The formation of small-scale umbral brightenings in sunspot atmospheres

Abstract

Sunspot atmospheres are highly inhomogeneous hosting both quasi-stable and transient features, such as `umbral micro-jets' and dark fibril-like events. We seek to understand the morphological properties and formation mechanisms of small-scale umbral brightenings (SSUBs; analogous to umbral micro-jets) and to understand whether links between these events and short dynamic fibrils, umbral flashes, and umbral dots can be established. An SST filtergram time-series sampling the Ca II H line and a CRISP full-Stokes 15-point Ca II 8542 A line scan dataset were used. The average lifetime and lengths of 54 SSUBs identified in the sunspot umbra are found to be 44.2 seconds (sigma=20 seconds) and 0.56" (sigma=0.14"). The spatial positioning and morphological evolution of these events was investigated finding no evidence of parabolic or ballistic profiles nor a preference for co-spatial formation with umbral flashes. The presence of Stokes V profile reversals provided evidence that these events could form through the development of shocks in the chromosphere. The application of the weak-field approximation indicated that changes in the line-of-sight magnetic field were not responsible for the modifications to the line profile and suggested that thermodynamic effects are the actual cause of the increased emission. Finally, a sub-set of SSUBs were observed to form at the foot-points of short dynamic fibrils. Overall, we found no correlation between the spatial locations where these events were observed and the occurrence of umbral dots and umbral flashes. SSUBs, however, have lifetimes and spectral signatures comparable to umbral flashes and are located at the footpoints of short dynamic fibrils, during or at the end of the red-shifted stage. It is possible, therefore, that these features form due to the shocking of fibrilar material in the lower atmosphere upon its return under gravity.

11 pages, 9 Figures

References in corpus (12)

• CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
• Dynamic Fibrils are driven by Magnetoacoustic Shocks
• High Resolution Observations and Modeling of Dynamic Fibrils
• CRISPRED: A data pipeline for the CRISP imaging spectropolarimeter
• An Open Source, Massively Parallel Code for Non-LTE Synthesis and Inversion of Spectral Lines and Zeeman-induced Stokes Profiles
• 3D Temperature Mapping of Solar Photospheric Fine Structure Using Ca II H Filtergrams
• Various Local Heating Events in the Earliest Phase of Flux Emergence
• The properties of penumbral microjets inclination
• High Resolution Observations of Chromospheric Jets in Sunspot Umbra
• The detection of upwardly propagating waves channelling energy from the chromosphere to the low corona
• Dynamics of subarcsecond bright dots in the transition region above sunspot and their relation to penumbral micro-jets
• Stable Umbral Chromospheric Structures

Cited by in corpus (10)

• Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST)
• The European Solar Telescope
• Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes
• Umbral chromospheric fine structure and umbral flashes modelled as one: the corrugated umbra
• The Propagation of Coherent Waves Across Multiple Solar Magnetic Pores
• Evolution Of Super-Sonic Downflows In A Sunspot
• Stability of two-fluid partially-ionised slow-mode shock fronts
• Downflowing umbral flashes as an evidence of standing waves in sunspot umbrae
• Evolution Of Downflows In The Transition Region Above A Sunspot Over Short Time-Scales
• Chromospheric plasma ejection above a pore