High-resolution X-ray spectra of solar flares

I discuss high-resolution solar flare soft X-ray spectra and also comment on some recent results from extreme ultraviolet (EUV) spectroscopy. Spectra of solar flares at these wavelengths have been recorded since the late 1960s, beginning primarily with the NASA Orbiting Solar Observatory (OSO) series of spacecraft. Knowledge of EUV flare spectra took a quantum leap with the NASA Skylab Apollo Telescope Mount spectrographs in the early 1970s. Knowledge of the X-ray spectrum took a similar leap in the 1980s with the US Department of Defense P78-1 spacecraft, the NASA Solar Maximum Mission spacecraft (SMM), and the Japanese ISAS Hinotori spacecraft. Investigations of flare X-ray spectra continued with the Bragg Crystal Spectrometer (BCS) experiment on the Japanese Yohkoh mission. EUV solar flare spectroscopy has been extended with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer and the Coronal Diagnostics Spectrometer (CDS) on the ESA Solar and Heliospheric Observatory (SOHO) spacecraft. Recently, more Bragg crystal spectra have become available from experiments such as the RESIK spectrometers on the Russian Coronas-F spacecraft. In addition to the above missions, significant earlier contributions were made with instrumentation on a number of other spacecraft, e.g., the Soviet Intercosmos X-ray spectrometers. Our knowledge of the physical conditions in solar flares has been greatly expanded from analyses of X-ray and EUV flare spectra. I discuss the general characteristics of the flare emission line and continuum spectra, and the physical processes that produce them. I summarize what we have learned about solar flares from the spectra, and highlight a few problems and prospects for future solar flare research.