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Astronomical Science
New Eyes on the Sun — Solar Science with ALMA
Sven Wedemeyer 1
1
University of Oslo, Norway
Corona (AIA94+335+193)
Height in the solar atmosphere
In Cycle 4, which starts in October
2016, the Atacama Large Millimeter/
submillimeter Array (ALMA) will be
open for regular observations of the Corona (AIA171)
Sun for the first time. ALMA’s impres-
sive capabilities have the potential
to revolutionise our understanding of
Chromosphere (AIA304)
our host star, with far-reaching impli
cations for our knowledge about stars
in general. The radiation emitted at
ALMA wavelengths originates mostly Photosphere (HMI)
from the chromosphere — a complex
and dynamic layer between the photo- Wedemeyer, SDO/NASA
sphere and the corona that is prominent
during solar eclipses. Despite decades Figure 1. The Sun as seen with NASA’s space-borne able. Amongst the most important, cur
Solar Dynamics Observatory (SDO) on 20 December
of intensive research, the chromo- rently used, diagnostics are the spectral
2014, exhibiting a prominence on the top right limb
sphere is still elusive due to its complex of the solar disc. The images on the right side show lines of singly ionised calcium (Ca II) and
nature and the resulting challenges close-ups of an active region observed in different magnesium (Mg II), and Hα. Examples
to its observation. ALMA will change filters, which essentially map plasma within different of observations of the chromosphere in
temperature ranges and thus at different height
the scene substantially by opening up a the Ca II spectral line at 854 nm are
ranges in the solar atmosphere. The lowest panel
new window on the Sun, promising shows the photosphere, whereas the top two shown in Figure 2 for different types of
answers to long-standing questions. images map the corona at temperatures of more region on the Sun, all of them exhibiting
than 6 × 105 K and at millions of Kelvin, respectively. a complicated structure shaped by the
ALMA will observe mostly the intermediate layer, the
interaction of magnetic fields and dynamic
chromosphere, which is displayed in red.
The Sun — A dynamic multi-scale object processes.
The impressive progress in ground- and produce flares, i.e., violent eruptions The real problem, however, lies in the
based and space-borne solar observa during which high-energy particles and interpretation of these observations
tions, together with numerical modelling, intense radiation at all wavelengths are because these spectral lines have com
has led to a dramatic change in our emitted. It is fascinating to realise that the plicated formation mechanisms, which
picture of the Sun. We know now that the Sun, a reliable source of energy sustain include non-equilibrium effects that are
solar atmosphere, i.e., the layers above ing life on Earth, is at the same time the a direct result of the intricate nature of
the visible surface, cannot be described source of the most violent and hazardous the chromosphere. This layer marks the
as a static stack of isolated layers. Rather, phenomena in the Solar System. transition between very different domains
it has to be understood as a compound in the atmosphere of the Sun. Many sim
of intermittent, highly dynamic domains, plifying assumptions that can be made
which are intricately coupled to one Observing the elusive solar chromo for the photosphere no longer hold for
another. These domains are structured sphere the rarer chromosphere. There, the mat
on a large range of spatial scales and ter is optically thick for some wavelength
exhibit a multitude of physical processes, The different radiation continua and ranges, but mostly transparent for others.
making the Sun a highly exciting plasma spectral lines across the whole spectrum The ionisation degree of hydrogen, the
physics laboratory. originate from different domains or layers major ingredient of the Sun’s plasma, is
within the solar atmosphere and probe clearly out of equilibrium due to hot, prop
High-resolution images and movies different, complementary plasma prop agating shock waves ionising the gas,
from modern solar observatories impres erties. Consequently, simultaneous multi- but recombination does not occur instan
sively demonstrate the Sun’s complexity, wavelength observations, as obtained taneously. Consequently, the observed
which is aesthetic and challenging at during coordinated campaigns with space- intensities of chromospheric diagnostics
the same time (see Figure 1). While most borne and ground-based instruments, usually depend on a large number of
of the Sun is covered by quiet (or quies are a standard in modern solar physics. factors, which are non-local and involve
cent) regions, a few active regions are the previous evolution of the plasma.
prominently apparent. They are charac Unfortunately, only a few suitable diag Deriving the true plasma properties from
terised by strong magnetic field concen nostic techniques for probing plasma an observable is therefore a complicated
trations, visible in the form of sunspots, conditions in the chromosphere are avail task with limited accuracy.
