Scientists identify source of supersonic downdrafts into sunspots

(ORDO NEWS) — Supersonic downdrafts (SD) in the solar transition region are a downward flow of mass towards sunspots. This phenomenon is commonly observed over most sunspots. The spectra of IRIS SD often show strong red secondary emission peaks at about 100 km/s, and they last for at least several hours.

How these long-lived SDs are formed and what mechanisms are responsible for their significant and stable mass has remained unclear since their discovery in the 1980s.

Recently, Dr. Li Leping of the National Astronomical Observatory of the Chinese Academy of Sciences (NAOC) and colleagues provided new clues to these unresolved questions. They suggested that SDs result from the cooling and condensation of hot coronal plasma in magnetic dips caused by magnetic reconnection.

Using joint observations from the China Solar Ground Telescope (NVST) and several satellites including SDO, STEREO and IRIS, the researchers studied the series of SD transition regions in NOAA AR 12740 and their associated coronal activity.

“We have clearly followed the formation of a quasi-stationary SD event for the first time,” said Dr. Chen Hechao of Peking University, the study’s first author.

Extreme ultraviolet (EUV) bi-perspective images have shown that these SDs result from the cooling and condensation of coronal plasma in dips along a large-scale closed system that connects the sunspot and the remote region.

Repetitive coronal rain forms in the depressions and continuously flows along these loops towards the sunspot, resulting in an SD event in the transition region.

“Joint observations from two perspectives from multiple telescopes provide us with a very valuable opportunity to directly trace the coronal origin of these SDs,” said Professor Tian Hui of Peking University, one of the study’s corresponding authors.

Based on observations and extrapolation of the magnetic field, the researchers proposed a reconnection-based SD formation scenario that promotes coronal condensation. Under this scenario, dips are formed slowly by reconnection between two sets of loops with opposite polarities.

Then the coronal plasma in the dips rapidly cools and condenses due to thermal instability. In this process, condensed materials accumulate as transient prominences in sinkholes and thus form a reservoir of mass available to feed a sustained stream of rain.

“Because rain is constantly flowing into the sunspot along different trajectories in the form of funnel-shaped structures, i.e. sunspot plumes, the funnel effect of this geometry further changes the shape of the coronal-altitude cloddy rain into a more elongated and stream-like shape when it reaches the lower layers atmosphere,” says Dr. Chen. “This leads to quasi-stationary SD.”

“This work shows a new extended application of our previous reconnection-induced coronal condensation scenario, which was originally proposed for coronal rain formation,” said Dr Li, the study’s other corresponding author.

The coronal rain drain and its resulting SD last for more than two hours, indicating a significant mass input from coronal condensation.

In the dip, the total mass of condensate and the rate of condensation were calculated. They are indeed large enough to support this long-lived SD event. “Thus, SDs play an important role in the chromospheric-corona mass cycle in the sunspot atmosphere,” said Dr. Lee.

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