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ALMA – The key to the Sun’s coronal heating problem.

Periodic Reporting for period 4 - SolarALMA (ALMA – The key to the Sun’s coronal heating problem.)

Reporting period: 2021-03-01 to 2021-08-31

The key question of the SolarALMA project is how are the outer layers of the Sun heated to high temperatures of a million degrees Celsius and more. The proximity of our Sun, which is the star closest to us, facilitates spatially resolved observations and thus detailed investigations of its atmospheric structure, dynamics, and activity. As such, the insights gained about the nature of our Sun have implications for understanding stars in general. Better understanding the Sun as our life-giving host star has also direct benefits for society, which depends more than ever on sensitive satellite infrastructure that is susceptible to the activity of the Sun.
The high temperatures in the outer layers of the Sun, as inferred from observations already in the 1930ies, imply that these layers are heated by some physical process. A large number of heating mechanisms have been proposed as explanation since but it remains open which mechanisms are most important. Answering this question requires and thus the key to solving the chromospheric/coronal heating problem lies therefore in accurate observations at high spatial, temporal and spectral resolution, facilitating the identification of the mechanisms responsible for the transport and dissipation of energy. This has so far been impeded by the small number of accessible diagnostics and the challenges with their interpretation. In 2016, the interferometric Atacama Large Millimeter/submillimeter Array (ALMA), located at an altitude of 5000m in the Chilean Andes, started to offer impressive complementary capabilities for observing the Sun. Due to the properties of the solar radiation at millimeter wavelengths, ALMA serves essentially as a linear thermometer, mapping the Sun’s outer layers at different heights. It can measure the thermal structure and dynamics of the solar chromosphere and with it signs of atmospheric heating.
The SolarALMA project aimed at utilizing the first observations of the Sun with ALMA for determining the thermal structure of the solar atmosphere at high temporal and (for this wavelength range) unprecedented spatial resolution. The instrumental complexity of ALMA, which combines up to 66 large antennas into one giant telescope operating under harsh high-altitude conditions, and the challenges of observing the Sun, being a highly complex and dynamic target, required that new strategies for processing the resulting data had to be developed. The use of state-of-the-art 3D numerical simulations of the Sun and corresponding artificial solar ALMA observations were crucial for the development and testing of these new strategies. The SolarALMA project succeeded to build the Solar ALMA Pipeline for the processing of such data and to collect the resulting time series of images in the Solar ALMA Science Archive (SALSA), which is open to the scientific community. The scientific analysis of these novel ALMA observations of the Sun revealed the signatures of propagating shock waves, various oscillations modes and other transient heating events that together contribute to the heating of the Sun’s outer layers. The most important result of the SolarALMA project, however, is the development of solar observations of the Sun with ALMA as a new complementary tool for quantitative studies of the Sun and the demonstration of the current and future diagnostic potential, which promises advances in our understanding of the Sun beyond the chromospheric/coronal heating problem.
The SolarALMA team has been involved in the first regular observations of the Sun with ALMA in 2016 and subsequent observing cycles, gathering an increasing number of data sets. The observational data delivered by ALMA need to be processed before the resulting time series of images of the solar chromosphere are ready for scientific analysis. A main focus during the first project half was therefore the development and optimisation of processing software for solar ALMA data. The resulting Solar ALMA Pipeline (SoAP) is now routinely producing data sets of high quality. The processing also includes the co-alignment of time series with space-borne co-observations with the IRIS and SDO satellites, which provide complementary information on various layers in the solar atmosphere.
As one of the first scientific analyses based on these rich data sets, ALMA and IRIS observations of a sunspot were compared in order to understand the differences in the probed atmospheric layers and properties and thus how to interpret ALMA observations. Several other publications address transient brightening events and oscillations, which are both connected to wave modes that are prime candidates for the energy transport and heating in the solar atmosphere. These studies were supported by detailed comparisons with synthetic ALMA observations that were produced via radiative transfer calculations based on 3D numerical models. The synthetic observations are used to further improve the SoAP with the aim to optimise the quality of ALMA image sequences and to maximise the accuracy of the measured temperatures in the future.
The combination of ALMA observations with 3D models and also more established 1D codes produced interesting and promising results: (1) The spatial resolution of the observations is high enough for clearly revealing the structure of the mapped atmospheric layer as function of time but also for the detection of small-scale dynamic events. (2) The simulations helped to identify the observed brightening events as signatures of propagating shock waves. (3) As the ALMA observations now directly indicate the temperature changes connected to such events, the combination of observations and simulations allows for estimating the contributions shock waves to the heating of the outer solar atmosphere. (4) The high time resolution and ability to observe oscillatory behaviour in different properties as brightness temperature, velocity, and feature size make ALMA a valuable new tool for studying oscillations in the solar atmosphere with first surprising results. The oscillatory properties vary significantly between observations of different solar regions depending on their underlying magnetic topology. The most magnetically quiescent datasets show a picture similar to other diagnostics, whereas lower oscillation frequencies dominate for regions with strong magnetic field concentrations when observed with ALMA. This effect, while yet to be understood, once more highlights the importance of highly complementary data sets to which ALMA now contributes in an unprecedented way.
The SolarALMA project has clearly progressed beyond the state of the art by significantly contributing to the development of ALMA’s novel solar observing mode into a complementary diagnostic tool with large scientific potential for studying the Sun. In particular, the processing of observational data from calibrated raw data into a science-ready state with the Solar ALMA Pipeline is a major accomplishment of the project as demonstrated by the ability to process a large number of data sets as now publicly provided in the project’s SALSA database.
In order to make the calculations of synthetic observations computationally feasible, the Advanced Radiative Transfer code (ART) was developed and optimised towards 100 times higher performance in the framework of a PRACE Preparatory Access project.