The Messenger 163 – March 2016 15
Astronomical Science Wedemeyer S. et al., New Eyes on the Sun — Solar Science with ALMA
Figure 2. Chromo the chromosphere changes on short
spheric images taken in
dynamic timescales, long integration
the core of the Ca II
infrared triplet line at a times result in smearing out the pattern,
wavelength of 854.2 nm, most notably on the smallest scales,
one of the currently most which tend to evolve fastest. The first
used chromospheric
ALMA observations in Cycle 4 will already
diagnostics. The obser
ALMA be able to allow for a time resolution of
vations were carried
primary out with the Swedish only 2 seconds, which enables the study
beam 1-metre Solar Telescope. of the complex interaction of magnetic
1 mm The field of view is about
fields and shock waves and yet-to-be-
1 × 1 arcminute (~1/30
of the solar diameter). discovered dynamical processes —
a b (a) An active region with features that otherwise would not be
an ongoing medium- observable. At the same time, the neces
class flare (inside the
sary high time resolution makes observ
circle). (b) The chromo
sphere above a sunspot ing the Sun different from many other
close to the limb. (c) A astronomical objects. Exploiting the
quiet region inside a Earth’s rotation for improving the Fourier
coronal hole close to the
u–v plane coverage is not an option
disc centre. (d) A decay
ing active region with a and thus other solutions are required for
pronounced magnetic adequate imaging of solar features.
network cell close to the
disc centre. Courtesy of
The continuum radiation received at a
L. Rouppe van der Voort
(see also Wedemeyer et given wavelength originates from a
c d al., 2015). narrow layer in the solar atmosphere with
the height depending on the selected
wavelength (see Figure 3). At the shortest
The combination of the complicated, chromosphere. In other words, the radia wavelengths accessible with ALMA, i.e.
dynamic and intermittent nature of the tion at millimetre wavelengths gives direct 0.3 millimetres, the radiation stems from
chromosphere, with its non-linear rela access to fundamental properties such the upper photosphere and lower
tions between observables and plasma as the gas temperature, which are other chromosphere, whereas the uppermost
properties and the small number of useful wise not easy to obtain. This unique chromosphere is mapped at ALMA’s
diagnostics, have hampered progress in capability comes, unfortunately, at a longest wavelength (just short of 1 centi
our understanding of this enigmatic layer. price, namely the comparatively long metre). Rapid scanning through wave
On the other hand, this challenge has wavelength and the resulting low reso length, which might be achieved via rapid
driven the development of instrumenta lution for a given telescope size com receiver band switching in the future,
tion and theory. Currently, ground-based pared to optical telescopes. Resolving thus implies scanning through height in
observations with the Swedish 1-metre the relevant spatial scales on the Sun the solar atmosphere. Such observation
Solar Telescope (SST), aided by adaptive would require enormous single telescope sequences could be developed into
optics and advanced image reconstruc apertures. The technical answer to this tomographic techniques for measuring
tion methods, achieve a spatial resolution problem lies in the construction of large the three-dimensional thermal structure
of ~ 0.1 arcseconds, which corresponds interferometric arrays composed of many of the solar chromosphere — a true
to about 70 kilometres on the Sun. The telescopes, mimicking a large aperture novelty with a significant scientific impact.
next generation of solar telescopes with and facilitating reliable imaging of an The polarisation, which ALMA can
mirror diameters of the order of 1.5 metres extended source like the Sun. Despite measure, provides a measure of the lon
currently, and 4 metres in the near future, many noteworthy efforts in the past, only gitudinal component of the magnetic
will continue pushing towards smaller ALMA can now achieve imaging with an field vector at the same time and in the
scales, which seems inevitably required effective spatial resolution, which is, at same layer as the continuum radiation. In
in order to explain many observed solar the shortest wavelengths, close to what the same way, a scan through wavelength
phenomenon not yet understood. is currently achieved at visible wave can be used to reconstruct the three-
lengths and thus is sufficient for investi dimensional magnetic field structure in
gating the small-scale structure of the the solar chromosphere. Measuring the
ALMA and a new view of the Sun solar chromosphere. magnetic field in this layer is in itself a hot
topic and has been tried many times.
