Energy Release in Solar Microflares and New Methods for X-ray Imaging Spectroscopy

Abstract

Hard X-ray (HXR) observation of the Sun is a powerful tool for characterizing the structure and dynamics of the solar corona. Energy released by magnetic reconnection between stressed coronal field lines causes plasma heating and particle acceleration. Both hot plasma and accelerated particle distributions cause emission in HXRs. Reconnection powers solar flares, from the largest, most dramatic events all the way down to the smallest observable transients. Additionally, tiny reconnection events too small to individually resolve (“nanoflares”) are a hypothesized mechanism for large-scale heating of the corona. Thus, HXR observation is crucial for understanding the corona as a whole.Direct-focusing instruments are a powerful tool for HXR solar observation. Combining advanced grazing-incidence optics with segmented semiconductor detectors, these instruments have significantly higher sensitivity and effective area than HXR instruments that utilize indirect imaging methods. The Nuclear Spectroscopic Telescope ARray (NuSTAR) is an astrophysical direct-focusing X-ray observatory, which can also observe some solar phenomena. This includes faint HXR microflares at or below the limit of what could be observed by indirect imaging instruments. This dissertation presents analysis of eleven solar microflares, the first NuSTAR study to examine a population of such events collectively. The correspondence between microflares and the typical behavior of larger flares is examined, in particular focusing on the incidence of non-thermal emission. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rockets implement direct-focusing HXR observation optimized for solar observation. Three FOXSI rockets have flown already, with the capabilities of the payload improved each time. For FOXSI-4, work is underway to increase the angular resolution of the optics, meaning that the instrument resolution will be limited by the detector strip pitch. This dissertation presents new methods for achieving sub-strip resolution in FOXSI detectors, via characterizing charge shared events where one photon deposits charge in multiple strips. We document FOXSI detector tests at a synchrotron beamline which served to characterize these events, and outline new imaging methods. Additionally, we use a model of the response of the FOXSI instrument (including both the optics PSF and knowledge of detector performance) to demonstrate our new ability to resolve independent sources located only one strip pitch apart. The power of direct-focused solar HXR observation motivates a spacecraft-based solar-dedicated direct-focusing instrument utilizing the heritage of the FOXSI rocket program. The HEXITEC detector system is a pixelated semiconductor detector being developed for application in such a project. HEXITEC experiences charge sharing, as well as a significant rate of fluorescence events in the CdTe bulk. A thorough analysis of both of these effects in a HEXITEC test dataset is also presented in this dissertation, increasing understanding of the nature of such phenomena and the capabilities of the system for solar observation. In total, this dissertation both presents new contributions to the state of knowledge regarding solar X-ray microflares, and additionally develops several new methods for improving the capabilities of solid state detectors for X-ray imaging spectroscopy. Thus, it both utilizes the power of current direct-focusing instruments for investigation of coronal dynamics, and additionally contributes significantly to the development of future instruments further optimized for this purpose.