文档介绍：Solar aspects of papers
High-temperature Emission from Solar Flares
h J. H. Phillips Email address: ******@
UCL-Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, United Kingdom
Abstract: Some of the most significant advances made with spacecraft instruments observing high-temperature emission from solar flares are reviewed. Solar flares have a basic geometry consisting of a loop or loop system. Model loops include conduction and radiation cooling mechanisms, and correctly predict some of the features, though the nature of bright loop-top sources is not explained: they may be associated with hard X-ray, nonthermal sources above the loop or loop system. Dielectronic satellites of Li-like Fe ions remain the best means of determining the temperature (~ 20 MK) of the hottest part of flares. A ponent (up to ~ 40 MK) is indicated by the presence of H-like Fe ion line emission. Densities have been determined from line ratio techniques to be as high as 1019 m-3 at flare peak, but more generally 1018 m-3, and 1017 m-3 during flare decay. Mass motions are observed at the flare onset; there is more evidence that upflows result from chromospheric evaporation at flare footpoints. Nonthermal electrons have been detected spectroscopically. Line ratios may eventually give a useful means of determining the lower limit of nonthermal electron distributions at the flare impulsive stage. Finally, the importance of the spectral region — nm (—10 keV) is emphasized. Fe XXV lines with 1s2-1snp (n>2) transitions with Fe XXIV satellites offer the means of determining the temperature of the hottest part of the flare and for detecting nonthermal excitation. Micro-calorimeter observation of this region could reveal much physics for large flares and for non-solar sources.
1 Introduction: Solar activity has many manifestations, but none so remarkable as that of solar flares. The first solar flare was observed in white light by Carrington in 18