Nature of small-scale Ca ii H brightenings associated with weak fields in a quiet-Sun region

Abstract

The chromospheric emission observed in the Ca ii H&K resonance lines is generally positively correlated with photospheric magnetic fields. However, a fraction of strong Ca ii H&K brightenings are found to occur over regions of low photospheric magnetic flux density. We wish to understand the origins of these chromospheric brightenings that are associated with weak fields in quiet-Sun regions. We used high-resolution and high-cadence observations in the Ca ii H line taken by the Sunrise Filter Imager (SuFI) along with the photospheric magnetograms recorded by the Imaging Magnetograph eXperiment (IMaX). These instruments were on board the first flight of the balloon-borne observatory Sunrise . We also used the atmospheric physical parameters inverted from the IMaX data by the code called Stokes-Profiles-INversion-O-Routines (SPINOR). We identified 217 Ca ii H brightenings associated with weak fields, 111 of which are located at the peripheries of strong-field regions in the magnetic canopies formed by the expansion of the kilogauss flux tube. The other brightenings are associated with either weak unipolar magnetic patches (25 brightenings) or with interactions between weak mixed-polarity magnetic fields (75 brightenings), or they lie in locations without clearly identifiable magnetic features (6 brightenings). For the latter two situations, the Ca ii H intensity is found to vary with a period of 3--5 minutes. This is accompanied by variations in the line-of-sight velocity, the temperature at łogτ = -2.5, and the magnetic flux with a similar period. About half of the strong Ca ii H brightenings associated with weak fields are located in magnetic canopies overlying very weak photospheric fields. These canopies are produced by the expansion of kilogauss flux tubes. About 10-12% of the brightenings are associated with small magnetic bright points where the magnetic field is probably not spatially resolved so that it appears weak, but it likely reaches kilogauss strength in reality. The remainder appear to be Ca ii H grains, which are overwhelmingly found to be associated with magnetic flux emergence or cancellation events, however. We conclude that although Ca ii grains are mainly driven by (magneto-)acoustic shock waves, the observations indicate that interactions between magnetic fields with opposite polarities also contribute to the formation of the brightest peaks.