Astronomical Science DOI: 10.18727/0722-6691/5065

Exploring the Sun with ALMA

Timothy S. Bastian1 18 Space Vehicles Directorate, Air Force and to gain an understanding of how Miroslav Bárta2 Research Laboratory, Albuquerque, mechanical and radiative energy are Roman Brajša3 USA transferred through that atmospheric Bin Chen4 19 National Astronomical Observatories, layer. Bart De Pontieu5, 6 Chinese Academy of Sciences, Beijing, Dale E. Gary4 China Much of what is currently known about Gregory D. Fleishman4 the chromosphere has relied on spectro- Antonio S. Hales1, 7 scopic observations at optical and ultra- Kazumasa Iwai8 The Atacama Large Millimeter/submilli- violet wavelengths using both ground- Hugh Hudson9, 10 meter Array (ALMA) Observatory opens and space-based instrumentation. While Sujin Kim11, 12 a new window onto the Universe. The a lot of progress has been made, the Adam Kobelski13 ability to perform continuum imaging interpretation of such observations is Maria Loukitcheva4, 14, 15 and spectroscopy of astrophysical phe- complex because optical and ultraviolet Masumi Shimojo16, 17 nomena at millimetre and submillimetre lines in the chromosphere form under Ivica Skokić2, 3 wavelengths with unprecedented sen­ conditions of non-local thermodynamic Sven Wedemeyer6 sitivity opens up new avenues for the equilibrium. In contrast, emission from Stephen M. White18 study of cosmology and the evolution the Sun’s chromosphere at millimetre Yihua Yan19 of galaxies, the formation of stars and and submillimetre wavelengths is more planets, and astrochemistry. ALMA also straightforward to interpret as the emis- allows fundamentally new observations sion forms under conditions of local 1 National Radio Astronomy Observatory, to be made of objects much closer to ­thermodynamic equilibrium and the Charlottesville, USA home, including the Sun. The Sun has source function is Planckian. Moreover, 2 Astronomical Institute, Czech Academy long served as a touchstone for our the Rayleigh-Jeans approximation is valid of Sciences, Ondřejov, Czech Republic understanding of astrophysical pro- (hn/kT << 1) and so the observed intensity 3 Hvar Observatory, Faculty of Geodesy, cesses, from the nature of stellar interi- at a given frequency is linearly propor- University of Zagreb, Croatia ors, to magnetic dynamos, non-radiative tional to the temperature of the (optically 4 Center for Solar-Terrestrial Research, heating, stellar mass loss, and ener- thick) emitting material. By tuning across New Jersey Institute of Technology, getic phenomena such as solar flares. the full suite of ALMA’s frequency bands Newark, USA ALMA offers new insights into all of it is possible to probe the entire depth of 5 Lockheed Martin Solar & Astrophysics these processes. the chromosphere. Lab, Palo Alto, USA 6 Rosseland Centre for Solar Physics, Wedemeyer et al. (2016) comprehensively University of Oslo, Norway ALMA solar science discuss the potential of ALMA in this 7 Joint ALMA Observatory (JAO), ­context. In brief, observations of thermal ­Santiago, Chile Radiation from the Sun at millimetre and emission from material at chromospheric 8 Institute for Space-Earth Environmental submillimetre wavelengths largely origi- temperatures will be a mainstay of Research, Nagoya University, Japan nates from the chromosphere, the rela- ALMA’s solar physics programme. Multi- 9 School of Physics and Astronomy, tively thin interface between the radia- band, high-resolution, time-resolved ­University of Glasgow, Scotland, UK tion-dominated photosphere and the observations of the chromosphere at milli­ 10 Space Sciences Laboratory, University magnetically dominated corona. The chro- metre and submillimetre wavelengths of California, Berkeley, USA mosphere is highly dynamic in nature as with ALMA will play a key role in con- 11 Korea Astronomy and Space Science magnetohydrodynamic (MHD) waves straining models of chromospheric and Institute, Daejeon, Republic of Korea propagate up from the photosphere below coronal heating. 12 University of Science and Technology, and form shocks that dissipate their Daejeon, Republic of Korea energy (for example, Wedemeyer, 2016). In addition, ALMA will be important for 13 Department of Physics and Astronomy, addressing puzzles associated with West Virginia University, Morgantown, The chromosphere remains an out­ solar filaments and prominences that USA standing problem in solar physics and, form along magnetic neutral lines. 14 Max-Planck-Institut für Sonnensystem- by extension, stellar physics. The central Although they are at chromospheric tem- forschung, Göttingen, Germany question is: why does the Sun’s atmos- peratures, they occur at coronal heights 15 Astronomical Institute, St. Petersburg phere increase in temperature above the and are therefore immersed in much University, Russia visible photosphere as one proceeds up ­hotter plasma. Quiescent prominences 16 National Astronomical Observatory of through the chromosphere and into the can remain stable for days or even weeks Japan (NAOJ), Tokyo, Japan corona? What are the heating mecha- but may then become unstable and 17 Department of Astronomical Science, nisms? How is energy transported? What erupt. ALMA offers new insights into the The Graduate University for Advanced is the role of the magnetic field? ALMA thermal structure and dynamics of Studies (SOKENDAI), Tokyo, Japan is needed to help establish the thermo­ ­prominences, their formation, and their dynamic structure of the chromosphere eventual loss of equilibrium.

