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

Periodic Reporting for period 3 - SolarALMA (ALMA – The key to the Sun’s coronal heating problem.)

Reporting period: 2019-09-01 to 2021-02-28

The key question of the SolarALMA project is how are the outer layers of the Sun heated to such high temperatures. A large number of heating mechanisms have been proposed as explanation but it remains open by which mechanisms are most important. The key to solving the chromospheric/coronal heating problem lies 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. The interferometric Atacama Large Millimeter/submillimeter Array (ALMA), located at an altitude of 5000m in the Chilean Andes, now offers impressive capabilities. Due to the properties of the solar radiation at millimeter wavelengths, ALMA serves as a linear thermometer, mapping narrow layers at different heights. It can measure the thermal structure and dynamics of the solar chromosphere and thus sources and sinks of atmospheric heating. Radio recombination and molecular lines (e.g. CO) potentially provide complementary kinetic and thermal diagnostics, while the polarisation of the continuum intensity and the Zeeman effect can be exploited for valuable chromospheric magnetic field measurements.
The SolarALMA team has been involved in the first regular observations of the Sun with ALMA in December 2016 and participated in all observing cycles since then, gathering access to an increasing number of different data sets. The observational data delivered by ALMA needs to be processed before the resulting time series of images of the solar chromosphere are ready for scientific analysis. The solar observations being different from other observations with ALMA and their ground-breaking nature made the initial processing of the observational data a very challenging task. A main focus during the first project half was therefore the development and optimisation of calibration and 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. These rich data sets are now being analysed and a comparison between ALMA and IRIS results for a sunspot observation has been published. In addition, several sub-projects focused on different aspects of the small-scale structure and dynamics of the observed atmospheric layers are currently in progress.

Detailed comparisons with numerical simulations are an essential part of the methodology. These comparisons are facilitated through radiative transfer calculations based on the numerical models, producing synthetic ALMA observations. In order to make the required radiative transfer calculations computationally feasible, the Advanced Radiative Transfer code (ART) was developed in cooperation with de la Cruz Rodriguez, Stockholm University, and optimized for performance in the framework of a PRACE Preparatory Access project. ART now performs 100 times more efficient than the codes used before and will have applications that go beyond the SolarALMA project.

Next to the continued contribution to the next generation of numerical codes for 3D simulations of the solar atmosphere, also more established 1D codes have been used in order to give a first impression of the detailed dynamics as it is observable with the currently available ALMA receiver bands. Publications are currently in preparation.

First comparisons of the processed ALMA observations and corresponding simulated data products yield interesting and promising results. 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. The novel aspect is that these observations now directly indicate the temperature changes connected to such events. The available (and future) data sets allow therefore the anticipated study of chromospheric heating with ALMA, which will be the main focus of the second project half.
The SolarALMA team will continue to apply for more observations during the remaining project period. The analysis of the existing and future data sets will have highest priority. The study of small-scale dynamic events and the search for heating events, will has begun, will be continued with the aim to compose a statistical sample for such heating events. The temperature changes measured by ALMA will be evaluated in combination with co-observations from other space-borne (and, if successful, ground-based) telescopes, which will allow to characterise and distinguish different types of heating events.

The analysis is strongly supported by numerical simulations as they predict the signatures of small-scale processes and how to best detect these in real observations. To this end, large simulation data sets with unprecedented degree of realism are currently in production and will be available for the anticipated analysis. The simulation data will also be used for further optimising the Solar ALMA Pipeline with the aim to optimize the quality of ALMA image sequences and to maximize the accuracy of the measured temperatures.

Furthermore, it is planned to use a novel data inversion code which determines the properties of the gas in the atmospheric layers observed with ALMA. This method provides a complementary way to identify and evaluate heating events and the structure and dynamics of the solar atmosphere as a whole.

Finally, the numerical simulations and tools will be employed for the evaluation of future observing modes for ALMA, especially for polarisation capabilities that will eventually facilitate measurements of the magnetic field in the solar chromosphere, which is in itself a highly anticipated result with large scientific impact.