Solar X‐Ray Physics

Abstract

This is a review of the research in solar x‐ray phenomena that has been done by American scientists in the last four years. This research ranges from new instrumentation, such as multilayer mirrors and high spectral resolution detectors, to refined theories and observations of the flare process. Contributions have been made in our understanding of x‐ray bright points, solar active regions, solar flares, and flare‐associated phenomena. The hard x‐ray spectra during the impulsive phase of solar flares have been examined in greater detail and found, among other things, to fit a double power law. Models for the hard x‐ray emissions have focused largely on electron acceleration by field‐aligned currents, although other models such as proton beams, waves, and neutral beams have also been investigated. Correlations between the hard x‐ray emission and radio, ultraviolet, and gamma‐ray emissions have been used to provide additional constraints on the flare models. The process known as chromospheric evaporation, which is seen as blueshifts of the soft x‐ray line profiles, has been explored through further analysis of the observations and the development of new models. New atomic calculations have been performed so that further information about the plasma properties during the decay phase of solar flares can be derived from the soft x‐ray line emissions. Spatially resolved observations have been used to identify new x‐ray features such as flaring arches and giant post‐flare arches. Other topics of investigation included the energy in solar flares and the long‐term variations in solar indices, the 155‐day periodicity of solar flares, and the distribution of flares on the solar disk. Finally, associations have been made between the solar x‐ray emission and the phenomena of coronal mass ejections and solar energetic particles, as well as the effect of this radiation on the earth's upper atmosphere.