# Atacama Large Aperture Submillimeter Telescope (AtLAST) Science: Solar and
stellar observations

Sven Wedemeyer  Rosseland Centre for Solar Physics, University of Oslo,
Postboks 1029 Blindern, N-0315 Oslo, Norway  Institute of Theoretical
Astrophysics, University of Oslo, Postboks 1029 Blindern, N-0315 Oslo, Norway
Miroslav Bárta  Astronomical Institute, The Czech Academy of Sciences, 251 65
Ondřejov, Czech Republic  Roman Brajša  Hvar Observatory, Faculty of Geodesy,
University of Zagreb, Kačićeva 26, HR-10000 Zagreb, Croatia  Yi Chai
Astronomical Institute, The Czech Academy of Sciences, 251 65 Ondřejov, Czech
Republic  Joaquim Costa  Centro de Rádio Astronomia e Astrofísica Mackenzie,
Escola de Engenharia, Universidade Presbiteriana Mackenzie, 01302-907, São
Paulo, Brazil.  Dale Gary  Center for Solar-Terrestrial Research, New Jersey
Institute of Technology, Newark, NJ 07102, USA  Guillermo Giménez de Castro
Centro de Rádio Astronomia e Astrofísica Mackenzie, Escola de Engenharia,
Universidade Presbiteriana Mackenzie, 01302-907, São Paulo, Brazil.  Stanislav
Gunár  Astronomical Institute, The Czech Academy of Sciences, 251 65 Ondřejov,
Czech Republic  Gregory Fleishman  Center for Solar-Terrestrial Research, New
Jersey Institute of Technology, Newark, NJ 07102, USA  Institute for Solar
Physics (KIS), Schoeneckstr 6, 79104 Freiburg, Germany  Antonio Hales
National Radio Astronomy Observatory (NRAO), 520 Edgemont Road,
Charlottesville, VA, 22903, USA  Joint ALMA Observatory (JAO), Alonso de
Córdova 3107, Vitacura, 763-0355, Santiago, Chile  Hugh Hudson  SUPA School of
Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK  Space
Sciences Laboratory, University of California, Berkeley, Berkeley, CA, 94720,
USA  Mats Kirkaune  Rosseland Centre for Solar Physics, University of Oslo,
Postboks 1029 Blindern, N-0315 Oslo, Norway  Institute of Theoretical
Astrophysics, University of Oslo, Postboks 1029 Blindern, N-0315 Oslo, Norway
Atul Mohan  The Catholic University of America, Washington, DC 20064, USA
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA  Galina Motorina
Astronomical Institute, The Czech Academy of Sciences, 251 65 Ondřejov, Czech
Republic  Central Astronomical Observatory at Pulkovo of Russian Academy of
Sciences, St. Petersburg, 196140, Russia  Alberto Pellizzoni  INAF - Cagliari
Astronomical Observatory, Via della Scienza 5, I–09047 Selargius (CA), Italy
Maryam Saberi  Rosseland Centre for Solar Physics, University of Oslo,
Postboks 1029 Blindern, N-0315 Oslo, Norway  Institute of Theoretical
Astrophysics, University of Oslo, Postboks 1029 Blindern, N-0315 Oslo, Norway
Caius L. Selhorst  Center for Solar-Terrestrial Research, New Jersey Institute
of Technology, Newark, NJ 07102, USA  NAT - Núcleo de Astrofísica,
Universidade Cidade de São Paulo, São Paulo, SP, Brazil  Paulo J. A. Simões
Centro de Rádio Astronomia e Astrofísica Mackenzie, Escola de Engenharia,
Universidade Presbiteriana Mackenzie, 01302-907, São Paulo, Brazil.  SUPA
School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
Masumi Shimojo  National Astronomical Observatory of Japan, Mitaka, Tokyo,
181-8588, Japan  Graduate University for Advanced Studies, SOKENDAI, Mitaka,
Tokyo, 181-8588, Japan  Ivica Skokić  Hvar Observatory, Faculty of Geodesy,
University of Zagreb, Kačićeva 26, HR-10000 Zagreb, Croatia  Davor Sudar  Hvar
Observatory, Faculty of Geodesy, University of Zagreb, Kačićeva 26, HR-10000
Zagreb, Croatia  Fabian Menezes  Centro de Rádio Astronomia e Astrofísica
Mackenzie, Escola de Engenharia, Universidade Presbiteriana Mackenzie,
01302-907, São Paulo, Brazil.  Stephen White  Space Vehicles Directorate, Air
Force Research Laboratory, Albuquerque, NM 87117 USA  Mark Booth  UK Astronomy
Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9
3HJ, UK  Pamela Klaassen  UK Astronomy Technology Centre, Royal Observatory
Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK  Claudia Cicone  Institute of
Theoretical Astrophysics, University of Oslo, Postboks 1029 Blindern, N-0315
Oslo, Norway  Tony Mroczkowski  European Southern Observatory, Karl-
Schwarzschild-Str. 2, 85748 Garching bei München, Germany  Martin A. Cordiner
Astrochemistry Laboratory, Code 691, NASA Goddard Space Flight Center,
Greenbelt, MD 20771, USA.  Luca Di Mascolo  Doug Johnstone  NRC Herzberg
Astronomy and Astrophysics, 5071 West Saanich Rd, Victoria, BC, V9E 2E7,
Canada  Department of Physics and Astronomy, University of Victoria, Victoria,
BC, V8P 5C2, Canada  Eelco van Kampen  European Southern Observatory, Karl-
Schwarzschild-Str. 2, 85748 Garching bei München, Germany  Minju Lee  Cosmic
Dawn Center (DAWN), Denmark  DTU-Space, Technical University of Denmark,
Elektrovej 327, DK2800 Kgs. Lyngby, Denmark  Daizhong Liu  Max-Planck-Institut
für extraterrestrische Physik, Giessenbachstrasse 1 Garching, Bayern, D-85748,
Germany  Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua
Road, Nanjing 210023, China  Thomas Maccarone  Department of Physics &
Astronomy, Texas Tech University, Box 41051, Lubbock TX, 79409-1051, USA  John
Orlowski-Scherer  Department of Physics and Astronomy, University of
Pennsylvania, 209 South 33rd Street, Philadelphia, PA, 19104, USA  Amélie
Saintonge  Department of Physics and Astronomy, University College London,
Gower Street, London WC1E 6BT, UK  Max-Planck-Institut für Radioastronomie
(MPIfR), Auf dem Hügel 69, D-53121 Bonn, Germany  Matthew Smith  School of
Physics & Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK
Alexander E. Thelen  Division of Geological and Planetary Sciences, California
Institute of Technology, Pasadena, CA 91125, USA.

