Coronal hole boundaries evolution at small scales - II. XRT view. Can small-scale outflows at CHBs be a source of the slow solar wind?

摘要

Aims. We aim to further explore the small-scaleevolution of coronal hole boundaries using X-ray high-resolution andhigh-cadence images. We intend to determine the fine structure anddynamics of the events causing changes of coronal hole boundaries andto explore the possibility that these events are the source of the slowsolar wind. Methods. We developed an automated procedure for theidentification of transient brightenings in images from the X-raytelescope on-board Hinode taken with an Al Poly filter in theequatorial coronal holes, polar coronal holes, and the quiet Sun withand without transient coronal holes. Results. We found that in comparison to the quietSun, the boundaries of coronal holes are abundant with brighteningevents including areas inside the coronal holes where closed magneticfield structures are present. The visual analysis of these brighteningsrevealed that around 70% of them in equatorial, polar and transientcoronal holes and their boundaries show expanding loop structuresand/or collimated outflows. In the quiet Sun only 30% of thebrightenings show flows with most of them appearing to be contained inthe solar corona by closed magnetic field lines. This strongly suggeststhat magnetic reconnection of co-spatial open and closed magnetic fieldlines creates the necessary conditions for plasma outflows to largedistances. The ejected plasma always originates from pre-existing ornewly emerging (at X-ray temperatures) bright points. Conclusions. The present study confirms our findingsthat the evolution of loop structures known as coronal bright points isassociated with the small-scale changes of coronal hole boundaries. Theloop structures show an expansion and eruption with the trapped plasmaconsequently escaping along the ``quasi'' open magnetic field lines.These ejections appear to be triggered by magnetic reconnection, e.g.the so-called interchange reconnection between the closed magneticfield lines (BPs) and the open magnetic field lines of the coronalholes. We suggest that these plasma outflows are possibly one of thesources of the slow solar wind. Key words: Sun: atmosphere - Sun: corona -methods: observational - methods: data analysis