The solar radiation continuum at milli High-resolution imaging is only one of
metre wavelengths, as will be observed several key capabilities that make ALMA Furthermore, the many spectral channels
with ALMA, may provide answers to so interesting for solar observing. In and the flexible set-up of the ALMA
many open questions because it serves addition, the high achievable temporal receiver bands opens up a number of new
as a nearly linear thermometer for and spectral resolution and the ability to possibilities. Radio recombination and
the plasma in a narrow layer of the solar measure polarisation are crucial. Since molecular lines at millimetre wavelengths
16 The Messenger 163 – March 2016
moss
Corona
Hot plasma
Torsional motion
Transition region
Spic
II
ule I
le
es
Spicu
Canopy domain Canopy domain
v
wa
Height
n
fvé
0.3 mm Wavelength 8.6 mm
Al
Chromo-
ril Fib
sphere Fib ril
Continuum formation height
ril
fib
~1.5 Mm Sub-canopy domain
ic
c s = cA Chromo-
m
na
sphere.
swirl Sub-canopy
Dy
~1 Mm domain
Shock waves
Current sheets Weak fields
~0.5 Mm Classical
temperature min. Small-scale canopies/HIFs
Photo-
Reversed granulation
sphere
0 Mm τ
500 = 1 p-modes /
Granulation Vortex flows
Convection g-waves
Network Internetwork Network Network
zone
Supergranulation
Figure 3. Schematic structure of the atmospheric Atmospheric heating Next to magnetic reconnection pro
layers in Quiet Sun regions, i.e. outside the strongest
Since the late 1930s, it has been known cesses and Ohmic heating, wave heating
magnetic field concentrations, exhibiting a multitude
of different phenomena. The black solid and dotted that the gas temperature in the outer processes emerge as the most likely
lines represent magnetic field lines. The arrow to the layers of the Sun rises, counter-intuitively, heating candidates outside flaring regions
left and coloured bars illustrate the height range from a temperature minimum of around (see, e.g., De Pontieu et al., 2007). The
mapped by ALMA. Aspect ratio is not to scale. From
4000 K at only a few hundred kilometres waves can be primarily distinguished
Wedemeyer et al. (2015).
above the visible surface to values in between acoustic and magnetohydrody
excess of a million degrees Kelvin in the namic (MHD) waves, where the latter are
are expected to have a large diagnostic corona (Edlén, 1943; see Figure 5). The subdivided into different wave modes,
potential, providing complementary obvious conclusion is that the outer layers including for example Alfvén waves. MHD
information about the thermal, kinetic of the Sun are heated, but more than waves can in general contribute directly
and magnetic state of the chromospheric 70 years later it is still not clear exactly or indirectly to heating the atmospheric
plasma and possibly adjacent layers. how. The same applies to the outer layers plasma, i.e., through perturbation of the
These unprecedented capabilities prom of other stars, too, making coronal and magnetic field resulting in damping of the
ise important new findings for a large chromospheric heating a central and long- waves and the associated release of
range of topics and progress in funda standing problem in modern astrophysics. magnetic and kinetic energy. The conse
mental questions in contemporary solar quences of this heating are, for instance,
physics, although the details need to be After many decades of research, a large observed in the form of the ubiquitous
investigated first. number of processes are known, which spicules at the solar limb.