###### Abstract

Observations at (sub-)millimeter wavelengths offer a complementary perspective
on our Sun and other stars, offering significant insights into both the
thermal and magnetic composition of their chromospheres. Despite the
fundamental progress in (sub-)millimeter observations of the Sun, some
important aspects require diagnostic capabilities that are not offered by
existing observatories. In particular, simultaneously observations of the
radiation continuum across an extended frequency range would facilitate the
mapping of different layers and thus ultimately the 3D structure of the solar
atmosphere. Mapping large regions on the Sun or even the whole solar disk at a
very high temporal cadence would be crucial for systematically detecting and
following the temporal evolution of flares, while synoptic observations, i.e.,
daily maps, over periods of years would provide an unprecedented view of the
solar activity cycle in this wavelength regime. As our Sun is a fundamental
reference for studying the atmospheres of active main sequence stars,
observing the Sun and other stars with the same instrument would unlock the
enormous diagnostic potential for understanding stellar activity and its
impact on exoplanets. The Atacama Large Aperture Submillimeter Telescope
(AtLAST), a single-dish telescope with 50 m aperture proposed to be built in
the Atacama desert in Chile, would be able to provide these observational
capabilities. Equipped with a large number of detector elements for probing
the radiation continuum across a wide frequency range, AtLAST would address a
wide range of scientific topics including the thermal structure and heating of
the solar chromosphere, flares and prominences, and the solar activity cycle.
In this white paper, the key science cases and their technical requirements
for AtLAST are discussed.

## Keywords

Sun: activity; Sun: atmosphere; Sun: filaments, prominences; Sun: flares; Sun:
magnetic fields; Sun: solar-terrestrial relations; Sun: sunspots;

## Plain language summary

Observations of our Sun and other stars at wavelengths of around one
millimeter, i.e. in the range between infrared and radio waves, present a
valuable complementary perspective. Despite significant technological
advancements, certain critical aspects necessitate diagnostic capabilities not
offered by current observatories. The proposed Atacama Large Aperture
Submillimeter Telescope (AtLAST), featuring a 50-meter aperture and slated for
construction at a high altitude in Chile’s Atacama desert, promises to address
these observational needs. Equipped with novel detectors that would cover a
wide frequency range, AtLAST could unlock a plethora of scientific studies
contributing to a better understanding of our host star. Simultaneous
observations over a broad frequency range at rapid succession would enable the
imaging of different layers of the Sun, thus elucidating the three-dimensional
thermal and magnetic structure of the solar atmosphere and providing important
clues for many long-standing central questions such as how the outermost
layers of the Sun are heated to very high temperatures, the nature of large-
scale structures like prominences, and how flares and coronal mass ejections,
i.e. enormous eruptions, are produced. The latter is of particular interest to
modern society due to the potentially devastating impact on the technological
infrastructure we depend on today. Another unique possibility would be to
study the Sun’s long-term evolution in this wavelength range, which would
yield important insights into its activity cycle. Moreover, the Sun serves as
a fundamental reference for other stars as, due to its proximity, it is the
only star that can be investigated in such detail. The results for the Sun
would therefore have direct implications for understanding other stars and
their impact on exoplanets. This article outlines the key scientific
objectives and technical requirements for solar observations with AtLAST.

##  Introduction

The continuum radiation emitted by the Sun at millimeter wavelengths
originates from the chromosphere, the layer of the solar atmosphere located
between the photosphere and the corona. The thermal and magnetic structure of
the chromosphere is complex and highly dynamic with a plethora of physical
processes at work, many of which induce notable deviations from equilibrium
conditions (for example in the ionisation state of hydrogen). The largest
features like filaments can span substantial fractions of the solar diameter
($\sim$30’), followed by Active Regions (often a few arcminutes), and magnetic
network cells as the chromospheric imprints of supergranulation cells (20” –
60”). Sunspots, including the penumbra, have diameters of typically 30” but
occasionally up to $\sim 1$’, whereas the atmosphere in the Quiet Sun regions
exhibits spatial scales of 1-2” and below. It is expected that there is small-
scale structure even smaller than $\sim 0.1$” scales resolved by modern
4m-class optical solar telescopes (Rimmele et al. 2020; Quintero Noda et al.
2022). Likewise, the temporal scales range from about a month for one solar
rotation, days for the evolution of sunspots, down to minutes and seconds for
small-scale and energetic phenomena.

Consequently, observing the chromosphere and characterising the physical
mechanisms that govern this dynamic environment is challenging. So far, there
were only a few suitable diagnostic tools available to examine the plasma
conditions in the chromosphere such as the spectral lines of singly ionized
calcium (Ca II) and magnesium (Mg II) as well as the H line. The
interpretation of observations in these spectral lines and deriving the
physical properties of the plasma in the mapped atmospheric regions is
complicated and limited by the complex line formation mechanisms that involve
non-local thermodynamic equilibrium effects. The situation is better for the
millimeter continuum radiation, which is assumed to be formed mostly under
local equilibrium conditions so that the measured brightness temperature
should be closely related to the local plasma temperature. Although available
telescopes for millimeter wavelengths made notable contributions to solar
physics in the past (e.g., Kundu 1959; Trottet et al. 2011; Loukitcheva et al.
2014, and references therein), they did not have sufficient spatial resolution
for resolving the small spatial scales in the solar chromosphere until the
Atacama Large Millimeter/submillimeter Array (ALMA) started regular
observations of the Sun in 2016 (Bastian 2002; Karlický et al. 2011; Shimojo
et al. 2017b; White et al. 2017). It should also be noted that the
chromosphere evolves on timescales well below 1 min, which meant that
meaningful observations are restricted to very short integration times
essentially leading to snapshot imaging. Techniques like Earth rotation
synthesis or combining observations in different array configurations are not
viable for solar observations.

Figure 1: ALMA Total Power (TP) maps in Band 6 (230 GHz) obtained by double-
circle scanning at two different times capturing a very quiescent state (left)
and a more active state with clearly visible Active Regions including sunspots
(right). (ALMA project IDs: 2017.1.00009.S, 2011.0.00020.SV.)

The high spatial and temporal resolution and also the prospects for utilising
polarisation measurements for measuring magnetic fields in the solar
chromosphere make (sub-)millimeter telescopes very powerful tools for
investigating the solar chromosphere and thus contributing to the solution of
many open questions (see Wedemeyer et al. 2016, and references therein).

It should also be noted that the continuum radiation received at a certain
frequency mostly originates from a narrow layer in the solar atmosphere with
the mapped height range depending on the used receiver band. Observations of
the Sun in different receiver bands thus map different layers in the Sun’s
atmosphere. Despite already large success of ALMA in advancing solar physics
(e.g. Shimojo et al. 2017a; Brajša et al. 2018; Rodger et al. 2019; Selhorst
et al. 2019; Valle Silva et al. 2021; Labrosse et al. 2022; Heinzel et al.
2022; Menezes et al. 2022; Oliveira e Silva et al. 2022; Skokić et al. 2023),
there are important science cases that require different observational
capabilities. One diagnostic limitation is that observations are limited to
one narrow frequency band and therefore to one atmospheric layer at a time,
while the solar atmosphere is a prime example of a dynamic, complex, and
inherently 3D phenomenon. Meaningful observations must therefore be carried
out strictly simultaneously with as many complementary diagnostics and in as
many layers as technically possible. This capability could be offered by a
large-aperture sub-millimeter facility telescope like the proposed Atacama
Large Submillimeter Telescope (AtLAST111http://atlast-telescope.org/, Klaassen
et al. 2020; Ramasawmy et al. 2022; Mroczkowski et al. 2023, 2024). Despite an
impressive aperture of 50 m, AtLAST would not reach the angular resolution
achieved with an interferometric array like ALMA but compensate with other
unique features such as a simultaneous wider frequency coverage, thus
unlocking complementary science cases. AtLAST would thus have the potential to
boost the impact of millimeter observations on solar physics beyond its
current state. In this article, key science cases and observing strategies for
solar observations with AtLAST are described.

