Astronomical Science

New Eyes on the Sun — Solar Science with ALMA

Sven Wedemeyer1

1 University of Oslo, Norway

Corona (AIA94+335+193) e 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 Height in the solar atmospher 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 of intensive research, the chromo- Solar Dynamics Observatory (SDO) on 20 December 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 the scene substantially by opening up a temperature ranges and thus at different height 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 ALMA ­diagnostics. The obser­ vations were carried be able to allow for a time resolution of 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 — ab (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 cd 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

Transition region Spicule I

orsional motion orsional T

Hot plasma

Canopy domain Canopy domain

Height Spicule II Chromo- Alfvén waves F i b r i l sphere F i b r i l ~1.5 Mm Sub-canopy domain c s = c A Chromo- sphere. swirl Sub-canopy Dynamic fibril ~1 Mm avelength 8.6 mm domain Shock waves

Current sheets Weak fields

0.3 mm W ~0.5 Mm Classical Continuum formation height temperature min. Small-scale canopies/HIFs Photo- Reversed granulation sphere

0 Mmτ = 1 500 Granulation p-modes / 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 m) e (M 0.0 z

omospher Chr with shock waves z = 1000 km e Height –2.4 ospher lation PhotospherPhot anu with granulation 8 Mm zone (1 ction z Gas temperature Brightness temperature 1 Conve 1ǥ) ǥ) (1 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 32 terns demonstrates that ALMA can serve as a linear ALMA will also be able to make substan­ 10 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 6 1)

10 = game-changer that would take an essen­ nm tial step towards answering the chromo­ 0 50 τ spheric/coronal heating problem. ( ” ) face ion

The answer to the heating problem would (K

ur have direct implications for the nature of 105 g l “s stellar atmospheres and their activity in re ca ion general. Observations of the activity as a ti Figure 5. Schematic

Op average gas tem­p­ mperature function of stellar type, e.g., by using the ansit erature of the Sun as Tr

s te function of height. observed flux density in the Ca II H + K e spectral lines as a chromospheric activity Ga 104 The lower curve illus­ trates how the temp­ indicator, reveal a lower limit, known as spher

o erature in the upper

basal flux (Schrijver, 1987). The exact ot layers would decline

source of this basal flux, however, is still Solar interior Ph Chromosphere Corona without heating, debated. It is most likely the combined (not to scale) whereas the upper product of heating due to acoustic waves curve is the actual average profile and processes connected to the atmos­ 01000 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 high- Solar Hi-res. mostly due to electron-ion free-free magnetic prominence legs builds up twist resolution radio absorption and H− free-free absorption, in the magnetic field of the overlying solar mm 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 106 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 the Sun allow for the development and that a new research field is emerging, American, European and East Asian ALMA Regional 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 paring key solar science with ALMA other wavelength ranges, combined with Edlén, B. 1943, Zeitschrift für Astrophysik, 22, 30 1 Emslie, A. G. et al. 2005, Journal of Geophysical through simulation studies: SSALMON 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 network has currently (as of early 2016) scientific results expected for the coming Schrijver, C. J. 1987, A&A, 172, 111 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