The Messenger 171 – March 2018 25 Astronomical Science Bastian T. S. et al., Exploring the Sun with ALMA

ALMA observations of non-thermal emis- tion, yet considerable work was needed shape and increased side lobes. There- sion will also be of crucial importance. in practice to implement solar observing fore, while solar filters will be needed for Although observations of small flares or modes — work that is still ongoing. Provi- observations of stronger solar flares, small eruptive events may be observed sions were made to ensure that ALMA the second approach was developed for using the existing modes described antennas could safely point at the Sun quiet Sun programmes. below, observations of solar flares are not without damaging the extremely precise yet feasible with ALMA in general. Unlike telescope hardware; in particular, the sur- Yagoubov (2013) pointed out that the quiescent solar emission, solar flares face panels were chemically roughened ALMA superconductor-insulator-super- ­produce intense non-thermal radiation as in order to scatter optical and infrared conductor (SIS) mixers could be de-tuned a result of energetic electrons interacting radiation and reduce the radiative heating or de-biased to reduce the mixer gain with the local magnetic field. ALMA will of the sub-reflector and other elements and effectively increase the level at which be sensitive to emission from the most to safe levels. However, there are addi- receivers saturate, thereby allowing solar energetic of these electrons. An impor- tional factors to consider when observing observing without the use of the solar tant discovery by Kaufmann et al. (2004) the Sun. While ALMA’s sensitive receivers ­filters. This idea is illustrated in Figure 1, is that some flares produce a spectral are not damaged by pointing at the Sun, which shows the SIS current gain (left component at millimetre/submillimetre they saturate when such an intense signal axis) and conversion gain (right axis) plot- wavelengths with an inverted spectrum is introduced into the system, resulting in ted against the voltage bias for ALMA — the so-called “sub-terahertz” com­ a strongly non-linear performance. Band 3. The normal voltage bias tuning is ponent, which is distinct from the non- on the first photon step where the gain thermal gyrosynchrotron component. There are two approaches to mitigating conversion is a maximum. However, the Krucker et al. (2013) discuss it at length, the problem: attenuate the signal intro- mixer still operates at other voltage bias yet the origin of the sub-terahertz flare duced into the receiver, or increase the settings. These produce lower conver- component remains unknown. ALMA “headroom” of the receiver to ensure that sion gain but, since the dynamic range will play a central role in understanding its response remains linear by reducing scales roughly inversely with gain, these its origin. the receiver gain. Both approaches are settings can handle larger signal levels possible with ALMA. The first approach before ­saturating. This operational mode Each of these broad science themes was implemented through the use of a is referred to as the mixer de-biased (MD) additionally informs the burgeoning field solar filter on the ALMA Calibration Device. mode. Observing both the source and of “Space Weather”, which is aimed at The solar filter attenuates the incident calibrators in a specific MD mode obviates understanding the drivers of disturbances signal by a frequency-dependent amount the need to explicitly measure the change in the solar corona and wind and their that allows solar observations in a given in system gain introduced by the mode. effects on Earth and the near-Earth frequency band. However, solar filters environment. have a number of undesirable properties Another consideration, however — for mapping the quiet (i.e., non-flaring) regardless of whether the solar filter or Sun: they greatly reduce the sensitivity the MD mode is used — is that the input Enabling solar observing with ALMA with which calibrator sources can be ob­­ power changes significantly as the anten- served; they cause frequency-dependent nas move from the (solar) source to a Building on preliminary work performed complex gain changes; and they intro- ­calibrator and back. Signal levels must in East Asia, Europe, and by the Joint duce significant wavefront errors into the remain within nominal limits along the ALMA Observatory, the Solar Develop- illumination pattern of the antenna, which path to the analogue-to-digital converters ment Team was formed in late 2013, sup- result in distortions to the antenna beam and the correlator. Stepped attenuators ported by the National Science Founda- tion, ESO and East Asia. Its aims were to enable solar observations with ALMA. 250 5 LSB Superconducting gap voltage The work is supported by an extensive USB science simulations effort — the Solar 200 0 Simulations for the Atacama Large Milli- B)

