT launches possible. We would like to particularly acknowledge the
following individuals. Tracy Gibb did a superb job managing the launch of the
VAULT payload. Frank Lau managed the development of the Mark 7 digital SPARCS
attitude control system. We would like to acknowledge the SPARCS team for
their superb efforts in the support of our launch. Jesus and Carlos Martinez
developed and operated the Mark 7 SPARCS. Richard Garcia, Shelby Elborn and
Kenneth Starr developed the VAULT telemetry section. The support from the
White Sands Missile Range and Wallops Flight Facility NASROC personnel was of
the highest caliber. The VAULT instrument development work has been supported
by the ONR task area SP033-02-43 and by NASA defense procurement request
S-84002F.

## References

  * Antiochos and Noci (1986) Antiochos, S.K., Noci, G.: 1986, Astrophys. J. 301, 440\. doi:10.1086/163912.
  * Bartoe and Brueckner (1975) Bartoe, J.D.F., Brueckner, G.E.: 1975, In: Bulletin of the American Astronomical Society, Bulletin of the American Astronomical Society 7, 432\.
  * Benz and Krucker (2002) Benz, A.O., Krucker, S.: 2002, Astrophys. J. 568, 413\. doi:10.1086/338807.
  * Berghmans, Clette, and Moses (1998) Berghmans, D., Clette, F., Moses, D.: 1998, Astron. Astrophys. 336, 1039\.
  * Bonnet and Tsiropoula (1982) Bonnet, R.M., Tsiropoula, G.: 1982, Solar Phys. 75, 139\. doi:10.1007/BF00153467.
  * Bonnet et al. (1980) Bonnet, R.M., Decaudin, M., Bruner, E.C. Jr., Acton, L.W., Brown, W.A.: 1980, Astrophys. J. 237, L47. doi:10.1086/183232.
  * Budnik et al. (1998) Budnik, F., Schroeder, K.P., Wilhelm, K., Glassmeier, K.H.: 1998, Astron. Astrophys. 334, L77.
  * Curdt et al. (2008) Curdt, W., Tian, H., Teriaca, L., Schühle, U., Lemaire, P.: 2008, Astron. Astrophys. 492, L9. doi:10.1051/0004-6361:200810868.
  * de Pontieu et al. (1999) de Pontieu, B., Berger, T.E., Schrijver, C.J., Title, A.M.: 1999, Solar Phys. 190, 419\. doi:10.1023/A:1005220606223.
  * de Pontieu et al. (2007a) de Pontieu, B., McIntosh, S., Hansteen, V.H., Carlsson, M., Schrijver, C.J., Tarbell, T.D., Title, A.M., Shine, R.A., Suematsu, Y., Tsuneta, S., Katsukawa, Y., Ichimoto, K., Shimizu, T., Nagata, S.: 2007, Publ. Astron. Soc. Japan 59, 655\.
  * de Pontieu et al. (2007b) de Pontieu, B., Hansteen, V.H., Rouppe van der Voort, L., van Noort, M., Carlsson, M.: 2007, In: Heinzel, P., Dorotovič, I., Rutten, R.J. (eds.) The Physics of Chromospheric Plasmas, Astronomical Society of the Pacific Conference Series 368, 65\.
  * de Pontieu et al. (2009) de Pontieu, B., Hansteen, V.H., McIntosh, S.W., Patsourakos, S.: 2009, Astrophys. J. 702, 1016\. doi:10.1088/0004-637X/702/2/1016.
  * De Pontieu et al. (2009) De Pontieu, B., McIntosh, S.W., Hansteen, V.H., Schrijver, C.J.: 2009, Astrophys. J. 701, L1. doi:10.1088/0004-637X/701/1/L1.
  * de Wijn and De Pontieu (2006) de Wijn, A.G., De Pontieu, B.: 2006, Astron. Astrophys. 460, 309\. doi:10.1051/0004-6361:20065864.
  * Dowdy, Rabin, and Moore (1986) Dowdy, J.F. Jr., Rabin, D., Moore, R.L.: 1986, Solar Phys. 105, 35\. doi:10.1007/BF00156374.
  * Emerich et al. (2005) Emerich, C., Lemaire, P., Vial, J.C., Curdt, W., Schühle, U., Wilhelm, K.: 2005, Icarus 178, 429\. doi:10.1016/j.icarus.2005.05.002.