##  Key science cases

The chromosphere is an integral part of the solar atmosphere and as such plays
an important role in the transport of energy and matter. These are essential
for understanding the heating of the corona, the origins of the solar wind,
and the drivers of solar activity and space weather. There is a plethora of
physical processes involved, which are entangled in complicated dynamic ways,
rendering the understanding of the workings of our Sun a challenging
undertaking.

As detailed in the sections below, AtLAST (Mroczkowski et al. 2024) opens up a
new window to science cases that cannot be addressed with existing sub-mm
telescopes. In addition, some solar science cases for ALMA (see Wedemeyer et
al. 2016; Bastian et al. 2018, and references therein; see also Fig. 1) and
LLAMA (Lépine et al. 2021) are potential solar use cases for AtLAST, too, thus
creating synergies between these observatories. These science cases concern
the thermal structure of the solar atmosphere including the thermal structure
of prominences and filaments, the related transport of energy and mass, and
phenomena related to solar activity. In the following, the key science cases
for AtLAST are highlighted.

###  Thermal structure and the atmospheric heating problem

One of the large open questions in solar physics is the coronal heating
problem, which has been known since the late 1930s when observations of
spectral lines due to elements in extremely high ionisation stages implied
temperatures in excess of a million Kelvin in the corona (Edlén 1943). As
pointed out in the introduction, deriving the plasma properties in the outer
solar atmosphere is difficult, which hampered identifying the processes
responsible for the transport of the required energy into the corona. As a
result, much of the coronal heating problem remains unsolved today. It should
be noted that the Sun is not unique in this sense but rather serves as a
reference for other stars that exhibit a corona, too, thus making the solar
coronal heating problem relevant for late-type stars in general.

After decades of research, numerous processes have been identified that are
capable of supplying the energy required to account for the high temperatures
observed in the Sun. The focus has shifted towards determining which of these
processes are most relevant and how exactly they transport and deposit energy.
It is likely that a combination of different processes contributes to
chromospheric/coronal heating, with their relative importance varying in
different solar regions characterized by distinct magnetic field environments
and activity levels, i.e. from Active Regions to Quiet Sun regions. Some
processes provide continuous heating, while others, such as solar flares,
produce a more variable and intermittent heating component due to their
transient nature.

Typically these phenomena are grouped into processes related to (i) magnetic
reconnection/Ohmic heating and (ii) wave heating processes, specifically
acoustic and magnetohydrodynamic (MHD) waves. MHD waves, including Alfvén
waves, can contribute to plasma heating by perturbing the magnetic field,
leading to wave damping and the release of magnetic and kinetic energy, but,
in turn, can also be excited by dynamic variations of magnetic field
structures. The resulting heating is evident in phenomena like spicules
observed at the solar limb. The multitude of possible heating mechanisms and
their potential entanglement has made it challenging to determine which ones
are predominant and how their contributions depend on the solar region type.
In principle, identifying these mechanisms and understanding their
contributions requires quantitative and precise measurements that capture the
thermal, magnetic, and kinetic state of chromospheric plasma over time and in
three spatial dimensions. AtLAST has the potential to provide such data.

###  Solar flares

AtLAST is poised to make significant contributions in addressing numerous
unresolved inquiries pertaining to solar flares, a prominent conundrum within
contemporary solar physics that continues to be a highly dynamic area of
investigation. Although numerous intricacies remain shrouded in mystery, it is
evident that solar flares manifest as a result of the dynamic reconfiguration
and interconnection of magnetic fields within the solar atmosphere.
Consequently, substantial amounts of energy previously stored within the
magnetic field are explosively released during these events, manifesting as
both radiation and high-energy particles. This emitted radiation encompasses
the entire electromagnetic spectrum, spanning from gamma and X-rays to mm and
radio waves.

Solar flares can differ by orders of magnitude in strength, which is typically
measured according to the emitted soft X-ray flux. The strongest solar flares
ever documented released energy on the order of $10^{32}$ ergs (Emslie et al.
2005) and are often accompanied by coronal mass ejections (CME) that can
impact space weather with notable effects on Earth. The effects manifest
themselves from beautiful aurorae to severe disturbances of power grids (like
the Quebec blackout in 1989) and satellite infrastructure. As a recent
example, we can mention the destruction of the majority of a batch of Starlink
satellites due to solar activity (e.g. Dang et al. 2022). And yet, it cannot
be ruled out that even stronger flares exceeding the Carrington event of 1859
could occur with disastrous consequences for our technology-dependent modern
society. Such so-called super-flares are observed for other (solar-like) stars
(Maehara et al. 2012) (see Sect. 2.5). The study of strong solar flares, to
which AtLAST can contribute in new ways, is thus of utmost importance.

The occurrence rate of a flare is roughly related to the released energy,
making the strongest events rare (although varying with the solar cycle) and
the much weaker micro- and nano-flares more frequent and ubiquitous. The
latter are therefore extremely important as potential contribution to the
heating of the corona (see Sect. 2.1) in a more continuous way across most of
the Sun, whereas strong flares are associated with Active Regions.

Despite many decades of research and significant progress, there are still
many open questions regarding central aspects of solar and stellar flares on
all scales including the particle acceleration mechanisms (e.g. Miller 1998),
quasi-periodic pulsations (e.g. Simões et al. 2015; Mohan 2021), and the
source of the still enigmatic emission at sub-THz frequencies (Kaufmann et al.
1985, 2004; Fleishman & Kontar 2010; Krucker et al. 2013), i.e. in the
frequency range to be covered by AtLAST.

Observations of flare-like brightenings with ALMA, in both interferometric
mode (Shimojo et al. 2017a; Rodger et al. 2019) and single-dish (Skokić et al.
2023), have revealed faint and localized sources, likely thermal in origin,
and well associated with the observed emissions in EUV and SXR. Observations
of such sub-mm flares might also provide key information about the formation
of hot onsets in solar flares (Hudson et al. 2021; da Silva et al. 2023).

The mm and sub-mm data can uniquely address several key questions regarding
solar flares (Fleishman et al. 2022). For example, in combination with other
instruments for higher frequencies (see Sect. 3.4), the spectral coverage in
the mid-IR to sub-mm range can help to unveil the thermal structure of the
flaring atmosphere. Radiative-hydrodynamic (RHD) modelling of flares suggest
that the emission at wavelengths longer than 50 m is optically thick, and
thus, the observed brightness temperature should directly provide estimates
for the electron temperature at the emitting chromospheric layer (Simões et
al. 2017). In Figure 2, we provide an example of the temporal evolution of a
synthetic flare spectrum from RHD modelling.

