Evidence for plasmoid formation related to a hot UV burst in the solar atmosphere

Abstract

Magnetic reconnection in the solar atmosphere drives energetic phenomena such as Ellerman bombs, UV bursts, and surges. We used coordinated observations from the Swedish 1-m Solar Telescope (SST), IRIS, SDO, and the Hinode/X-ray Telescope (XRT) to analyze an active region with strong magnetic flux emergence. This region displayed long duration UV burst activity, occurrence of multiple Ellerman bombs, and recurrent ejection of surges. Chromospheric imaging show sthe presence of a dome-like structure that overarched the region where magnetic flux was emerging. The curved chromospheric fibrils were closely associated with bright and similarly curved extensions from a UV burst in the IRIS 1400 A channel. We observed an episode of formation of a bright sheet along the curved fibrils in H-beta that subsequently fragmented into plasmoid-like blobs with peak emission at a Doppler offset of +43 km/s. IRIS Si IV lines in the same region show complex and non-Gaussian line profiles that display asymmetric extensions to high Doppler offsets. Some profiles have clear components with Doppler offsets in excess of 150 km/s on both sides of the nominal line center. The optically thin SDO/AIA channels, including 94 A, show intermittent and repeated occurrences of the burst. Emission measure and filter ratio analysis indicate that the burst is multi-thermal and can attain temperatures beyond 1 MK. AIA and Hinode/XRT observations reveal a localized EUV and soft-X-ray counterpart of the burst/dome system, supporting the presence of intrinsically multithermal plasma that reaches coronal, and likely multi-MK, temperatures.