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 space­borne 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 close­ups 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 multi­scale object processes. The impressive progress in ground­ and produce flares, i.e., violent eruptions The real problem, however, lies in the based and space­borne solar observa­ during which high­energy 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 non­equilibrium 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 High­resolution 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 ground­based 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 non­local 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 1­metre 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 ob servable. 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, chromo sphere. 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 non­linear rela­ access to fundamental properties such the upper photosphere and lower tions between observables and plasma as the gas temperature, which are other­ chromo sphere, 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, ground­based pared to optical telescopes. Resolving thus implies scanning through height in observations with the Swedish 1­metre 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 three­dimensional 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 polari sation, 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 small­scale 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 High­resolution 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.