metre Observatory Network (Wedemeyer (d

et al., 2015). For example, Heinzel et al. (μA) 150 –5 gain Figure 1. Plot of SIS cur-

t (2015) simulated high-resolution obser­ First photon step rent and conversion gain below the gap vations of fine structures in solar promi- rsion as a function of voltage curren setting for ALMA Band 3 nences at millimetre wavelengths. Exten- 100 Second photon step –10 nve at a local oscillator fre- SI S sive testing and validation were carried below the gap co Second photon step quency of 100 GHz. The out in 2014 and 2015, leading to the below the gap SI S arrowed ellipses indicate acceptance of solar observing with ALMA 50 First photon step –15 the relevant ordinate: left in Cycle 4 (see below). below the gap for the SIS current and right for the conversion 0 –20 gain. See Shimojo et al. Support for solar science was part of the 5678910 11 12 13 14 15 16 (2017a) for additional ALMA science programme from its incep- SIS voltage (mV) details.

26 The Messenger 171 – March 2018 are used for this purpose. A concern was Figure 2. Example of the whether the variable attenuators them- l = 30 ­double-circle pattern used to 1000 perform single-dish fast-scan selves would introduce unacceptable total power maps of the solar (­differential) phase variation between the disc, with a map spacing of source and calibrator settings, thereby 30 arcseconds. The dotted line corrupting phase calibration referenced indicates the solar limb (radius 500 960 arcseconds). The rainbow against suitable sidereal calibrators. A ) colour variation indicates the second concern was whether there are progress of the pointing pattern with time and the dashed differences between the spectral window seconds bandpass response between source and rc line indicates the track of the (a 0 centroids of the individual calibrator scans as a result of attenuator ew minor circles in the pattern. vi settings. It is a testament to the system See White et al. (2017) for addi- design that neither concern proved to d of tional details. Fiel be a significant issue. Extensive testing –500 in 2014 showed that the different attenu- ator settings used to observe calibrator sources and the Sun do not introduce significant phase errors or distort the fre- –1000 quency bandpass. MD observing modes were therefore adopted as the basis for –1000 –500 0 5001000 observations of the quiet Sun. Field of view (arcseconds)

Two additional challenges are posed by correlator. In order to recover the Sun’s particularly important on long interfero- solar observations. First, the complex brightness distribution on the largest metric baselines for correcting differential brightness distribution of the Sun contrib- angular scales, all interferometric obser- phase variations. Unfortunately, the WVRs utes significant power on angular scales vations were supplemented by full-disc saturate when ALMA’s antennas are ranging from sub-arcsecond scales to fast-scan total power maps (Phillips et pointed at the Sun and WVR measure- the diameter of the solar disc, the details al., 2015) in the relevant frequency bands ments are therefore unavailable for solar of which vary on short timescales (tens (Figures 2 and 3). These can be combined observations. As a consequence, obser- of seconds). Good instantaneous sam- with the interferometric data via “feather- vations of the Sun with long-baseline pling of the aperture plane is needed to ing” to produce photometrically accurate antenna configurations cannot currently recover measurements over the full range maps of the Sun’s brightness distribution. be supported. of angular scales. The 7-metre antennas in the fixed Atacama Compact Array were Second, water vapour radiometers The extensive solar development efforts therefore included as well as those in the (WVRs) are used on each ALMA antenna required to bring solar observing modes 12-metre array so as to sample a broader to measure variations in the electrical to the solar community are documented range of angular scales. All antennas path length introduced by the overlying in two papers. Shimojo et al. (2017a) dis- were processed through the baseline atmosphere. These measurements are cuss the steps necessary to implement

Figure 3. Left: Example of a full-disc fast-scan total power map in Band 9 (0.45 mm) on 18 December 2015 using a scan pattern similar to that shown in ­Figure 1. Right: The corre- sponding image in Ha.

