The Micro Solar Flare Apparatus (MiSolFA) and High-Energy Solar Observations From Multiple Perspectives

Abstract

Space-based observations of high-energy emission from solar flares have contributed greatly to our understanding of the Sun, its atmosphere, and its most dynamic phenomena. This thesis investigates the intricacies of such observations, from maximizing scientific impact in new instrument design, to understanding how even very similar instruments may observe different parts of solar flares, to utilizing multiple perspectives and wavelengths to deduce plasma properties of a single flare. Solar flares are the result of energy stored in magnetic fields being suddenly and explosively released, accelerating particles to near light speed and releasing total energies of up to 10$^{32}$J in a single event. Accelerated electrons emit bremsstrahlung radiation in the form of high-energy hard X-rays when they encounter denser regions of the solar atmosphere, mainly at flare footpoints in the chromosphere. Slower particles lose energy through thermal collisions, resulting in soft X-ray emission often found in the magnetic loops overarching the flaring region. Because X-rays are signatures of the presence of hot, dense material in the normally thin solar corona, observing them is a crucial component to studying problems from magnetic reconnection to coronal heating. For sixteen years between 2002 and 2018, X-ray imaging and spectroscopy of solar flares was made possible by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Using a pair of diffraction gratings and temporal modulation via satellite rotation, images are reconstructed by Fourier transform. The next generation X-ray imaging spectrometer is the Spectrometer/Telescope for Imaging X-rays (STIX), orbiting the Sun rather than the earth. Like RHESSI, it relies on Fourier transform imaging, although through spatial modulation made possible through \moire{} interference patterns. A major drawback of indirect imagers like RHESSI and STIX is the limited dynamic range of the images. In the realm of solar physics, this means that coronal X-ray sources will mostly be invisible compared to the bright chromospheric footpoints. One solution to this issue is joint observation from different perspectives; stereoscopic X-ray imaging. The Micro Solar-Flare Apparatus (MiSolFA), an imaging spectrometer designed for a small 6U micro-satellite, is designed with this purpose in mind. As a relatively inexpensive yet capable Earth-orbiting instrument, MiSolFA must be able to image sources with energies between 10 and 100 keV, with 10" angular resolution. Working together with STIX, the two instruments would provide a three-dimensional view of all X-ray emitting regions of a flare -- not just the footpoints or, if the footpoints are hidden behind the solar disk, the coronal source. In order to fit an X-ray telescope into a 6U satellite, the diffraction gratings themselves must be extremely fine-pitched, while maintaining a thickness necessary to absorb high-energy X-rays. This is the main technological challenge of an instrument like MiSolFA. A large portion of this thesis is dedicated to the design, testing, and characterization of the gratings commissioned for the MiSolFA imager. Even when using two very similar instruments, joint observations present a challenge. Both the Geostationary Operational Environmental Satellite's (GOES) X-ray Sensor (XRS) and the MErcury Surface, Space ENvironment, GEochemistry, and Ranging's (MESSENGER) Solar Assembly for X-rays (SAX) were designed independently to observe solar soft X-rays. In its Mercury orbit, MESSENGER observed almost 700 solar flares at energies from 1.5 -- 8 keV. Earth-orbiting GOES observes in two channels, 1--8 \AA{}, the peak flux in which defines a solar flare's classification on a log scale, and 0.5--4 \AA{}, which overlap the energy range of SAX. Due to its different viewing angle, SAX can be used as a GOES proxy, to determine the classes of partially or fully occulted flares as seen from Earth. For flares with GOES classes above C2 seen on-disk for both instruments, an empirical relationship was found that could be used as such a proxy, accurate to within a factor of two. Agreement between the two soft X-ray spectrometers was far from perfect because the energy response of each instrument was quite different, with GOES far more sensitive to low-temperature emission than SAX. This, combined with the fact that flare plasmas do not consist of only one single temperature component, accounted for why even similar instruments looking at the same flare can measure very different quantities. With a good understanding of each instrument's biases, however, these different perspectives can offer a wealth of knowledge. One example of this is the behind-the-limb solar flare of 1 May 2013 (SOL2013-05-01T02:32). This medium-sized, M-class flare was accompanied by a ($\sim$ 400 km/s) CME and observed by several space-based observatories with different viewing angles, most notably RHESSI and Mars Odyssey, which was equipped with a hard X-ray spectrometer. Thanks to the footpoints being occulted, RHESSI imaging revealed hard X-ray emissions that originated high in the corona, at least 0.1\solrad{} above the flare site. This showed both a hot, dense, extended (11 MK, $10^{9}$ cm$^{-3}$, >60") thermal source from the escaping CME core and a non-thermal source originating from an even larger area ($\sim$100 ") at lower densities ($10^{8}$ cm$^{-3}$) located above the hot core, but still behind the CME front. Because of their relative locations, it was inferred that the non-thermal electrons were not responsible for heating the CME core itself. Were Odyssey an imaging spectrometer, this could have been definitively confirmed, along with the interpretation that heating of the core occurred before the flare became visible to RHESSI. Spectra from Odyssey did support the conclusion that only $\sim$0.1 - 0.5\% of the non-thermal electron population $>$20 keV were due to the coronal source, while the rest stayed within the chromospheric flare ribbons. The detection of high coronal hard X-ray sources in this moderate size event suggests that such sources are likely a common feature within solar eruptive events. This is good news for an observing pair such as MiSolFA and STIX; with a statistical sample size of these types of observations, it would become apparent how frequent such sources are. Whether they are large or small, the next generation of hard X-ray instruments is sure to bring many new discoveries.