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The idea for this text emerged over several years as the authors participated in research projects related to analysis of data from NASA's RHESSI Small Explorer mission. The data produced over the operational lifetime of this mission inspired many investigations related to a specific science question: the when, where, and how of electron acceleration during solar flares in the stressed magnetic environment of the active Sun. A vital key to unlocking this science problem is the ability to produce high-quality images of hard X-rays produced by bremsstrahlung radiation from electrons accelerated during a solar flare. The only practical way to do this within the technological and budgetary limitations of the RHESSI era was to opt for indirect modalities in which imaging information is encoded as a set of two-dimensional spatial Fourier components. Radio astronomers had employed Fourier imaging for many years. However, differently than for radio astronomy, X-ray images produced by RHESSI had to be constructed from a very limited number of sparsely distributed and very noisy Fourier components. Further, Fourier imaging is hardly intuitive, and extensive validation of the methods was necessary to ensure that they produced images with sufficient accuracy and fidelity for scientific applications. This book summarizes the results of this development of imaging techniques specifically designed for this form of data. It covers a set of published works that span over two decades, during which various imaging methods were introduced, validated, and applied to observations. Also considering that a new Fourier-based telescope, STIX, is now entering its nominal phase on-board the ESA Solar Orbiter, it became more and more apparent to the authors that it would be a good idea to put together a compendium of these imaging methods and their applications. Hence the book you are now reading.
Solar X-ray, extreme ultraviolet, H[alpha] and radio emission were studies to determine what solar radiation is best suited for an automatic flare alarm system aboard a satellite for the detection of the start of a solar flare. Although hard X-rays ([lambda] “1 Å), centimeter-wavelength solar radio bursts, and flashes at certain EUV wavelengths usually have faster rise times and peak earlier than soft X-rays in the 2-16 Å range, the data available to date show that on the average the start time of the 2-16 Å X-rays occurs earlier than the start times for these other types of data. The early start times and large percentage increase of 2-16 Å X-rays make this radiation the best suited for the automatic detection of solar flares for the present state of the art of solar radiation measurements.
High time and intensity resolution satellite measurements of X-ray and extreme ultraviolet (EUV) radiation during solar flares are studied to determine the wavelength dependence of the flare radiation responsible for sudden frequency deviations (SFD). SFD's measure the flare-induced effects in the E and F1 regions of the ionosphere and are in effect like a broadband (1-1030 Å) detector for impulsive flare enhancements. He II 303.8 Å, O V 629.7 Å, H Ly [upsilon] 972.5 Å, C III 977.0 Å, and H Ly [alpha] 1215.7 Å were found to have essentially the same time dependence as the total ionizing radiation producing SFD's, except that they decay faster than the net 1-1030 Å radiation. Flare enhancements of Fe XV 284.1 Å, Fe XVI 335.3 Å, Si XII 499.3 Å. Mg X 625.3 Å, and Ne VIII 770.4 Å, which are normally coronal lines, appear to have a much slower time dependence than the radiation responsible for SFD's. X-rays in the 0.5-3 Å range are slightly slower than the radiation responsible for SFD's during the decay stage; 1-8 Å X-ray flares are slower, especially during the decay stages; and 8-20 Å flare radiation enhancements are slower throughout the entire SFD.
This report describes research performed under the Phase 3 Compton Gamma-Ray Observatory (CGRO) Guest Investigator Program. The objective of this work is to study different mechanisms of solar flare heating by comparing their predictions with simultaneous hard and soft X-ray observations. The datasets used in this work consist of hard X-ray observations from the CGRO Burst and Transient Source Experiment (BATSE) and soft X-ray observations from the Bragg Crystal Spectrometer (BCS) and Soft X-ray telescope (SXT) on the Japanese Yohkoh spacecraft. Zarro, Dominic M. Goddard Space Flight Center NASA-CR-204530, NAS 1.26:204530, R97-257 NASA Order S-57783-F...