Addressing these aspects with AtLAST would require high spectral, spatial and
temporal resolution and preferably full polarisation, which then would allow
for probing the dynamic thermal structure and magnetic structure of the solar
atmosphere before, during, and after a flare with potentially ground-breaking
insights. While already the thermal free-free radiation produced mostly due by
electron-ion free-free absorption and H- free-free absorption (e.g. Heinzel &
Avrett 2012) contains essential information about the physical mechanisms
behind flares, the non-thermal radiation component due to gyrosynchrotron and
resonance processes would be essential for constraining the acceleration of
charged particles, which is central to understanding flares.

Figure 2: Evolution of a synthetic sub-mm and IR flare $T_{b}$ spectrum from
RHD modelling (from Simões et al. 2017). From these simulations, for
$\lambda>50\mu$m, $T_{b}$ should provide the electron temperature in the
optically-thick upper chromosphere.

###  Solar prominences

Multi-scale observations of solar prominences, i.e. extended structures in the
solar atmosphere that stretch upward from the visible surface into the corona
(Vial & Engvold 2015), are another area where AtLAST can make a significant
impact. The magnetic field in a prominence supports denser prominence plasma
against gravity. The prominence material has a much lower temperature (around
104 K) while being surrounded by coronal plasma (around $10^{6}$ K). Here, the
prominence magnetic field provides the insulation, which allows the prominence
plasma to cool radiatively. Prominences appear as bright when viewed above the
solar limb but dark when viewed against the solar disk’s brightness (when they
are called filaments). Quiescent prominences can extend over a few hundred
thousand kilometers, i.e. a significant fraction of the Sun’s diameter. These
prominences are among the longest-lasting solar phenomena with lifetimes of
several days to weeks. The large field of view of AtLAST, combined with its
good spatial resolution and the ability to observe the Sun on daily bases for
extended periods, will offer us opportunities to study the questions of the
prominence origin, evolution, and eventual instability. These are important
questions, because the drivers that make the prominence magnetic field
configurations unstable and erupt are still not well understood. Erupting
prominences, as one of the main drivers of space weather, are then capable of
hurling large amounts of magnetized plasma into interplanetary space at high
speeds. However, it is unclear exactly what sets off these eruptions.

The simultaneous multi-band coverage, which is a capability that no other
facility than AtLAST can provide with sufficient resolution, will allow us to
answer also important questions about the thermal properties and energy
balance of prominence plasma and to uncover the nature of the prominence
plasma transition from cool cores to the hot corona. The advantage of
observing at millimetre wavelengths lies in their easier interpretation (e.g.
Labrosse et al. 2022; Heinzel et al. 2022). The spectral lines in the optical
and UV used for prominence observations are typically optically thick so that
detailed radiative transfer calculations are necessary for their analysis. In
contrast, the prominence plasma is mostly optically thin at AtLAST
wavelengths, which makes the interpretation much simpler due to relation
between the observed flux and the plasma temperature of the emitting region
(see, e.g., Rodger & Labrosse 2017; Gunár et al. 2016, 2018). Moreover,
simultaneous observations in multiple bands, which only AtLAST can provide,
allow detailed measurements of the kinetic temperature distribution of the
prominence plasma (see Gunár et al. 2016, 2018).

Extremely valuable will be the joint observations of prominences with AtLAST
and ALMA. ALMA interferometry is capable of very high spatial resolution, thus
extending the observable domain towards the shortest resolvable scales. On the
other hand, the interferometric (Fourier-based) observations are insensitive
to the larger-scale structures (larger than the single antenna primary beam).
In order to fill the gap around zero in the Fourier space, ALMA uses
supplementary single-dish (SD) scanning when observing a multi-scale
structures, like prominences. However, the ALMA SDs have the same size of 12m
and, consequently, there is a little overlap between the domains of scales
covered by the high-resolution interferometric data and those obtained form
the SD observations. With the much larger dish, AtLAST would provide an
excellent bridge connecting both domains and allowing for a proper match of
the interferometric and SD data. This is very true in general, but for the
limb observations (prominences, spicules) even yet more vital.

###  The solar activity cycle

As stated above, millimetre observations have the advantage that the
brightness temperatures corresponding to the observed continuum flux are
thought to be a close proxy for the chromospheric plasma temperature with the
formation height increasing with wavelength. Measuring the polarisation allows
for deriving the line-of-sight component of the magnetic field. Daily full-
disk maps of the Sun would permit the study of how the temperature (and hence
the energy content) evolves in Active Regions, Quiet Sun regions and coronal
holes. These data can be compared with other diagnostics, and in particular
the evolution of magnetic fields in the solar atmosphere. The solar
chromosphere is responsible for most of the UV emission that plays a major
role in structuring the Earth’s upper atmosphere, and evolution of the
millimeter emission will in turn reflect the drivers of solar UV emission. As
observations in different frequency bands probe different depths in the solar
atmosphere, comparing the evolution in these different layers is important for
understanding the transport of energy through the chromosphere. Repeated sets
of such observations obtained over a period of years will reveal how the
millimeter emission responds to the solar cycle. A noteworthy demonstration of
the scientific potential is the sequence of observations with the Solar
Submillimeter-wave Telescope (SST) that covers a period longer than one solar
cycle (Menezes et al. 2021).

Measuring the solar radius at subterahertz frequencies (submm/mm) allows one
to probe the solar atmosphere, since these measurements show the height above
the photosphere at which most of the emission at a determined observation
frequency is generated (Menezes & Valio 2017). Changes in the solar radius
show it can be modulated with the 11-year activity cycle (mid-term variations)
as well as longer periods (long-term variations). The equatorial radius time
series was found to be positively correlated to the solar cycle, since the
equatorial regions are more affected by the increase of active regions during
solar maxima, making the solar atmosphere warmer in these regions; on the
other hand, the anticorrelation between polar radius time series and the solar
activity proxies could be explained by a possible increase of polar limb
brightening during solar minima (Menezes et al. 2021). In the context of the
study on the subterahertz solar atmosphere, spicules can affect the
measurement of the solar radius and cause the solar limb to appear more
diffuse, which can lead to an overestimation of the solar radius. In Menezes
et al. (2022), the discrepancies between measured limb brightening values and
model predictions highlight the need for further studies to improve our
understanding of the solar atmosphere. Therefore, it is important to take into
account the presence of spicules when measuring the solar radius and limb
brightening at different frequencies. Currently, however, there are no full-
disk observations of the Sun in the mm range apart from occasional ALMA TP
maps a few times a year.

Please note that the Sun’s rotation depends on latitude with a period of down
to 24 days at the equator (see, e.g., Schou et al. 1998; Snodgrass 1983). Any
long-lived feature like an Active Region would thus move across the visible
disk of the Sun in less than two weeks. However, at the resolution anticipated
for AtLAST, features like Active Regions and the sunspots (Solanki 2003)
therein would evolve significantly on a daily timescale. This use case would
therefore be addressed best via full-disk mapping of the Sun at multiple
frequency bands, once per day. Already an initial 1-month campaign covering a
full solar rotation would allow for addressing the fundamental unsolved
problems mentioned above including the nature of coronal and chromospheric
heating problems and solar flares, whereas much longer sequences are required
for addressing the solar activity cycle.

New insights regarding the Sun’s activity variations and thus the long-term
evolution of our host star could be transferred to other stars. This Sun-as-a-
star approach has recently received increasing attention in the context of
next-generation exoplanet observations for which it is crucial to separate
observable exoplanet signatures from the host star’s “background radiation”.

