Small-scale coronal brightenings as seen by Solar Orbiter
Abstract
The corona of the Sun shows variability over a wide range of scales, in space, time and energy. The power-law-like distributions small-scale coronal brightenings events have been used to propose self-similarity of the involved processes. Already during the cruise phase, Solar Orbiter was close enough to the Sun so that images provided by the Extreme Ultraviolet Imager (EUI) are among the highest resolution coronal data acquired so far. The small brightenings found in the quiet Sun, now often termed campfires, could be considered as the small end of the distribution of coronal transients. Mostly, these coronal brightenings occur very low in the atmosphere, essentially just above the chromosphere. Still they show a variety of morphology, ranging from dot-like to loop-like with propagating disturbances, small jets, or miniature flux-rope eruptions. This variety of the phenomenology on the smallest resolvable scales points towards a conclusion that there is not one single process that can drive small-scale brightenings. This is supported by studies relating the EUV brightenings to the underlying magnetic field: a part of the cases shows a relation to changes of the surface magnetic flux, while in other cases it is very hard to find any connection to the magnetic field. In the quiet Sun these small brightenings are mostly found at the edges of bright elements of the chromospheric network, which they have in common with another class of brightenings seen at lower temperatures in the transition region, namely explosive events. These transients have been proposed as being due to reconnection, have a similar size and lifetime as the small brightenings, but mostly seem to lack a component at high temperatures. It might well be that these explosive events are related to one particular type of the campfires, e.g. the jet-types, but that remains to be studied. Modelling work on small brightenings is not yet very abundant. One 3D MHD model of the quiet Sun shows coronal brightenings that share properties with the observations. Here the brightening is caused (mostly) by component reconnection, but also one case of a twisted flux rope is found in the simulation data, other (future) models most likely will reveal that also different processes could produce similar brightenings. Because of timing and the mission profile, so far remote sensing observations have been taken mostly in regions of quiet Sun. With the perihelion in March 2022 Solar Orbiter will not only be closer than 0.3 AU from the Sun, providing coronal observations at even higher resolution than before, but also active region observations are planned. These might extend the zoo of the small-scale coronal transients to small features in active regions that might show properties similar or different from the quiet Sun coronal brightenings. Either way, this will provide a challenge for our understanding of the small-scale corona.
- Publication:
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44th COSPAR Scientific Assembly. Held 16-24 July
- Pub Date:
- July 2022
- Bibcode:
- 2022cosp...44.1323P