  * Engvold (1998) Engvold, O.: 1998, In: Webb, D.F., Schmieder, B., Rust, D.M. (eds.) IAU Colloq. 167: New Perspectives on Solar Prominences, Astronomical Society of the Pacific Conference Series 150, 23\.
  * Feldman (1983) Feldman, U.: 1983, Astrophys. J. 275, 367\. doi:10.1086/161539.
  * Feldman (1987) Feldman, U.: 1987, Astrophys. J. 320, 426\. doi:10.1086/165556.
  * Feldman, Dammasch, and Landi (2009) Feldman, U., Dammasch, I.E., Landi, E.: 2009, Astrophys. J. 693, 1474\. doi:10.1088/0004-637X/693/2/1474.
  * Feldman, Dammasch, and Wilhelm (2000) Feldman, U., Dammasch, I.E., Wilhelm, K.: 2000, Space Science Reviews 93, 411\.
  * Fontenla, Reichmann, and Tandberg-Hanssen (1988) Fontenla, J., Reichmann, E.J., Tandberg-Hanssen, E.: 1988, Astrophys. J. 329, 464\. doi:10.1086/166392.
  * Fontenla, Avrett, and Loeser (2002) Fontenla, J.M., Avrett, E.H., Loeser, R.: 2002, Astrophys. J. 572, 636\. doi:10.1086/340227.
  * Gouttebroze (2004) Gouttebroze, P.: 2004, Astron. Astrophys. 413, 733\. doi:10.1051/0004-6361:20031570.
  * Gouttebroze, Vial, and Tsiropoula (1986) Gouttebroze, P., Vial, J.C., Tsiropoula, G.: 1986, Astron. Astrophys. 154, 154\.
  * Gunár et al. (2006) Gunár, S., Teriaca, L., Heinzel, P., Schühle, U.: 2006, In: SOHO-17. 10 Years of SOHO and Beyond, ESA Special Publication 617.
  * Innes et al. (2009) Innes, D.E., Genetelli, A., Attie, R., Potts, H.E.: 2009, Astron. Astrophys. 495, 319\. doi:10.1051/0004-6361:200811011.
  * Judge (2008) Judge, P.: 2008, Astrophys. J. 683, L87. doi:10.1086/591470.
  * Judge and Centeno (2008) Judge, P., Centeno, R.: 2008, Astrophys. J. 687, 1388\. doi:10.1086/590104.
  * Korendyke et al. (2001) Korendyke, C.M., Vourlidas, A., Cook, J.W., Dere, K.P., Howard, R.A., Morrill, J.S., Moses, J.D., Moulton, N.E., Socker, D.G.: 2001, Solar Phys. 200, 63\.
  * Koza, Rutten, and Vourlidas (2009) Koza, J., Rutten, R.J., Vourlidas, A.: 2009, Astron. Astrophys. 499, 917\. doi:10.1051/0004-6361/200810710.
  * Krucker and Benz (1998) Krucker, S., Benz, A.O.: 1998, Astrophys. J. 501, L213. doi:10.1086/311474.
  * Landi and Feldman (2004) Landi, E., Feldman, U.: 2004, Astrophys. J. 611, 537\. doi:10.1086/422169.
  * Langangen et al. (2008) Langangen, Ø., Carlsson, M., Rouppe van der Voort, L., Hansteen, V., De Pontieu, B.: 2008, Astrophys. J. 673, 1194\. doi:10.1086/524377.
  * Lemaire et al. (2004) Lemaire, P., Emerich, C., Vial, J.C., Curdt, W., Schühle, U., Wilhelm, K.: 2004, In: 35th COSPAR Scientific Assembly, COSPAR, Plenary Meeting 35, 510\.
  * Lin, Engvold, and Wiik (2003) Lin, Y., Engvold, O.R., Wiik, J.E.: 2003, Solar Phys. 216, 109\. doi:10.1023/A:1026150809598.
  * Millard, Vial, and Vourlidas (2009) Millard, A.A., Vial, J.C., Vourlidas, A.: 2009, Astron. Astrophys., submitted.
  * Miller, Mercure, and Rense (1956) Miller, S.C. Jr., Mercure, R., Rense, W.A.: 1956, Astrophys. J. 124, 580\. doi:10.1086/146263.
  * Ofman and Wang (2008) Ofman, L., Wang, T.J.: 2008, Astron. Astrophys. 482, L9. doi:10.1051/0004-6361:20079340.
