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.
