Dynamics of fine structure in the atmosphere of solar active regions

Abstract

This thesis presents an observational study of the dynamics of fine structure in the (mostly lower) atmosphere of solar active regions. The considered phenomena – Ellerman bombs, flocculent flows and coronal rain – are diverse, but ubiquitous in active regions and are testament to the need for high-resolution observations of the solar atmosphere. Employing data obtained with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar Telescope (SST), they are studied at a higher spatial, spectral and temporal resolution than ever before. Multi-diagnostic analysis of Ellerman bombs provides evidence, both through their flame-like morphology in Hα 6563Å and Ca II 8542 Å, and from underlying Fe I 6301Å polarimetry, that they constitute a photospheric reconnection phenomenon between small-scale, but strong magnetic concentrations. Their fine structure displays dynamic changes on the timescale of seconds, enhancing their flaring character as they move along intergranular lanes. Simultaneous shortwavelength data from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory reveal no consistent upper atmosphere response, but suggest that Ellerman bomb detection in 1700Å may prove feasible in identifying small-scale reconnection in larger active regions and possibly even full-disc images. High-cadence Hα CRISP data have also enabled the discovery of small elongated absorptions that propagate along curved trajectories, following the canopy of superpenumbral fibrils surrounding a small sunspot, in a seemingly footpointto- footpoint motion. Based on their morphology, dynamics and recurrence along similar paths, we have dubbed them “flocculent flows”. Both their quantitative dynamics and their Doppler signature seem to argue for actual mass motion, rather than propagating waves, while their size is comparable to, but smaller than the morphologically similar coronal rain. We suggest they may be part of a siphon flow, where the condensations result from either recurring heating events or a thermal instability aggravated by wave interactions with the flow channels. Finally, a study of coronal rain in Hα data aims at expanding the statistics on this phenomenon to cases that are only observed on-disc, at different viewing angles. Their on-disc characterisation is much more difficult and may partly explain the tendency we find for the condensations to move slower, be more elongated and colder than their off-limb counterparts. Nevertheless, their properties generally agree well with earlier studies, while being sufficiently different from those of flocculent flows for these morphologically similar phenomena to be distinguished