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Sun: chromosphere – Sun: corona – Sun: activity – Sun: magnetic fields – Methods: observational, data analysis
Abstract:
Context.We investigate the chromospheric counterpart of small-scale coronal loops constituting a coronal bright point (CBP) and itsresponse to a photospheric magnetic-flux increase accompanied by co-temporal CBP heating.Aims.The aim of this study is to simultaneously investigate the chromospheric and coronal layers associated with a CBP, and in sodoing, provide further understanding on the heating of plasmas confined in small-scale loops.Methods.We used co-observations from the Atmospheric Imaging Assembly and Helioseismic Magnetic Imager on board the SolarDynamics Observatory, together with data from the Fast Imaging Solar Spectrograph taken in the Hαand Caii8542.1 Å lines. Wealso employed both linear force-free and potential field extrapolation models to investigate the magnetic topology of the CBP loopsand the overlying corona, respectively. We used a new multi-layer spectral inversion technique to derive the temporal variations of thetemperature of the Hαloops (HLs).Results.We find that the counterpart of the CBP, as seen at chromospheric temperatures, is composed of a bundle of dark elongatedfeatures named in this work Hαloops, which constitute an integral part of the CBP loop magnetic structure. An increase in thephotospheric magnetic flux due to flux emergence is accompanied by a rise of the coronal emission of the CBP loops, that is a heatingepisode. We also observe enhanced chromospheric activity associated with the occurrence of new HLs and mottles. While the coronalemission and magnetic flux increases appear to be co-temporal, the response of the Hαcounterpart of the CBP occurs with a smalldelay of less than 3 min. A sharp temperature increase is found in one of the HLs and in one of the CBP footpoints estimated at 46%and 55% with respect to the pre-event values, also starting with a delay of less than 3 min following the coronal heating episode.The low-lying CBP loop structure remains non-potential for the entire observing period. The magnetic topological analysis of theoverlying corona reveals the presence of a coronal null point at the beginning and towards the end of the heating episode.Conclusions.The delay in the response of the chromospheric counterpart of the CBP suggests that the heating may have occurred atcoronal heights.