###  The solar-stellar connection

The Sun, which can be observed spatially resolved, serves as a fundamental
reference case for studying the dynamic nature of stellar atmospheres and
solar/stellar activity including the occurrence of flares (see Sect. 2.2). In
addition to serendipitous detection as a by-product of other observations and
surveys, dedicated stellar observations are essential for understanding the
structure and activity of stellar atmospheres in general. The sample by Mohan
et al. (2021) demonstrates the potential of stellar mm observations and, at
the same time, illustrates the scarcity of suitable observations. AtLAST could
make substantial contributions in this regard, either through targeted
observations or also through coincidental (or even archival) detections. In
particular, observations of stellar flares in the (sub-)mm range are extremely
rare (MacGregor et al. 2018, 2020), and yet are crucial for unveiling the
physics of these events including particle acceleration and plasma heating and
for evaluating the impact of flares on the habitability of exoplanets (see,
e.g., Lammer et al. 2003; Segura et al. 2010; Konings et al. 2022; Bellotti et
al. 2023, and references therein). At millimeter wavelengths, the flux density
of the few detected stellar flares tends to be 10 - 1000 times higher than the
quiescent flux. Assuming a typical quiet chromospheric temperature of $\sim
10^{5}$ K (at 100 GHz) as seen in cool stars (Mohan et al. 2021), the flux
density during a flare can vary in the range $\sim$ 0.5 - 50 Jy for a star at
a distance of 1 pc. It is important to emphasise that the flux varies on
timescales of a few seconds to minutes. Observing the temporal evolution of a
flare, which is essential for exploring the physical mechanisms, thus requires
time sequences with accordingly high temporal cadence. Such data sets are
typically not generated from a serendipitous detection or as part of a survey
but require a dedicated sit-and-stare observation of the same region in the
sky.

AtLAST would be capable of such sit-and-stare observations, providing crucial
time-resolved light curves for stellar flares. Given the high sensitivity of
AtLAST, stellar flares in cool main sequence stars at distance of up to
$\sim$50 pc should be detectable with integration times of a few seconds, thus
resolving the relevant timescales. In addition, the anticipated unique
capability of observing at multiple frequencies simultaneously, which is
typically not provided by other sub-mm instruments and surveys, especially
when combined with polarisation capabilities, would enable AtLAST to utilise
essential diagnostics such as the variation of the mm spectral index, an
indicator of stellar atmospheric activity (Mohan et al. 2022), and strong
constraints to the emission mechanism and estimates of the magnetic field
strengths in the flaring active regions.

The full potential of flare observations at millimeter wavelengths is only
unlocked through coordinated, strictly co-simultaneous observations with other
telescopes covering as large a wavelength range across the electromagnetic
spectrum as possible. Such coordinated campaigns, which have become common in
multi-messenger astronomy and are basically standard procedure for
observations of the Sun, demand for PI-driven operations. The scarcity of such
coordinated stellar observing campaigns is among the main reasons why there is
still no confident detection of a stellar coronal mass ejection (CME) – an
essential component of exo-space weather. The success of such campaigns would
be greatly enhanced by AtLAST’s large FOV and high angular resolution, thus
being able to observe multiple stellar targets at the same time while
separating the flare source(s) from other components such as close-by active
companion stars or disks, making AtLAST ideal for time-resolved coordinated
observations of stellar flares. See also the time-domain science cases for
AtLAST (Orlowski-Scherer & et al. 2024).

##  Observing the Sun with AtLAST

Figure 3: The ALMA sunspot mosaic (left) that was taken during the ALMA SV
campaign in on December 18, 2015 in Band 6 230 GHz at original interferometric
resolution (left) and after convolution with the expected AtLAST beam (right).
For reference, the ALMA single-dish beam (dashed line) and the AtLAST beam
(solid line) are plotted in the lower left corner.

###  Diagnostic capabilities and instrumental requirements

There are two major constraints for solar observations at millimeter
wavelengths: (i) As the Sun is by far the brightest millimeter source in the
sky, sensitivity is not an issue and very short integration times and thus
even on-the-fly scanning is possible. (ii) The Sun evolves on short time
scales which prohibits long integration/scan times. Imaging of solar data thus
needs to be done as snapshot imaging or over short time windows that should
not exceed 1 min for the expected resolution (on the Sun, temporal and spatial
scales tend to correlate). As will be detailed in this section, the
combination of high angular resolution, high temporal cadence (or at least
short integration time), wide frequency coverage, and large dynamic range are
crucial for meaningful solar observations with AtLAST (Mroczkowski et al.
2024). In addition, full-Stokes polarimetry would greatly enhance AtLAST
diagnostic potential for the Sun.

####  High angular resolution and spatial mapping

High spatial resolution is of utmost importance for solar observations as much
of the relevant dynamics occurs on small spatial scales. Achieving
sufficiently high resolution is challenging at millimeter wavelengths, in
particular for single-dish telescopes such as AtLAST. While AtLAST’s aperture
sets a limit, the achievable resolution is still in the relevant range and
superior to any previous single-dish observation of the Sun. With an aperture
of 50 m, the achievable angular resolution would be 6.5” at a wavelength of
1.3 mm (ALMA Band 6) and down to just 1.5” at 0.3 mm (ALMA Band 10) (see Fig.
4).

##### Fast on-the-fly scanning

techniques (with a single beam) are used for creating maps of the whole solar
disk or limited regions by, e.g, ALMA (White et al. 2017), and could be
implemented for AtLAST, too (Kirkaune et al. in prep.). The scanning motion
could be accomplished by moving the primary or secondary reflector. Scanning
the whole disk with an ALMA TP antenna in a double-circle pattern (Phillips et
al. 2015) takes on the order of 10 min. See Fig. 1 for examples. The roughly 4
times larger aperture of AtLAST compared to an ALMA 12-m antenna results in an
accordingly much smaller ($\sim$6 %) beam area. Scanning the whole disk of the
Sun would take considerably longer, which would be sufficient for synoptic
studies with just one scan per day but prohibitive for science cases that rely
on high temporal cadence. In the latter case, scans would then need to be
limited to smaller region, although a compromise between size of the mapped
region and overall cadence can be made. In either case, for a time series, the
scanning could be continuous.

##### A multi-pixel detector

would significantly enhance the diagnostic possibilities for solar
observations with AtLAST. Even for a modest setup with a detector with 1000
pixels in a concentric setup with no overlap, spaced by one beam width, the
resulting instantaneously covered region on the Sun (hereafter referred to as
the instantaneous field of view, FOV) would cover a region with a radius of
about 36 beams. While a single AtLAST beam would be a factor $50/12$ smaller
than the beam of a 12 m ALMA antenna, using 1000 pixels would effectively make
the instantaneous FOV $36\times\frac{12}{50}\approx 9$ times larger. As shown
in Fig. 4, the diameter of the FOV would range from $\sim 45$” at 950 GHz
(ALMA Band 10) to $\sim 450$” at 100 GHz (ALMA Band 3). Such a FOV would cover
a complete sunspot even at the highest frequencies and whole Active Regions at
the lower frequencies. The angular resolution of the final imaging products
could be increased by means of a rapid small scan pattern (on scales
comparable to the beam size and below) that compensates for the above
described pixel spacing. The diameter of the instantaneous FOV would be halved
again for a denser pixel setup that allows for Nyquist sampling, which then
would not require the mentioned scan pattern. Please see Sect. 3.3 for a
detailed discussion of the detector requirements for solar observations.

