EUV and X-ray Spectroscopy of the Active Sun

TL;DR

This thesis advances understanding of solar activity by analyzing solar flares and coronal mass ejections through multi-instrument observations, hydrodynamic modeling, and sensitivity analysis of EUV imagers, revealing new insights into their mechanisms.

Contribution

It provides new evidence on flare heating mechanisms, CME initiation processes, and a novel method for assessing EUV instrument sensitivity across different plasma temperatures.

Findings

Evidence for pre-flare heating and chromospheric evaporation.

CME triggered by internal tether-cutting, not breakout reconnection.

A new technique to calculate EUV imager sensitivity to various plasma temperatures.

Abstract

This thesis strives to improve our understanding of solar activity, specifically the behaviour of solar flares and coronal mass ejections. An investigation into the hydrodynamic evolution of a confined solar flare was carried out using RHESSI, CDS, GOES and TRACE. Evidence for pre-flare heating, explosive and gentle chromospheric evaporation and loop draining were observed in the data. The observations were compared to a 0-D hydrodynamic model, EBTEL, to aid interpretation. This led to the conclusion that the flare was not heated purely by non-thermal beam heating as previously believed, but also required direct heating of the plasma. An observational investigation in to the initiation mechanism of a coronal mass ejection and eruptive flare was then carried out, again utilising observations from a wide range of spacecraft: MESSENGER/SAX, RHESSI, EUVI, Cor1 and Cor2. Observations provided evidence of CME triggering by internal tether-cutting and not by breakout reconnection. A comparison of the confined and eruptive flares suggests that while they have different characteristics, timescales and topologies, these two phenomena are the result of the same fundamental processes. Finally, an investigation into the sensitivity of EUV imaging telescopes was carried out. This study established a new technique for calculating the sensitivity of EUV imagers to plasmas of different temperatures for four different types of plasma: coronal hole, quiet sun, active region and solar flare. This was carried out for six instruments: Proba-2/SWAP, TRACE, SOHO/EIT, STEREO A/EUVI, STEREO B/EUVI and SDO/AIA. The importance of considering the multi-thermal nature of these instruments was then put into the context of investigating explosive solar activity.