In the past several years, X-ray observations of the Sun made from rockets and satellites have demonstrated the existence of high temperature (~20 × 106 – ~100 × 106 K), low density plasmas associated with solar flare phenomena. In the hard X-ray range (λ≲ 1 Å), spectra of the flaring plasma have been obtained using proportional and scintillation counter detectors. It is possible from these data to determine the evolution of the hard X-ray flare spectrum as the burst progresses; and by assuming either a non-thermal or thermal (Maxwellian) electron distribution function, characteristic plasma parameters such as emission measure and temperature (for a thermal interpretation) can be determined. Thermal interpretations of hard X-ray data require temperatures of ~100 × 106 K.

Incontrasi, the soft X-ray flare spectrum (1 Å<λ<30Å) exhibits line emission from hydrogenlike and helium-like ions, e.g. Ne, Mg, Al, Si, … Fe, that indicates electron energies more characteristic of temperatures of ~20 × 106 K. Furthermore, line intensity ratios obtained during the course of an event show that the flare plasma can only be described satisfactorily by assuming a source composed of several different temperature regions; and that the emission measures and temperatures of these regions appear to change as the flare evolves. Temperatures are determined from line ratios of hydrogen-like to helium-like ions for a number of different elements, e.g., S, Si, and Mg, and from the slope of the X-ray continuum which is assumed to be due to free-free and free-bound emission. There is no obvious indication in soft X-ray flare spectra of non-thermal processes, although accurate continuum measurements are difficult with the data obtained to date because of higher order diffraction effects due to the use of crystal spectrometers.

Soft X-ray flare spectra also show satellite lines of the hydrogen-like and helium-like ions, notably the 1s22s2S-1s2s2p2P transition of the lithium-like ion, and support the contention that in low density plasmas these lines are formed by dielectronic recombination to the helium-like ion. Also, series of allowed transitions of hydrogen-like and helium-like ions are strong, e.g., the Lyman series of S up to Lyman-ε, and ratios of the higher member lines to the Lyman-α line can be compared with theoretical calculations of the relative line strengths obtained by assuming various processes of line formation.

This review will discuss the X-ray spectrum of solar flares from ~250 keV to ~0.4 keV, but will be primarily concerned with the soft X-ray spectrum and the interpretation of emission lines and continuum features that lie in this spectral range.

Using the measurements of impulsive solar X-rays made with the OGO-5 satellite to identify the flash phase electron acceleration in solar flares of Hα-importance ≲ 1, the satellite and ground based observations are analyzed to study the origin of the different groups of non-thermal electrons responsible for the impulsive X-ray, impulsive microwave, type III radio and interplanetary electron emission.

While the merging of the higher lines of the Balmer series emitted by solar flares is solely determined by the electron density, the merging of the high Lyman lines is determined both by the electron density, through Stark broadening of the line absorption coefficient, and by the total number of hydrogen atoms in the flare, through the effect of self-absorption. Preliminary results of model calculations indicate that two or more intensity measurements, each midway between two consecutive lines of the Lyman series (lines 3–4, 4–5, 5–6) allow the determination of the column density of hydrogen atoms in the ground state provided that the electron density is known. One can believe that in between the Lyman lines only the flare elements contribute to the measured intensity since excited interflare matter of much lower electron density produces line profiles of substantially smaller width. Thus the data in between the lines can be reasonably compared to the Ne values deduced from high members of the Balmer series.

Soft X-ray enhancements of solar radiation are on many occasions recorded by Solrad 9 which is instrumented with several photometers sensitive to X-ray emission in the range 2–25 keV. A non-thermal interpretation of X-ray bursts for energy larger than 10–20 keV is usually given in the literature, while for the soft X-ray region below 10 keV the common interpretation is a thermal one.

Recent observations of the energetic particles produced in solar flares indicate that the production of electrons, with energies up to about 100 keV, is a fairly common feature of small flares. In those flares the acceleration of protons and other nuclei does not extend beyond about 1 MeV.

The X-ray emission often exhibits two distinct components of which the first one is produced by non-thermal, the second by thermal electrons through bremsstrahlung collisions with the ambient ions. Along with these X rays, radio emission, in the microwave region, is observed. This radio emission is usually interpreted as due to gyrosynchrotron radiation from the same electrons.

In this review a discussion is presented of the processes occurring in solar flares with special reference to the acceleration and radiation processes.

The catalog of X-ray sources now includes types I and II super-novae remnants, at least one pulsar, other periodic or quasi-periodic sources, starlike objects which emit primarily in X rays, normal galaxies, radio galaxies, Seyfert galaxies, a quasar, and an apparently isotropic extragalactic background. There is ample evidence that X-ray emission is characteristic of many of the most interesting objects in astronomy, and the background may have cosmological implications. This should not be too surprising, since significant X-ray emission occurs whenever high energy electrons interact, and high energy electron production is usually associated with explosive phenomena.

The most useful energy range for X-ray observations extends from about 200 eV to perhaps 10 keV. The low energy limit results from the absorption by the interstellar media, which of course varies from object to object; some typical cutoffs are given in Table I. The high energy limit, which is much more arbitrary, results from the usually observed rapidly decreasing emission with increasing energy, and also from the lack of important characteristic emission or absorption features above this energy range. In many cases, however, observations outside of this energy range are required to definitively identify a dominant source mechanism.

In order to resolve clusters of X-ray lines emitted by flares (Fritz et al, 1967; Neupert and Swartz, 1970) and to determine line shapes, a considerable resolving power is needed. The angular size of flares, typically about 2’ will not allow the necessary resolution to be obtained without a very fine slit system combined with the flat crystal X-ray spectrometer. In stellar X-ray spectroscopy large plane diffracting crystals will be associated with large counters, which in turn would act as a source of considerable noise.

Nine years of rocket and balloon observations, considerably enhanced in the last few months by the survey experiment on the Uhuru satellite, have revealed a large number – now approaching 100 – of cosmic x-ray sources, remarkable in both their luminosities (Lx= 1036–1046 ergs s-1) and apparent variety. Spectral data, expected eventually to be of crucial importance in revealing the emission processes and dynamics of individual sources, have been of generally inadequate quality to date (e.g. Figure 1).