##### Multi-frequency synthesis

might offer means to further increase AtLAST angular resolution. However, as
the formation height range of the mm continuum depends on the frequency, only
a moderate frequency range should be considered. Depending on the receiver
design, multi-frequency synthesis could be an option that could be applied
flexibly on AtLAST data sets.

####  Frequency coverage and spectral setup

An essential aspect to the continuum radiation is that the formation height
range from where most of the emission emerges depends on the observing
frequency or wavelength. At the highest frequency accessible with ALMA, i.e.
950 GHz (or a wavelength of 0.3 mm), the continuum radiation stems from the
upper photosphere and lower chromosphere, whereas the continuum radiation at
the shortest frequency of 35 GHz (equivalent to a wavelength just short of 1
cm) originates from the uppermost chromosphere with possible contributions
from the transition region. Covering a large frequency range simultaneously or
at least rapidly scanning through frequency has thus the potential to map a
large height range in the solar atmosphere. In case of scanning through
frequency, the scanning speed must be short compared to the dynamical
timescales in the solar atmosphere, which are down to seconds and generally
well below 1 min (depending also on the achieved spatial resolution).

Observations that cover extended regions on the Sun (or even the whole Sun)
across a large frequency range would enable the reconstruction of the three-
dimensional thermal structure of the solar chromosphere via tomographic and
inversion techniques. Producing a time series at sufficiently high cadence
would then enable to assess the temporal variation of the 3D structure — a
data product with the potential of truly ground-breaking scientific impact for
a very large range of essential topics in solar physics.

The observable frequency range should extend at least from 90 GHz to 660 GHz.
Observing at higher frequencies of up to 1 THz would provide higher angular
resolution but adequate observing conditions may not occur every day, making
these bands unreliable in the context of daily synoptic observations. Even for
continuum observations, a high frequency resolution is desirable but already a
frequency increment of $\sim 100$ GHz would allow for excellent scientific
results.

While the aforementioned considerations apply to continuum observations, a
large number of spectral channels with high spectral resolution and a flexible
set-up of the receiver bands would offer additional possibilities. While it
has yet to be seen if spectral lines, mostly hydrogen recombination lines and
potentially CO, can be detected at millimeter wavelengths, they would have
large diagnostic potential for assessing the thermal, kinetic and magnetic
state of the chromospheric plasma. It is difficult to answer right now which
frequency setup would be necessary to exploit the diagnostic potential of
spectral lines but, based on tentative simulations, hydrogen recombination
lines might be spread out over one or a few GHz but would require nonetheless
a spectral resolution better than 15.6 MHz (as currently offered by ALMA) in
order to exploit the full diagnostic potential (e.g., Doppler shifts for line-
of-sight velocities).

####  Sensitivity, integration time, dynamic range

The Sun is by far the brightest millimeter source in the sky with typical
brightness temperatures of several thousand Kelvin. Consequently, sensitivity
is not a concern and the integration time can be very low, allowing for ultra-
high cadence, which is of essence for a range of science cases. The
integration time can be much below 1 s depending on possible attenuation
strategies (e.g., solar filter or detuning). Important for solar observations,
however, is a large dynamic range. ALMA observations in Band 3 and 6 show a
wide range of brightness temperatures from as low as 3000 K to above 13 000 K
(see, e.g., data sets in the Solar ALMA Science Archive, Henriques et al.
2022). In general, mean brightness temperature of the Sun depends on the
frequency because the atmospheric height range of the mapped layer depends on
frequency and thus on the thermal stratification of the solar atmosphere. In
principle, the highest ALMA receiver bands map the transition between the
upper photosphere and lower chromosphere (i.e., the classical “temperature
minimum” region) with comparatively low temperatures, whereas the lowest ALMA
bands map radiation emerging from the on average much hotter upper
chromosphere close to the transition region with its steep jump in
temperature. However, as mentioned in Sect. 2.2, solar flares are a key
science case for solar observations with AtLAST. Depending on the strength
(i.e. overall energy release) of a flare, plasma temperatures exceed $10^{6}$
K or even $10^{7}$ K. A flare thus emits orders of magnitude more radiation
than the average (quiescent) Sun as discussed above. It should be noted that
the corresponding peak brightness temperature also depends on the observing
frequency and the beam size, which is typically too large to resolve the
source. From observations of a (very strong) X-class flare with the
1.5m-aperture Solar Submillimeter Telescope (SST, Kaufmann et al. 2008) at 212
GHz an excess brightness temperature of $1.75\times 10^{6}$ K for an assumed
source size of 25”, which would remain unresolved (Giménez de Castro et al.
2018). In comparison, AtLAST would have an angular resolution of $\sim$7” at
212 GHz and would thus be able to resolve the source and thus brightness
temperatures in excess of $10^{6}$ K. At shorter wavelengths in the sub-mm
range, we rely on simulations to estimate the expected brightness of flares
(see Figure 2).

Addressing the full range of science cases would thus require to account for a
brightness temperature range from typically 3000 K up to 14 000 K for the non-
flaring Sun, depending on frequency and resolution, but up to millions of K
for flares. Consequently, obtaining a large dynamic range for solar
observations and at the same a high sensitivity for non-solar observations is
a big challenge for designing of a receiver system that is intended to cover
all science cases.

####  Polarisation

Magnetic fields play an essential role in the Sun’s structure, dynamics, and
activity. Their impact is clearly visible in Active Regions most notably in
the form of sunspots and as large-scale prominences but also the Sun’s Quiet
Regions harbour magnetic fields in various configurations and strengths. The
latter becomes immediately obvious from high-resolution H observations (see,
e.g., Rutten et al. 2008; Rouppe van der Voort et al. 2023). High-resolution
magnetograms obtained from the spectral lines in the photosphere (i.e., the
Sun’s surface layer) have become a staple in contemporary solar physics
relevant to a large range of scientific topics but often also just for
context. Reliably measuring the magnetic field in the solar chromosphere
above, i.e. the layer that would be observed with AtLAST, is still a current
technological challenge, again with much diagnostic potential. The measured
polarization state provides a measure for the longitudinal component of the
magnetic field vector in the same layer as the continuum radiation. In the
same way, a scan through wavelength (and thus through formation height) can be
used to reconstruct the three-dimensional magnetic field structure in the
solar chromosphere. Measuring the magnetic field in this layer is in itself a
hot topic with potentially ground-breaking results.

Early observations with ALMA at a frequency of 100 GHz, which were obtained as
science verification data in Band 3 (ADS/JAO.ALMA#2011.0.00011.E), indicate
that the circular polarization degree is at least 1% . This value is
considered a minimum, as the observations were carried out with the 12m-Array
only (Shimojo et al. 2024). First regular ALMA observations of the Sun with
full polarisation in Band 3 are scheduled for Cycle 10. Important lessons are
expected from these observations, which would have implications for full
polarisation observations with AtLAST. In general, circular polarization
levels of up to 5% are expected for the Sun at mm wavelengths.

