Properties of ubiquitous magnetic reconnection events in the lower solar atmosphere

Abstract

Context. Magnetic reconnection in the deep solar atmosphere can give rise to enhanced emission in the Balmer hydrogen lines, a phenomenon referred to as Ellerman bombs. Recent high-quality Hβ observations indicate that Ellerman bombs are more common than previously thought, and it was estimated that at any time, about half a million Ellerman bombs are present in the quiet Sun.

Aims. We performed an extensive statistical characterization of the quiet-Sun Ellerman bombs (QSEBs) in these new Hβ observations.

Methods. We analyzed a 1 h dataset of the quiet Sun observed with the Swedish 1-m Solar Telescope that consists of spectral imaging in the Hβ and Hα lines as well as spectropolarimetric imaging in Fe I 6173 Å. We used the k-means clustering and the 3D connected component labeling techniques to automatically detect QSEBs.

Results. We detected a total of 2809 QSEBs. The lifetimes vary between 9 s and 20.5 min, with a median of 1.14 min. The maximum area ranges between 0.0016 and 0.2603 Mm2, with a median of 0.018 Mm2. The maximum brightness in the Hβ wing varies between 1.06 and 2.76 with respect to the average wing intensity. A subset (14%) of the QSEBs displays enhancement of the Hβ line core. On average, the line core brightening appears 0.88 min after the onset of brightening in the wings, and the distance between these brightenings is 243 km. This gives rise to an apparent propagation speed ranging between −14.3 and +23.5 km s−1, with an average that is upward propagating at +4.4 km s−1. The average orientation is nearly parallel to the limbward direction. QSEBs are nearly uniformly distributed over the field of view, but we find empty areas with the size of mesogranulation. QSEBs are located more frequently near the magnetic network, where they are often larger, live longer, and are brighter.

Conclusions. We conclude that QSEBs are ubiquitous in the quiet Sun and appear everywhere, except in areas of mesogranular size with the weakest magnetic fields (BLOS ≲ 50 G). Our observations support the interpretation of reconnection along vertically extended current sheets.