Spectroscopic Signatures of Solar Flare Onset

The Sun is our closest star, and with space now firmly established as part of our society's environment, its unique proximity has inescapable consequences for us. While solar radiation provides the energy source of our whole ecosystem, our understanding of how the variations in that radiation control, e.g. our climate, still contains huge gaps. As well as the long-term variations in the solar output, the Sun exhibits a cycle of activity the constituents of which are explosive events which release energy. This explosive energy release occurs on a myriad of scales, from nanoflares to huge eruptive flares, which are accompanied by the bulk eruption of plasma and magnetic field known as coronal mass ejections (CMEs) and whose impacts can be seen globally across the Sun and throughout the heliosphere. The most extreme of these events constitute the largest examples of explosive energy release within our solar system, during which upwards of 1026 J of energy is released. Solar flares comprise a key component of space weather, and yet despite their key importance and the extensive range of observations available from space and the ground, the precise mechanisms that lead to their onset remain an open question.
Emission lines measured in the solar atmosphere routinely show widths in excess of both the thermal Doppler and instrumental line widths. Measurements of these so-called 'non-thermal' widths provide additional information on the state of the emitting plasma, including the possible existence of turbulence, or effects due to pressure and opacity broadening. X-ray non-thermal line widths have long been observed to show substantial enhancements during flaring activity, peaking early in the impulsive phase and increasing up to tens of minutes before the flare peak, a result that has been confirmed and refined with EUV observations from Hinode EIS (for which Sarah Matthews is the Principal Investigator). Such an early response to the flaring suggests that this parameter is potentially significant in the early trigger phase of flares. X-ray and EUV spectral lines are formed in plasmas with temperatures that represent conditions in the solar corona, where the standard model indicates energy is released. However, recent work exploring the profiles of spectral lines formed in the lower atmosphere suggests correlations between profile shape and flare energy deposition. These initial results have yet to be compared with coronal line profiles, however, which would enable the energy release and deposition process to be tracked throughout the entire solar atmosphere enabling new insights into flare onset and possibilities for future prediction techniques.
The project will initially utilise spectroscopic data from Hinode EIS and IRIS, incorporating new data from Solar Orbiter as it becomes available from the EUI, SPICE and STIX instruments. The research undertaken will also form part of the solar group's preparatory work for the Solar C EUVST mission currently under development and scheduled for launch in 2028. The student undertaking this project would thus have the opportunity to participate both in the international Hinode EIS and Solar C EUVST teams, including attending team meetings and collaborative visits.