  * Okamoto et al. (2007) Okamoto, T.J., Tsuneta, S., Berger, T.E., Ichimoto, K., Katsukawa, Y., Lites, B.W., Nagata, S., Shibata, K., Shimizu, T., Shine, R.A., Suematsu, Y., Tarbell, T.D., Title, A.M.: 2007, Science 318, 1577\. doi:10.1126/science.1145447.
  * Patsourakos, Gouttebroze, and Vourlidas (2007) Patsourakos, S., Gouttebroze, P., Vourlidas, A.: 2007, Astrophys. J. 664, 1214\. doi:10.1086/518645.
  * Peter (2001) Peter, H.: 2001, Astron. Astrophys. 374, 1108\. doi:10.1051/0004-6361:20010697.
  * Prinz (1974) Prinz, D.K.: 1974, Astrophys. J. 187, 369\. doi:10.1086/152641.
  * Prinz and Brueckner (1977) Prinz, D.K., Brueckner, G.E.: 1977, J. Geophys. Res. 82, 1481\. doi:10.1029/JA082i010p01481.
  * Purcell and Widing (1972) Purcell, J.D., Widing, K.G.: 1972, Astrophys. J. 176, 239\. doi:10.1086/151626.
  * Puschmann and Sailer (2006) Puschmann, K.G., Sailer, M.: 2006, Astron. Astrophys. 454, 1011\. doi:10.1051/0004-6361:20053918.
  * Rutten, van Veelen, and Sütterlin (2008) Rutten, R.J., van Veelen, B., Sütterlin, P.: 2008, Solar Phys. 251, 533\. doi:10.1007/s11207-008-9116-9.
  * Schrijver (2001) Schrijver, C.J.: 2001, Solar Phys. 198, 325\. doi:10.1023/A:1005211925515.
  * Schrijver et al. (1997) Schrijver, C.J., Title, A.M., van Ballegooijen, A.A., Hagenaar, H.J., Shine, R.A.: 1997, Astrophys. J. 487, 424\. doi:10.1086/304581.
  * Stenborg, Vourlidas, and Howard (2008) Stenborg, G., Vourlidas, A., Howard, R.A.: 2008, Astrophys. J. 674, 1201\. doi:10.1086/525556.
  * Teriaca et al. (2005) Teriaca, L., Schühle, U., Solanki, S.K., Curdt, W., Marsch, E.: 2005, In: Innes, D.E., Lagg, A., Solanki, S.A. (eds.) Chromospheric and Coronal Magnetic Fields, ESA Special Publication 596.
  * Title et al. (1989) Title, A.M., Tarbell, T.D., Topka, K.P., Ferguson, S.H., Shine, R.A., SOUP Team: 1989, Astrophys. J. 336, 475\. doi:10.1086/167026.
  * Trujillo Bueno, Shchukina, and Asensio Ramos (2004) Trujillo Bueno, J., Shchukina, N., Asensio Ramos, A.: 2004, Nature 430, 326\. doi:10.1038/nature02669.
  * Tsiropoula et al. (1986) Tsiropoula, G., Alissandrakis, C., Bonnet, R.M., Gouttebroze, P.: 1986, Astron. Astrophys. 167, 351\.
  * Vernazza, Avrett, and Loeser (1981) Vernazza, J.E., Avrett, E.H., Loeser, R.: 1981, ApJS 45, 635\. doi:10.1086/190731.
  * Vourlidas et al. (2001) Vourlidas, A., Klimchuk, J.A., Korendyke, C.M., Tarbell, T.D., Handy, B.N.: 2001, Astrophys. J. 563, 374\. doi:10.1086/323835.
  * Wilhelm (2000) Wilhelm, K.: 2000, Astron. Astrophys. 360, 351\.
  * Wilhelm et al. (1995) Wilhelm, K., Curdt, W., Marsch, E., Schühle, U., Lemaire, P., Gabriel, A., Vial, J.C., Grewing, M., Huber, M.C.E., Jordan, S.D., Poland, A.I., Thomas, R.J., Kühne, M., Timothy, J.G., Hassler, D.M., Siegmund, O.H.W.: 1995, Solar Phys. 162, 189\. doi:10.1007/BF00733430.
  * Woods et al. (1995) Woods, T.N., Rottman, G.J., White, O.R., Fontenla, J., Avrett, E.H.: 1995, Astrophys. J. 442, 898\. doi:10.1086/175492.
  * Xia et al. (2005) Xia, L.D., Popescu, M.D., Doyle, J.G., Giannikakis, J.: 2005, Astron. Astrophys. 438, 1115\. doi:10.1051/0004-6361:20042579.