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

摘要

Context. We study the magnetic properties of small-scale transients in coronal holes and a few in the quiet Sun identified in X-ray observations and spectroscopic data in two previous papers. Aims. We aim to investigate the role of small-scale transients in the evolution of the magnetic field in an equatorial coronal hole. Methods. Two sets of observations of an equatorial coronal hole and another two in quiet-Sun regions were analysed using longitudinal magnetograms taken by the Solar Optical Telescope. An automatic feature tracking program, SWAMIS, was used to identify and track the magnetic features. Each event was then visually analysed in detail. Results. In both coronal holes and quiet-Sun regions, all brightening events are associated with bipolar regions and are caused by magnetic flux emergence followed by cancellation with the pre-existing and/or 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. In the coronal hole, the magnetic features with the largest flux are those of the dominant polarity, while the opposite polarity is distributed in weaker features. The number of magnetic features of the dominant polarity is four times grater than the opposite polarity. The supergranulation configuration appears to preserve its general shape during approximately nine hours of observations although the large concentrations (the dominant polarity) in the network did evolve and/or were slightly displaced, and their strength either increased or decreased. The emission fluctuations/radiance oscillations seen in the X-ray bright points are associated with recurring 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 find 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. Conclusions. Our study demonstrates that the magnetic flux in coronal holes is continuously processed 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. The question of whether this open flux is transported as a result of the formation and evolution of these transient events, however, still remains open.