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

Abstract

Context. Chromospheric emission observed in the Ca II H and 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. Aims. We aim to understand the origin of these chromospheric brightenings associated with weak fields in quiet Sun regions. Methods. We use Ca II H observations taken by the SuFI along with the photospheric magnetograms recorded by the IMaX, instruments onboard the first flight of the balloon-borne observatory Sunrise. We also use the atmospheric physical parameters inverted from IMaX data by the SPINOR code. Results. 217 Ca II H brightenings associated with weak fields are identified. 111 of these are located at the peripheries of strong field regions, in the magnetic canopies formed by the expansion of the kG flux tube. The other brightenings are associated with either weak unipolar magnetic patches, with interactions between weak mixed-polarity magnetic fields, or at locations with no clear magnetic features. For the latter two situations, the Ca II H intensity varies with a period of 3-5 mins, accompanied by variations in the line-of-sight velocity, the temperature, and the magnetic flux with a similar period. Conclusions. About half of the strong Ca II H brightenings associated with weak fields are located in magnetic canopies overlying very weak photospheric fields. About 10-12% are associated with magnetic bright points where the magnetic field is probably not spatially resolved. The remainder appears to be Ca II H grains, which are overwhelmingly associated with magnetic emergence or cancellation events. We conclude that although Ca II grains are mainly driven by (magneto-)acoustic shock waves, interactions between opposite-polarity magnetic fields also contribute to the formation of the brightest peaks.