Magnetized supersonic downflows in the chromosphere

Abstract

The chromosphere above active regions (ARs) on the Sun hosts magnetized supersonic downflows. Studies of these supersonic downflows help to decipher the magnetic fine structure and dynamics of the chromosphere. We perform a statistical analysis of the magnetized supersonic downflows in a number of ARs at different evolutionary stages and survey their characteristics. We analyze spectro-polarimetric scans of parts of 13 ARs obtained in the infrared He I 10 830 Å triplet formed in the upper chromosphere recorded with the GREGOR Infrared Spectrograph mounted at the GREGOR solar telescope. We retrieve the line-of-sight velocities and the magnetic field vector using the HELIX+ inversion code that assumes Milne-Eddington atmospheres. We find magnetized supersonic downflows in all the ARs, with larger area coverage by such flows in ARs observed during their emerging phase. The fact that supersonic downflows were detected in all scans, albeit only covering a small fraction, 0.2–6.4%, of the observed field-of-view, suggests that they are a comparatively common phenomenon in the upper chromospheres of ARs. The supersonic downflows are found to be associated with many AR features, such as pores, sunspot umbrae, sunspot penumbrae, light bridges, plages, He I loops as part of arch filament systems characteristic of emerging fields, and filaments. Although several mechanisms are identified to be causing the supersonic downflows, by far the most common one appears to be the draining of plasma along the legs of rising magnetic loops. The loops mainly drain into forming pores. The line-of-sight velocities of the supersonic downflows reach values of up to 49 km s−1, and the velocity distribution shows multiple populations. Almost 92% of these supersonic downflows coexist with a subsonic flow component. The weaker, more horizontal fields associated with the supersonic component suggests that it is formed above the subsonic component.