Ignasi Josep Soler Poquet

arXiv2026

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

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.

astro-ph.SR↗

arXiv2026

Investigating white-light flare mechanisms via the Paschen jump using high-resolution continuum observations from the Swedish 1-m Solar Telescope

The continuum is understood to contain a large portion of the energy emitted by a solar flare. The optical continuum, known as white light (WL), is particularly relevant since it may be observed by ground-based instruments. We measured the WL enhancements short- and longward of the Paschen jump in order to gain insights into the possible mechanism(s) behind the creation of these increases in our two case studies. We took measurements from the Swedish 1-m Solar Telescope of the pseudo-continuum around the Ca II 8542 Å line as well as the true continuum around the K I 7699 Å and the Fe I 6173 Å line, providing us with observations on both sides of the Paschen jump. We observe WL enhancements of over 40% against the dark (pen-) umbral background in both flares. The WL excess in flare 1 is co-temporal with the derivative of the GOES soft X-ray and hard X-ray (HXR) measurements from the Advanced Space-based Solar Observatory (ASO-S), and the flare is compatible with the Neupert effect. For flare 2, a preceding smaller flare may be the cause of the temporal discrepancy. Signatures of chromospheric evaporation and condensation are found in the WL area for both flares. The ratio of intensities blueward and redward of the Paschen jump in flare 1 is below one for most WL pixels. This is in disagreement with the accepted WL formation mechanisms. We believe this is a consequence of the Ca II 8542 Å pseudo-continuum being affected by line wing opacity changes. The co-temporality of WL and HXR enhancements suggests that the WL emission enhancements in flare 1 (and parts of flare 2) are a result of direct electron precipitation. We conclude that more reliable continuum measurements free of any nearby line influence are necessary in order to obtain conclusive evidence for the formation mechanism(s) behind optical continuum enhancements from such analysis as presented in this work.

astro-ph.SR↗