could potentially provide the amount
of energy required to explain the high A large number of observations and theo
Central questions in solar physics temperatures deduced from observa retical models suggest further potential
tions. The question has therefore shifted candidates for relevant heating mecha
The chromosphere plays an important to which processes exactly are the most nisms, for example magneto-acoustic
role in the transport of energy and matter relevant. It seems plausible to assume shocks, gravity waves, (magneto-acoustic)
throughout the solar atmosphere and that a mix of different processes is high frequency waves, transverse kink
influences a number of currently poorly responsible and that their heating contri waves, multi-fluid effects and plasma
understood phenomena. Given ALMA’s butions vary for the different types of instabilities, to name just a few. The large
unique capabilities, there is little doubt region on the Sun, each of which have number of possible heating mechanisms
that ALMA will advance our knowledge different magnetic field environments and has made it difficult to determine which
of the chromosphere in all its different thus different activity levels. Some pro of them actually dominate and how their
flavours ranging from quiet solar regions cesses provide continuous heating, pro heating contributions depend on the type
to flares. In particular, substantial pro ducing a basal contribution, while others of region on the Sun. Knowledge of their
gress can be expected regarding central (e.g., flares, see below) have by nature characteristic properties, e.g., their spec
questions in contemporary solar physics, a more transient effect and cause more tral signatures, would in principle allow
which we describe below. variable and intermittent heating. the different mechanisms to be identified,
The Messenger 163 – March 2016 17
Astronomical Science Wedemeyer S. et al., New Eyes on the Sun — Solar Science with ALMA
Horizontal
2.0 slice
Height z (Mm)
ere
0.0
ro m osphk waves
Ch th shoc km
wi 000
z=1
–2.4 ere n
ho t ospahnulatio
P th gr
wi
8
M zo n e
m
(11 v e c tion Gas temperature Brightness temperature
) Con (11 ) at a height of 1000 km at a wavelength of 1 mm
8 Mm
Figure 4. Illustration of a 3D numerical model of a Sun may serve as a Rosetta Stone for radiation covers the whole electro
small region in the solar atmosphere (left). The gas
deciphering the various contributions from magnetic spectrum from gamma-rays
temperature in a horizontal cross-section in the
chromosphere at a height of 1000 km (centre) corre the different phenomena to stellar activity, and X-rays to radio waves. The strongest
sponds closely to the brightness temperature at a and ALMA would be the tool of choice. solar flares observed occur in active
wavelength of 1 mm (right) as derived from a detailed regions with strong magnetic fields and
radiative transfer calculation involving the whole
Solar flares release energy on the order of a few
model atmosphere. The close match between pat
terns demonstrates that ALMA can serve as a linear ALMA will also be able to make substan 1032 ergs, equivalent to a few billion
thermometer for the chromospheric plasma. tial contributions to answering many open megatons of TNT (Emslie et al., 2005).
questions concerning solar flares, which
can be considered as one of the major Not all flares are equally strong, but they
the disentangling of their contributions problems in contemporary solar physics span a large range of strengths. The
and could reveal the sources and sinks of and thus a very active field of research. much weaker micro- and nano-flares
heating in the chromosphere through While many details are as yet unknown, it occur on small spatial scales and may
which all energy has to pass on its way is clear that solar flares are produced contribute to the heating of the corona in
into the corona. This crucial task, how by the violent reconfiguration and recon a more subtle and continuous way. In
ever, ultimately requires quantitative and nection of the magnetic field in the solar contrast, the strongest flares are some
precise measurements, which can pro atmosphere. As a result, large amounts times accompanied by coronal mass
duce datasets that completely and unam of energy, which were stored in the mag ejections (CME) and can lead to the ejec
biguously describe the thermal, magnetic netic field prior to an event, are released tion of solar plasma into interplanetary
and kinetic state of the chromospheric explosively in the form of radiation and space. Such space weather events can
plasma in a way that is time-dependent high-energy particles, which are accel have notable effects on Earth, ranging
and in three spatial dimensions. ALMA erated to very high speeds. The emitted from beautiful aurorae to disruptions of
has the potential to deliver just such
impressive datasets, making it a potential
Optical “surface” (τ500 nm = 1)
game-changer that would take an essen 10 6
tial step towards answering the chromo
spheric/coronal heating problem.
The answer to the heating problem would
Transition region
Gas temperature (K)
have direct implications for the nature of 10 5
stellar atmospheres and their activity in
Figure 5. Schematic
general. Observations of the activity as a average gas temp
function of stellar type, e.g., by using the erature of the Sun as
observed flux density in the Ca II H + K function of height.
