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The Solar Flare Plasma: Observation and Interpretation

Published online by Cambridge University Press:  12 April 2016

G. A. Doschek
G. A. Doschek*
Affiliation:
E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, D.C. 20390, U.S.A.

Abstract

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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.

Type
Section V / The Solar Flare Plasma
Information
International Astronomical Union Colloquium , Volume 14: Special Issue: Ultraviolet and X-Ray Spectroscopy of Astrophysical and Laboratory Plasmas , November 1972 , pp. 765 - 821
Copyright
Copyright © Reidel 1972

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