Ultraviolet observations of spectral variability and dynamic processes during solar flares

Abstract

EUV and HXR observations of solar flares provide key constraints on energy release and transport in the flaring solar atmosphere. This thesis investigates high-cadence, spectrally resolved observations of the transition-region H I Lyman-alpha (1216 Å) and Si III (1206 Å) lines from SOLSTICE to examine their spectral variability during flares, with a further investigation into the role of erupted material in flare irradiance enhancements being made with EUV imaging data from AIA.

An initial analysis compared Lyman-alpha spectral variability to HXR emission and UV imaging during two M-class flares. Both events exhibited enhancements across the full Lyman-alpha profile coincident with HXR emission, consistent with chromospheric heating by nonthermal electrons. One event displayed enhanced blue-wing emission that was interpreted as blueshifted emission from the eruption.

Following this, Doppler shifts in Si III were measured during 11 flares, with two showing significant velocities: a redshift of 201.36±21.94 km s⁻¹ and a blueshift of -39.75±11.00 km s⁻¹. The blueshifted event corresponded to the same eruptive flare that exhibited enhanced blue-wing emission in Lyman-alpha. The redshift during the other event was instead interpreted as a signature of chromospheric condensation.

Finally, the contribution of erupting material to flare irradiance was quantified for nine eruptive events using spatially resolved AIA observations. Eruptions were found to contribute, on average, between 9% and 24% of the total excess radiated energy in different EUV passbands. HXR imaging showed little overlap between erupted material and HXR sources, indicating that their emission was not primarily driven by local nonthermal electron heating.

This work demonstrates that spectral variability UV lines, and dynamic motions in the chromosphere, during solar flares are driven primarily by heating of the chromosphere by nonthermal electrons, with a significant contribution also being made by emission from erupted material above in numerous events.

Date of Award	Jul 2026
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Department for the Economy (DfE) Higher Education Research Capital Grant Funding
Supervisor	Ryan Milligan (Supervisor) & Michail Mathioudakis (Supervisor)