The Messenger 171 – March 2018 27 Astronomical Science Bastian T. S. et al., Exploring the Sun with ALMA

Figure 4. Right: A white-light continuum image of add tremendous scientific value to ALMA to correct for the Sun’s differential the Sun obtained by the Helioseismic and Magnetic observations. For Cycle 4, these include rotation. Imager on board the Solar Dynamics Observatory showing a sunspot imaged by ALMA. Centre: A detail the Interface Region Imaging Spectro- of the same. Left: Map of the sunspot in Band 6 graph (IRIS; De Pontieu et al., 2014), which An observing strategy that mitigates, in (1.25 mm). The map was formed by a mosaic of 149 is led by NASA and operates at ultraviolet part, some of the complexity associated individual antenna pointings. The data were acquired wavelengths, and the Hinode mission, with scheduling solar programmes is to as part of science verification on 18 December 2015. See Shimojo et al. (2017) for additional details. which is led by the Japan Aerospace execute them during an “observing cam- Exploration Agency (JAXA) in cooperation paign”. That is, in coordination with rele- with NASA, the UK Science and Technol- vant missions and telescopes, a fixed interferometric imaging of the Sun, and ogy Facilities Council (STFC), ESO, and window of time is designated in one or White et al. (2017) describe fast-scan total the Norwegian Space Centre (Kosugi et more antenna configurations during which power mapping of the Sun. Examples of al., 2007), and which operates at optical, the bulk of the ALMA solar observing is maps obtained by ALMA during science extreme ultraviolet (EUV), and soft X-ray discharged. This worked well for Cycle 4 verification are shown in Figures 3 and 4. wavelengths. Numerous ground-based observing programmes from an opera- optical telescopes also participated. tional standpoint, although the commu­ nication strategy between the ALMA ALMA Cycle 4 Emission from the Sun can change dra- ­Principal Investigator, the ALMA duty matically on short timescales. Even so- astronomer and ALMA operations needs Solar observing was first made available called “quiet” Sun emission from non-­ further refinement. to the community in Cycle 4 with the call flaring active regions may evolve signifi- for proposals in March 2016. Cycle 4 solar cantly over the course of a day. Meeting programmes were restricted to continuum the science goals of a particular pro- Early science observations in two frequency bands, gramme may therefore pose operational Band 3 (3 mm) and Band 6 (1.25 mm) and scheduling challenges. The need to ALMA science verification data were using MD mode observing. Since WVR respond quickly to changing targeting released to the community in January measurements are not possible, solar requests is of paramount importance. In 2017 3, 4. These data4 validated solar observing programmes were further order to image a specific target on the observing modes released to the com- restricted to the use of the three most Sun, ALMA must correctly track the munity for Cycle 4 observing and have compact 12-metre antenna configurations Sun’s physical ephemeris, offsets relative served as the basis for a number of (C40-1, C40-2, and C40-3). Using C40-3, to the physical ephemeris, and its rota- recent scientific papers. Several studies the maximum possible angular resolutions tion. ALMA can accommodate observa- made full use of the interferometric and for Bands 3 and 6 were approximately tions of ephemeris objects such as the fast-scan total power data and others 1.5 arcseconds and 0.63 arcseconds, Sun through user-provided files that made use of the superb fast-scan total respectively. specify the exact target coordinates as power maps of the full disc of the Sun a function of time. A convenient tool, the alone: Nearly 50 solar proposals were received ALMA solar ephemeris generator — in Cycle 4 and roughly a third of these based on the Jet Propulsion Laboratory – Shimojo et al. (2017b) studied the erup- were approved and allocated observing (JPL) HORIZONS web interface1, 2 — was tion of a plasmoid in a solar active time at priority A or B. The solar physics developed by Ivica Skokić to generate region jointly at 3 mm, in the EUV (Solar community is inherently multi-wavelength files like these quickly. A solar observer Dynamics Observatory) and in soft in practice. A wide variety of ground- and can now specify the target offset relative X-rays (Hinode), demonstrating the space-based assets are available that to the centre of the Sun, and use a model ­utility of both the time-resolved, snap-

