title: The energetics of small solar flares and brightenings
Creators: name: Wright, Paul James
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Sun, solar, physics, solar physics, flares, corona, coronal heating, X-ray, NuSTAR.

description: This thesis concentrates on a long-standing problem in heliophysics—the coronal heating problem. The work in this thesis investigates this problem in two main ways. The first has been to initiate the novel, yet highly successful, approach of trying to observe the smallest distinguishable heating events with a highly sensitive hard X-ray telescope, optimised for targets outside of the solar system (NuSTAR; Chapters 4, 5); the second approach has been to study the unresolved ensemble of even smaller events by studying the properties of pixel-level time series (Chapters 6, 7, 8). Chapter 1 provides a background of the coronal heating problem, with specific emphasis on the observational signatures of nanoflare heating. This is followed by an overview of the advancements of coronal imaging and spectroscopy, and the instruments used throughout this thesis are introduced (Chapter 2). Chapter 3 introduces the basics of atomic line spectroscopy and the concept of the differential emission measure. A number of techniques that are utilised throughout this thesis to recover the thermal properties of the corona are described and demonstrated with both spectroscopic and narrowband EUV observations. Chapter 4 provides an introduction to the NuSTAR hard X-ray astrophysics telescope. While NuSTAR is an astrophysics mission, this chapter demonstrates that NuSTAR is able to provide scientifically useful observations of the Sun, and these observations have advanced our understanding of various coronal phenomena including active region microflares. Chapter 5 presents observations and analysis of the first NuSTAR microflare where the combination of SDO/AIA, Hinode/XRT, and NuSTAR observations have allowed the differential emission measure to be constrained over an unprecedented temperature range. While no non-thermal emission was observed in this microflare, upper-limits consistent with a null detection are derived, and thermal energetics are discussed. From Chapter 6, this thesis concentrates on the unresolved nanoflare ensemble. Chapter 6 describes the concept of time-lag analysis, a technique to determine the temporal offset of two channels by investigating EUV light curves from SDO/AIA. Time-lag analysis is presented on a number of active region features and the 94 Å (Fe XVIII) – 335 Å time-lag is introduced in order to separate the hot and cool components of the SDO/AIA 94 Å channel. Chapter 7 builds on the work of Chapter 6 by introducing the concept EM time-lag analysis, where time-lag analysis is performed on the emission measure distribution in order to understand if the true time-lag signature (in temperature) is masked by the multi-thermal nature of SDO/AIA. For the channel pair used within this chapter, EM time-lag analysis appears to generally agree with SDO/AIA time-lag analysis however, there are a number of differences that will be discussed. Finally, Chapter 8 presents a test case demonstration of wavelet analysis on EUV time series. Wavelet analysis is first introduced, followed by wavelet coherency, and local intermittency measure, each of which could be valuable tools in the search for the signatures of the coronal heating mechanism.