###  Observing modes

In order to address the aforementioned key science cases (see Sect. 2), it
would be best to combine 1) long-term synoptic observations with a very low
daily load on observing time with 2) campaign-based observations. The synoptic
observations could consist of one daily full-disk map or preferably a 10 min
long time sequence of maps across a wide predefined frequency range. The
resulting long-term data set would be of essential value for studying the
solar cycle and all related variations imprinted in the Sun’s chromosphere
with fundamental implications for stellar activity cycles and their impact on
the habitability of exoplanets (White et al. 2023). The campaign-based
observations would be more adjusted towards the requirements of individual
science cases. The scheduling for observations that require the existence of
an Active Region could be planned with just a few days ahead as the co-
ordination with other space-borne and ground-based observatories would boost
the scientific impact of such AtLAST data. Alternatively, a ToO mode (also
possibly confined to a campaign) would ensure to observe flares whenever a
suitable Active Region appears.

####  Solar observing mode 1 – Synoptic full-disk scans

A field-of-view (FOV) that would cover the whole Sun plus a margin around,
which is important to include the Sun’s outer atmosphere, has a diameter of
$\sim 2000$”. Scanning such a region with the comparatively small beam size of
AtLAST would require long scan times. Therefore we expect that multi-pixel
detectors will be needed for solar disk mapping as well as other non-solar
large FOV studies. The corresponding time needed for scanning the whole FOV
would be reduced basically by the number of simultaneously used beams (pixels)
and thus the resulting size of the instantaneously covered FOV (see also Sect.
3.1.1). The required scan time for the whole solar disk would scale
accordingly with respect to the single-dish Total Power scans with ALMA
antennas, which take about $\sim 10$ min. Consequently, a full-disk scan at
the lower frequencies could potentially be completed in a few minutes or less,
aided by the anticipated high slew speed of 3 deg/s. A smaller number of
pixels, e.g. $\sim 250$ pixels, would directly affect the covered FOV,
resulting in correspondingly longer scan times. Please note that very short
integration times are sufficient as the Sun is the brightest mm source on the
sky, and correspondingly mapping speed is not at all dependent on sensitivity
requirements, rather it depends solely on the receiver FOV and antenna slew
rates. See Sect. 3.3 for more considerations regarding multi-pixel detectors.

The short times for completing a scan of the whole solar disk would allow for
scheduling a daily solar map, which would have enormous potential for studying
the long-term evolution and the solar activity cycle (see Sect. 2.4).

Figure 4: Left: AtLAST beam diameter as a function of observing frequency. The
vertical dashed lines mark frequencies of 90 GHz and 660 GHz. Right: The
resulting diameter of the instantaneous FOV as a function of number of
detector pixels and observing frequency (see color legend). The resulting
ratio of the FOV area and the whole disk of the Sun is overlaid as red
contours. The dashed lines mark selected frequencies and detector numbers for
reference. The pixels are assumed to have the size of the corresponding beam
and are packed into a circle with no overlap. See Sect. 3.3 for details.

####  Solar observing mode 2 – High-resolution time-dependent observations

A selected target, e.g. a sunspot, would be followed in time for one or
several hours (compensating for motion on the sky plus solar rotation) and
observed with preferably high cadence. For comparison, ALMA currently observes
the Sun at 1 s cadence. This mode, which is equivalent to observations at
visible wavelengths at limited field of view, is suitable for a large range of
science cases that require a combination of high spatial and temporal
resolution.

####  Solar observing mode 3 – Regional scan sequences

As a compromise between full-disk scans (mode 1) and single-target tracking
(mode 2), AtLAST could perform scans of extended regions (or mosaics). The
size of the covered region on the Sun would depend on the instantaneous field
of view of the multi-pixel detector (see Sect. 3.3) and the maximum acceptable
temporal cadence. Such scans could be repeated continously for some time, e.g.
one hour, providing good temporal cadence of a large field of view. This mode
could be useful for observations of large-scale features such as filaments or
entire Active Regions. Such observations would facilitate the detection of
flares and filament eruptions and at the same time capture their evolution at
adequate temporal resolution.

###  Instrumentation

The detectors described in this section assume technological progress on a
time scale of about 10 years. The specifications are provided by the AtLAST
instrument working group222https://www.atlast.uio.no/memo-series/memo-
public/instrumentationwgmemo4_29feb2024.pdf. Please note the proposed
instruments would have different operation modes and would also be suited for
slightly different science cases. In Figure 4, the instantaneously covered FOV
of multi-pixel detectors with $n$ pixels are illustrated. The pixels are
assumed to the same diameter as the beam diameter $d_{\mathrm{beam}}$ at a
given observing frequency and are packed with no overlap. Treated as a circle
packing problem, the resulting diameter of the FOV is approximated as
$d_{\mathrm{fov}}\approx\sqrt{\frac{2}{3}\pi/\sqrt{3}}\ \
d_{\mathrm{beam}}\sqrt{n}$.

####  Multi-chroic continuum camera

A first-generation multi-chroic camera for continuum observations could have
30 000 pixels covering 3 frequencies simultaneously while a second-generation
camera could be extended to possibly 300 000 pixels covering 6 frequencies
simultaneously. The frequencies would be preferably spread equally over the
range from $\sim 100$ GHz to 700 GHz but adjusted according the atmospheric
transmission at the AtLAST site333https://almascience.eso.org/about-alma/alma-
site, e.g., 100, 220, 340, 460, 615, and 700 GHz. Full polarisation capability
would substantially enhance the diagnostic possibilities (see Sect. 3.1.4).

The details of the detector setup have to be investigated in detail. That
includes the spacing between pixels, the total number of pixels, and the
resulting instantaneously covered FOV. A fast small circular scanning pattern
could be used to fill intermediate pointings. Combining data from circular
scans (if possible on a second time scale or at least <10 s) would then allow
to produce data with higher angular resolution. A detailed study is currently
in preparation (Kirkaune et al. in prep.).

####  Multi-pixel heterodyne receiver system

A full heterodyne system with 64 spectral pixels (spaxels) and 16 GHz
bandwidth seems technologically feasible, whereas, on a 10-years perspective,
systems with at least 256 spectral pixels (spaxels) and 30 GHz bandwidth might
become possible. Such a detector would allow for exploiting the spectral
domain but comes at the price of a much smaller number of detector elements as
compared to multi-chroic continuum cameras (see Sect. 3.3.1), strongly
limiting the instantaneously covered region on the Sun. In combination with
spatial scanning strategies, multi-pixel heterodyne receiver system would
require compromises between covered field of view and overal temporal cadence,
which would be needed to adjusted to the needs of individual science cases.
These limitations would only make sense if spectral lines are discovered on
the Sun that would justify these limitations for accessing the diagnostic
potential of spectral lines (see Sect. 3.1.2).