Photosphere
spectral lines as a chromospheric activity 10 4 The lower curve illus
trates how the temp
indicator, reveal a lower limit, known as erature in the upper
basal flux (Schrijver, 1987). The exact layers would decline
source of this basal flux, however, is still Solar interior Chromosphere Corona without heating,
debated. It is most likely the combined (not to scale) whereas the upper
curve is the actual
product of heating due to acoustic waves average profile
and processes connected to the atmos 0 1000 2000 3000 implied by observa
pheric magnetic field. In this respect, the Height above solar surface (km) tions.
18 The Messenger 163 – March 2016
spectral, spatial and temporal resolution are thus one of the primary sources of
Space-borne for observations in the sub-THz range, space weather. It is not clear what exactly
solar observations together with the ability to probe the triggers such eruptions. Among the pro
thermal structure and magnetic fields in cesses, which may contribute and could
flaring regions, promise ground-breaking be studied with ALMA, are (giant) solar
Ground- discoveries in this respect. Already the tornadoes, which form at some of the
based thermal free-free radiation, which is legs of prominences. The rotation of the
Solar Hi-res. high- mostly due to electron-ion free-free magnetic prominence legs builds up twist
resolution
radio solar mm absorption and H− free-free absorption, in the magnetic field of the overlying
optical/
astronomy
astronomy UV/IR provides much needed information. In prominence, which may eventually cause
solar addition, however, ALMA can also detect a destabilisation of the whole structure.
observations the non-thermal radiation component
produced by gyro-synchrotron and gyro- Other important questions, which could
resonance processes, which become be addressed with ALMA, concern: the
Numerical
important in the vicinity of strong mag thermal structure of prominence bodies
simulations netic fields and can thus provide impor with their spatial fine-structure, which is
tant clues about the acceleration of composed of fine sub-arcsecond threads;
charged particles during flares. Another and the transition to the ambient coronal
Figure 6. Shaping a new research field: high- intrinsic feature of flares, which can be medium, with resulting implications for
resolution solar millimetre astronomy.
exploited by ALMA, are quasi-periodic the energy balance of prominences.
pulsations, which constrain the physical Equally, it is not yet settled what the ele
power grids (like the Quebec blackout in mechanisms behind the accumulation mentary magnetic field structures are or
1989) and the satellite infrastructure on and release of magnetic energy and the how they connect to the field at the solar
which our modern society so sensitively acceleration of particles. surface. In this respect, ALMA observa
depends. tions at high spatial and temporal resolu
Solar prominences tions will help to track the changes of
The strong flares observed so far may ALMA is an ideal tool for probing the the prominence plasma and the magnetic
not mark the upper end of the scale. The cool plasma in the solar atmosphere field, giving clues on how solar promi
possible occurrence of super-flares is and will therefore be able to contribute nences form, how they evolve and even
discussed (Maehara et al., 2012), which to answering many still-open questions tually diminish or erupt.
may release more energy than the infa about solar prominences. Solar promi
mous Carrington event of 1859. This nences are extended structures in the The advantage of using ALMA lies again
powerful geomagnetic storm was the solar atmosphere that reach up from the in the easier interpretation of the obser
result of a CME hitting the Earth’s magne visible surface into the corona (Vial & vations. The spectral lines in the optical
tosphere associated with a strong solar Engvold, 2015). Prominences appear and ultraviolet currently used for promi
flare, resulting in aurorae unusually far bright when seen above the solar limb nence observations are optically thick, so
from the poles, visible in Hawaii, Cuba, (Figure 1, top right) and as (dark) fila that detailed radiative transfer calcula
Liberia and Queensland, but also respon ments when seen against the bright solar tions are necessary for their analysis. In
sible for the failure of telegraph systems. disc. The gas contained in a prominence contrast, the prominence plasma is opti
A CME of similar strength occurred in is much cooler (some 104 K) and denser cally thin at ALMA wavelengths, which
2012 but luckily missed the Earth. Such than the surrounding 10 6 K coronal makes the interpretation much simpler
super-energetic events are certainly rare, plasma and is supported by magnetic and straightforward. In addition, observa
but they would have truly devastating fields against gravity. tions of waves and oscillations in promi
consequences for modern society and nences provide essential information and
must therefore be studied. Super-flares Quiescent prominences exhibit large- constraints on the magnetic field struc
have been observed on other stars and scale structures that can measure a few ture. Prominences have already been
red dwarf stars even exhibit mega-flares, 100 000 kilometres, thus stretching over seen with ALMA during test observations
which can be more than a thousand times a significant portion of the Sun. These and will certainly be an exciting target
stronger than their strongest (known) prominences can remain stable for many both for high-resolution studies of their
solar analogues and thus exceed the days or weeks, making them one of the dynamic fine-structure and for mosaics
bolometric luminosity of the whole star in longest-lived solar phenomena. Their fine capturing their large-scale structure.
