Coronal hole boundaries at small scales: IV. SOT view Magnetic field properties of small-scale transient brightenings in coronal holes

TL;DR

This study investigates the magnetic properties of small-scale transient brightenings in coronal holes, revealing their association with bipolar magnetic flux emergence and cancellation, and highlighting the role of magnetic reconnection in these phenomena.

Contribution

It provides detailed analysis of the magnetic flux behavior in small-scale brightenings, emphasizing the continuous magnetic flux recycling via reconnection in coronal holes.

Findings

Brightening events are linked to bipolar magnetic regions.

Magnetic flux cancellation correlates with X-ray brightening.

Magnetic flux cancellation rate is higher during jets.

Abstract

We study the magnetic properties of small-scale transients in coronal hole. We found all brightening events are associated with bipolar regions and caused by magnetic flux emergence followed by cancellation with the pre-existing and newly emerging magnetic flux. In the coronal hole, 19 of 22 events have a single stable polarity which does not change its position in time. In eleven cases this is the dominant polarity. The dominant flux of the coronal hole form the largest concentration of magnetic flux in terms of size while the opposite polarity is distributed in small concentrations. In the coronal hole the number of magnetic elements of the dominant polarity is four times higher than the non-dominant one. The supergranulation configuration appears to preserve its general shape during approximately nine hours of observations although the large concentrations in the network did evolve and were slightly displaced, and their strength either increased or decreased. The emission fluctuations seen in the X-ray bright points are associated with reoccurring magnetic cancellation in the footpoints. Unique observations of an X-ray jet reveal similar magnetic behaviour in the footpoints, i.e. cancellation of the opposite polarity magnetic flux. We found that the magnetic flux cancellation rate during the jet is much higher than in bright points. Not all magnetic cancellations result in an X-ray enhancement, suggesting that there is a threshold of the amount of magnetic flux involved in a cancellation above which brightening would occur at X-ray temperatures. Our study demonstrates that the magnetic flux in coronal holes is continuously recycled through magnetic reconnection which is responsible for the formation of numerous small-scale transient events. The open magnetic flux forming the coronal-hole phenomenon is largely involved in these transient features.