High-resolution solar interferometry with ALMA: Advanced deconvolution techniques

Advisor: Miroslav Bárta (ASU CAS)

Funding: Fully funded

Website: https://www.asu.cas.cz/en/departments/solar-department

Contact: miroslav.barta@asu.cas.cz

High-resolution imaging represents a key to understanding many processes in astrophysics that are inherently of multi-scale nature. The Sun, as our closest lab of stellar physics, is not an exception – energy release and conversion is expected to run in cascades, whose parameters have not yet been determined by observations.

The ALMA Observatory has opened a completely new spectral window in mm and sub-mm domain, which is believed to provide a new view also on many open questions in solar physics, namely on the still unsolved problems like chromospheric and coronal heating. ALMA is observing the Sun since 2017, nevertheless, spatial resolution reached in the solar interferometry mode has been insofar severely limited in comparison with the deep-universe targets. There are multiple reasons for that: (1) phase delays caused by atmospheric turbulence, for whose there have been, until very recently, no way of compensation and correction, and (2) sparse coverage of the Fourier space of solar images with interferometric visibilities – because of the short dynamic scale of the processes at the Sun (~1sec) we cannot utilize aperture synthesis with Earth rotation. The situation is further complicated by complex, truly multi-scale structure of the solar images.

While the first issue has been recently to a large extent successfully addressed in frame of the ESO ALMA Development Study, allowing now 5× better resolution than insofar, the latter remains open, preventing us in further improvement. The problem is related to the limits of traditional CLEAN algorithm used (in many sub-variants) in interferometric deconvolution. The way out can be seen in utilization of completely different approach to imaging/deconvolution of the sparse Fourier data, using more advanced deconvolution techniques. Actually, new-approach algorithms (resolve/BRAIN) have been recently used for imaging of the back-hole M87 EHT data, which suffers exactly by the same issue of highly diluted, time-varying visibilities. Inspired by that success, this project aims at investigation of application of the new cleaning algorithm from this broader family to the solar data with high spatial resolution.

Research will be carried out in the Observatory of ASU CAS in Ondrejov, which hosts one of the seven nodes of the European ALMA Regional Center; the node in Ondrejov being the Europe-unique expertise center for solar ALMA interferometry and the leader of two ESO ALMA Development Studies targeted at solar ALMA observations. Appointed PhD researcher will closely collaborate also with Fabrizia Guglielmetti (ESO) and group of Torsten Enßlin (MPA/DZA).

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