28 The Messenger 171 – March 2018 shot imaging capabilities of ALMA and Sun’s millimetre radiation in comparison magnetic fields. Ultimately, measurements the use of multi-wavelength observa- with the chromospheric and coronal of Stokes V (circular polarisation) to a tions to constrain the properties of the emission seen at optical and EUV wave- ­precision of 0.1 % are needed to fully plasmoid. lengths, finding a high degree of corre- exploit polarimetric observations of the lation, even including millimetre coun- Sun. Details of how solar polarimetric – Bastian et al. (2017) compared ALMA terparts to coronal X-ray bright points. observations will be exploited and the 1.25-mm observations of an active requirements for ALMA are discussed by region with the corresponding observa- These early results already anticipate the Loukitcheva et al. (2017b). tions of the Mg II ultraviolet emission richness and diversity of the solar obser- made by IRIS. The ALMA data com- vations to come under regular observing. Imaging spectroscopy prised a 149-point mosaic of a solar With the support of additional frequency Support for spectral-line-mode observing active region that was feathered with bands and new capabilities, the breadth is required to detect and exploit obser­ the corresponding fast-scan total of solar science that can be addressed vations of radio recombination lines power map. Although believed to form by ALMA will be fully realised. We con- (RRLs) and, possibly, other atomic and at similar heights in the chromosphere, clude with a brief discussion of future molecular transitions in the solar atmos- there are distinct differences between capabilities for solar observing. phere. Clark et al. (2000a, b) reported line the millimetre brightness temperature widths of order 500 MHz for the hydro- and the ultraviolet radiation tempera- gen RRLs H21a and H19a, suggesting ture, which are attributed to regional Future capabilities that relatively low-­resolution “time division dependencies of the formation height mode” obser­vations with ALMA may be and/or an increased degree of coupling To date, ALMA has barely scratched sufficient for early exploitation of RRLs. between the ultraviolet source function the surface of the scientific potential of and the local gas temperature in hotter millimetre and submillimetre observations Solar flares and denser areas of the active region. of the Sun. In addition to analysing and The strategy employed for observing the publishing the wealth of results from quiet Sun in Bands 3 and 6 using the MD – Iwai et al. (2017) discovered a signifi- Cycle 4 observations in Bands 3 and 6, receiver modes will not be feasible for cant 3-mm brightness enhancement in much work remains to enable observa- solar flares, which can produce intense the centre of a sunspot umbra that is tions in additional frequency bands and radiation that far exceeds the ubiquitous coincident with enhancements in the to deploy new observing modes. These, thermal background of the solar chromo- 1330 Å and 1400 Å ultraviolet contin- in turn, will allow the full power of ALMA sphere. For solar flare observations solar uum images observed by IRIS. The to be brought to bear on the fundamental filters must be used. The East Asia team enhancement may be intrinsic to sun- science questions outlined above. has previously demonstrated the use of spot umbrae at chromospheric heights, solar filters for flare observations but or alternatively could be the millimetre Additional frequency bands detailed calibration procedures have yet counterpart to a polar plume. The frequency range sampled by ALMA to be fully defined. offers a powerful probe of the solar chro- – Loukitcheva et al. (2017a) made mosphere. At present, observations in Science subarrays detailed comparisons of ALMA obser- Band 3 and Band 6 are supported. The The spectrum of continuum radiation vations of a sunspot at 1.25 and 3 mm intention of the Solar Development Team from the Sun is a key observable. Given with models of sunspot umbrae and is to provide support for observations in the dynamic nature of solar emissions at penumbrae, finding that none of the Band 7 (0.85 mm) and Band 9 (0.45 mm) millimetre and submillimetre wavelengths, extant models gives a satisfactory fit in Cycle 7 to allow deeper layers of the observations are needed in as many fre- to the dual-band high-resolution ALMA chromosphere to be observed. Evalua- quency bands as possible on a timescale observations. Observations between tion of Bands 7 and 9 for solar observing commensurate with the phenomenon 1.25 and 3 mm (Bands 4 and 5) is currently under way. In future years, of interest. In practice, this is a challenge. are needed, as well as additional multi- observations in frequency bands filling It may be possible to timeshare between band observations of many more the gaps between Bands 3, 6, 7, and 9 two or more frequency bands on times- sunspots. will be enabled. cales of tens of seconds for some pro- grammes, but others (for example, obser- – Allisandrakis et al. (2017) exploited fast- Polarimetry vations of solar flares) will require strictly scan total power maps at both 1.25 A key capability of ALMA for all scientific simultaneous observations in two or and 3 mm to assess the brightness communities is to fully support the quan- more frequency bands. This will require variation of the quiet Sun from the cen- titative characterisation of the polarisation dividing the array into two or more sub­ tre to the limb, inverting the transfer properties of the observed millimetre/ arrays that are each capable of observing equation to infer the dependence of submillimetre radiation, usually in the the Sun independently. plasma temperature on optical depth. form of the Stokes polarisation parame- ters. Of fundamental importance to under- Fast regional mapping – Brajša et al. (2018) also used fast-scan standing a range of physical processes Solar-mode observing currently includes total power maps to characterise the is the measurement of chromospheric support of full disc total power mapping