minutes (Hawley & Pettersen, 1991). structures, on the other hand, change
on timescales of a few minutes. In con
Open questions about solar and stellar trast, active prominences live for a much Preparing the future
flares on all scales concern, for example, shorter time, exhibit large-scale motions
the particle acceleration mechanisms and can erupt within hours. Erupting As in many other fields of astrophysics,
and the source of the still-enigmatic prominences can propel substantial numerical simulations have become an
emission component, which is observed amounts of magnetised plasma at high essential tool in solar physics. They help
at sub-THz frequencies. ALMA’s high speeds into interplanetary space and to simulate what ALMA might observe
The Messenger 163 – March 2016 19
Astronomical Science Wedemeyer S. et al., New Eyes on the Sun — Solar Science with ALMA
(Figure 4). Such artificial observations of In essence, these developments illustrate out by the solar development teams of the North
American, European and East Asian ALMA Regional
the Sun allow for the development and that a new research field is emerging,
Centres (ARCs) and the Joint Astronomical Obser
optimisation of observing strategies, which could be named high-resolution vatory (JAO), and as part of recent SSALMON publi
which are quite different for the dynamic solar millimetre astronomy (schematically cations. Special thanks in connection with this article
Sun than for most other ALMA targets. shown in Figure 6). This new field brings go to H. Hudson and N. Labrosse.
For this purpose, and in connection with together solar radio astronomy, which
solar ALMA development studies, an was previously limited to lower spatial References
international network was initiated in resolution, and ground-based and space-
2014, which aims at defining and pre borne high-resolution observations at De Pontieu, B. et al. 2007, Science, 318, 1574
Edlén, B. 1943, Zeitschrift für Astrophysik, 22, 30
paring key solar science with ALMA other wavelength ranges, combined with
Emslie, A. G. et al. 2005, Journal of Geophysical
through simulation studies: SSALMON1 state-of-the-art numerical modelling. Research (Space Physics), 110, A9, 11103
(Solar Simulations for the Atacama Large This is a truly golden era for studies of the Hawley, S. L. & Pettersen, B. R. 1991, ApJ, 378, 725
Millimeter Observatory Network). The solar chromosphere with many exciting Maehara, H. et al. 2012, Nature, 485, 478
Schrijver, C. J. 1987, A&A, 172, 111
network has currently (as of early 2016) scientific results expected for the coming
Vial, J.-C. & Engvold, O. 2015, A&SS Library,
77 members from 18 countries around years. Vol. 415, (Switzerland: Springer International
the world. Furthermore, the SolarALMA Publishing)
project, funded by the European Research Wedemeyer, S. et al. 2015, SSRv, Online First
Council from 2016 to 2021 and hosted Acknowledgements
at the University of Oslo, aims at address Future solar observing with ALMA is made possible Links
ing the heating problem through a syn due to the dedicated work of many colleagues as
thesis of ALMA observations and numeri part of Commissioning and Science Verification/
1
SSALMON: http://ssalmon.uio.no
cal simulations. Extension of Capabilities (CSV/EOC) activities carried
ESO/C. Malin
A meteor burning up in the Earth’s atmosphere was
captured on camera during a time-lapse exposure of
the Atacama Large Millimeter/submilimeter Array
(ALMA) at Chajnantor.
20 The Messenger 163 – March 2016
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