####  Integrated Field Unit

While multi-chroic continuum cameras (see Sect. 3.3.1) and heterodyne receiver
systems (see Sect. 3.3.2) are the extreme cases considered here in terms of
field of view versus spectral resolution, Integrated Field Unit could provide
a compromise. First generation units are expected to have a spectral resolving
power of $R=\lambda/d\lambda=500$ with 10 000 detectors (number of spaxels
$\times$ number of channels) and a frequency range of 70-690 GHz, second
generation units could reach $R=2000$ with 50 000 detectors covering an even
wider frequency range. While a resolving power of $R=2000$ would open
diagnostic possibilities for spectral line observations in a narrow frequency
range, it would only make sense if spectral lines at mm wavelengths are
observable in the Sun at all. Integrated Field Units would be less efficient
than multi-chroic continuum cameras for observing a large frequency range and
thus a large height range in the solar atmosphere due to the still limited
number of spaxels. With even only 10 frequencies, only 5000 spatial pixels
would be available, which is considerably lower than what is expected for
multi-chroic cameras. As for multi-pixel heterodyne receiver systems, IFUs
would likely only make sense if spectral lines can be observationally
exploited. In any case, a realistic evaluation of the scientific potential of
IFUs will require a dedicated study.

###  Synergies for development and co-observing

Given the complex and dynamic nature of the solar atmosphere, coordinated
multi-wavelength multi-instrument observing campaigns are the default modus
operandi. This typically involves ground-based and space-borne observatories,
which provide complementary data exploiting different continua and spectral
lines across the whole spectrum, thus providing large data sets that probe the
(ideally complete) thermodynamic and magnetic state of the solar plasma across
preferably all atmospheric layers. While such coordinated campaigns are
subject to the different time zones of participating instruments, there are
many successful examples including ALMA observations of the Sun, which are
typically complemented with space-borne observations with the Solar Dynamics
Observatory (SDO) and the Interface Region Imaging Spectrograph (IRIS). See
the Solar ALMA Science Archive (SALSA, Henriques et al. 2022) for examples.

Solar observations with AtLAST will be no different. The idea behind the
suggested synoptic full-disk observations (see Sect. 3.2.1) is to provide a
reference for complementary observations and to become a cornerstone of future
solar observations very much like SDO today.

The development of AtLAST can be fundamental to cover key ranges of the sub-mm
spectrum for joint flare observations, along with instruments already in
operation, such as the 30 THz cameras at Mackenzie University in São Paulo and
at CASLEO, Argentina (Kudaka et al. 2015), the new High Altitude THz Solar
photometer at 15 THz (HATS, Giménez de Castro et al. 2020) and future
telescopes, such as the Solar Submillimeter Telescope Next Generation (SSTng,
Giménez de Castro et al. 2023).

##  Summary of telescope requirements

AtLAST (Mroczkowski et al. 2024) observations of the Sun would produce a large
range of valuable contributions to science cases that are difficult if not
impossible to address with other existing or planned observatories. The need
of high temporal, spatial, and to some extent spectral resolution makes
adequate solar observations challenging. At the same time, resolution in one
dimension can be sacrificed to boost the resolution in another dimension,
e.g., lowering the temporal cadence for increased spatial resolution.
Consequently, the best approach combines a multi-pixel detector with fast on-
the-fly scanning. Appropriate imaging strategies could be developed during the
commissioning phase and through simulations at an earlier stage. Additionally,
in case that spectral lines at mm wavelengths are discovered in the Sun,
heterodyne receiver systems of IFUs might have important scientific
applications but require dedicated studies to realistically assess the
feasibility and applicability given the limited possible detector sizes. Since
the Sun is a very bright mm source, it could even be possible to split the
optical path and feed multiple instruments that are optimised for different,
complementary diagnostic purposes. Also, it has yet to be seen which solar
science cases require instruments specifically designed for solar observing
and which cases could be addressed with general instruments.

To fully unlock AtLAST’s potential in this respect, the following instrumental
properties are essential:

  * •

Sufficient instantaneous field-of-view: A multi-pixel detector is a
fundamental requirement as the instantaneously covered region on the sky with
a single beam would be insufficient for most solar science cases.

  * •

Wide frequency coverage: A wide (quasi-)simultaneous frequency coverage beyond
the current setup of ALMA’s receiver bands, possibly even covering the whole
frequency range from (at least) 90 GHz to 660 GHz, has large scientific
potential as it facilitates simultaneous mapping of an extended height range
in the solar atmosphere. A multi-chroic camera with a large number of pixels
seems to be a promising choice for continuum observations.

  * •

Full polarisation capabilities are desirable in order to provide information
on chromospheric magnetic fields.

  * •

High temporal cadence: While integration times are very short for a bright
object like the Sun, spatial (or spectral) scanning in order to cover an
adequate spatial (or spectral) region is constrained by the short dynamic
timescales. Preferably, the temporal cadence should be less than 1 min but the
shorter the better. High detector readout rates are required, especially for
fast scanning.

  * •

Adequate brightness temperature range and accuracy: As the Sun is the
brightest mm source in the sky and can vary in brightness significantly, in
particular during flares, a solar filter with well-defined characteristics and
large dynamic range for the detector are needed. A brightness temperature
accuracy of 100 K or better is important for solar science cases.
Consequently, the need for detector cooling will depend on the achievable
signal-to-noise ratio.

  * •

Spectral resolution: Spectral lines – if successfully detected in the Sun –
would enormously increase the diagnostic capabilities. Designing a suitable
instrument, possibly based on Integrated Field Units, would require a
dedicated study.

## Grant information

This project has received funding from the European Union’s Horizon 2020
research and innovation programme under grant agreement No. 951815 (AtLAST).
In addition, we acknowledge support by the Research Council of Norway through
the EMISSA project (project number 286853) and the Centres of Excellence
scheme, project number 262622 (“Rosseland Centre for Solar Physics”). GF
acknowledges support from NSF grants AGS-2121632 and AST-2206424 and NASA
grant 80NSSC23K0090. FM acknowledges financial support from grants
#2022/12024-0 and #2013/10559-5, São Paulo Research Foundation (FAPESP).
L.D.M. acknowledges support by the French government, through the UCAJ.E.D.I.
Investments in the Future project managed by the National Research Agency
(ANR) with the reference number ANR-15-IDEX-01. M.L. acknowledges support from
the European Union’s Horizon Europe research and innovation programme under
the Marie Skłodowska-Curie grant agreement No 101107795. PJAS acknowledges
support from Conselho Nacional de Desenvolvimento Científico e Tecnológico
(CNPq) (contract 305808/2022-2), Fundo Mackenzie de Pesquisa e Inovação
(MackPesquisa) project 231017 and Fundação de Amparo à Pesquisa do Estado de
São Paulo (FAPESP) contract 2022/15700-7.

## Software

The calculations used to derive integration times for this paper were done
using the AtLAST sensitivity calculator, a deliverable of Horizon 2020
research project ‘Towards AtLAST’, and available from this link.

## Acknowledgements

This article, which was composed by the Working Group for Solar Science with
AtLAST (SoSA) as part of the AtLAST design study, makes use of the following
ALMA data: ADS/JAO.ALMA#2011.00020.SV, ADS/JAO.ALMA#2017.1.00009.S. ALMA is a
partnership of ESO (representing its member states), NSF (USA) and NINS
(Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), and KAS
(Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA
Observatory is operated by ESO, AUI/NRAO and NAOJ.

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