The Messenger 171 – March 2018 29 Astronomical Science Bastian T. S. et al., Exploring the Sun with ALMA

as an adjunct to the interferometric ob­­ American contribution) and ESO (for the European Loukitcheva, M. et al. 2017a, A&A, 601, A43 servations. The total power maps have contribution). The help and cooperation of engi- Phillips, N. et al. 2015, Revolution in Astronomy with neers, telescope operators, astronomers-on-duty, ALMA: The 3rd Year, 499, 347 scientific value in their own right and, for the Extension and Optimisation of Capabilities (EOC) Shimojo, M. et al. 2017a, Sol. Phys., 292, 87 certain applications, may be preferable — formerly the Commissioning and Science Verifica- Shimojo, M. et al. 2017b, ApJ, 841, L5 to interferometic observations. The ALMA tion) team, and staff at the ALMA Operations Sup- Wedemeyer, S. et al. 2015, Advances in Space Solar Development Team has demon- port Facility were crucial for the success of solar Research, 56, 2679 commissioning campaigns in 2014 and 2015. We are Wedemeyer, S. 2016, The Messenger, 163, 15 strated that fast-scan mapping could be grateful to the ALMA project for making solar obser- Wedemeyer, S. et al. 2016, SSRv, 200, 1 performed on sub-regions of the Sun at a vations in the millimetre and submillimetre possible. White, S. M. et al. 2017, Sol. Phys., 292, 88 relatively high cadence (tens of seconds). Yagoubov, P. A. 2013, 38th International Continuous fast-scan mapping of regions Conference on Infrared, Millimeter, and Terahertz References Waves (IRMMW-THz), 1, DOI: 10.1109/IRMMW- on scales of just a few arcminutes using THz.2013.6665775 two or more total power antennas to pro- Alissandrakis, C. E. et al. 2017, A&A, 605, A78 vide multi-band imaging would be a valu- Bastian, T. S. et al. 2017, ApJ, 845, L19 able mode to observe certain aspects of Brajša, R. et al. 2018, A&A, accepted Links De Pontieu, B. et al. 2014, Sol. Phys., 289, 2733 solar flares and eruptive phenomena. Clark, T. A., Naylor, D. A. & Davis, G. R. 2000a, 1 The ALMA solar ephemeris generator: http://celes- A&A, 357, 757 tialscenes.com/alma/coords/CoordTool.html Clark, T. A., Naylor, D. A. & Davis, G. R. 2000b, 2 JPL Horizons web interface: https://ssd.jpl.nasa. Acknowledgements A&A, 361, 60 gov/horizons.cgi Heinzel, P. et al. 2015, Sol. Phys., 290, 1981 3 ALMA solar observations: http://www.almaobser- The ALMA solar commissioning effort was sup- Iwai, K. et al. 2017, ApJ, 841, L20 vatory.org/en/announcement/alma-starts-observ- ported by ALMA development grants from the Kaufmann, P. et al. 2004, ApJ, 603, 121 ing-the-sun/ National Astronomical Observatory of Japan (NAOJ; Kosugi, T. et al. 2007, Sol. Phys., 243, 3 4 ALMA science verification data:https://almasci - for the East Asia contribution), the National Radio Krucker, S. et al. 2013, A&ARv, 21, 58 ence.nrao.edu/alma-data/science-verification Astronomy Observatory (NRAO; for the North Loukitcheva, M. et al. 2017b, ApJ, 850, 35 D. Kordan/ESO D.

ALMA in 2017, just after a particularly harsh winter.

30 The Messenger 171 – March 2018