Time Flares

Over the last decade we entered a new exploration phase of solar flare physics, equipped with powerful spacecraft such as Yohkoh, SoHO, and TRACE that pro vide us detail-rich and high-resolution images of solar flares in soft X-rays, hard X -rays, and extreme-ultraviolet wavelengths. Moreover, the large-area and high sensitivity detectors on the Compton GRO spacecraft recorded an unprecedented number of high-energy photons from solar flares that surpasses all detected high energy sources taken together from the rest of the universe, for which CGRO was mainly designed to explore. However, morphological descriptions of these beau tiful pictures and statistical catalogs of these huge archives of solar data would not convey us much understanding of the underlying physics, if we would not set out to quantify physical parameters from these data and would not subject these measurements to theoretical models. Historically, there has always been an unsatisfactory gap between traditional astronomy that dutifully describes the mor phology of observations, and the newer approach of astrophysics, which starts with physical concepts from first principles and analyzes astronomical data with the goal to confirm or disprove theoretical models. In this review we attempt to bridge this yawning gap and aim to present the recent developments in solar flare high-energy physics from a physical point of view, structuring the observations and analysis results according to physical processes, such as particle acceleration, propagation, energy loss, kinematics, and radiation signatures.

Sudden Ionospheric Disturbances resulting from an interaction of the Solar Flare radiation with the constituents of the upper atmosphere constitute one of the three major aspects of ground level monitoring of solar flares -the other two being optical observations of flares, and the observations of solar bursts in radio wavelengths. SIDs, therefore, form a major part of flare monitoring programme in many observatories. Unlike the other two, however, the ionospheric effects of flares provide one major additional source of interest - the reaction of the ionospheric plasma to an impulsive ionization. The high atmosphere provides a low pressure laboratory without walls in which a host of reactions occur between electrons, ions and neutral particles. The resulting products and their distributions may bear no resemblance to those of the primary neutral constituents or their direct ionization products. The variations with the time of the day, with season and with solar activity that form the bulk of the ionospheric measurements are too slow to allow any insight into the nature of these ionospheric reactions whose lifetimes are often very short. The relaxation time of the ionospheric ionization is only a few minutes or fraction of a minute in the lower ionosphere and in the E-region and is about 30 min to an hour at 300 km. The flares provide a sudden short impulse comparable to these time scales.

High time and intensity resolution satellite measurements of X-ray and extreme ultraviolet (EUV) radiation during solar flares are studied to determine the wavelength dependence of the flare radiation responsible for sudden frequency deviations (SFD). SFD's measure the flare-induced effects in the E and F1 regions of the ionosphere and are in effect like a broadband (1-1030 Å) detector for impulsive flare enhancements. He II 303.8 Å, O V 629.7 Å, H Ly [upsilon] 972.5 Å, C III 977.0 Å, and H Ly [alpha] 1215.7 Å were found to have essentially the same time dependence as the total ionizing radiation producing SFD's, except that they decay faster than the net 1-1030 Å radiation. Flare enhancements of Fe XV 284.1 Å, Fe XVI 335.3 Å, Si XII 499.3 Å. Mg X 625.3 Å, and Ne VIII 770.4 Å, which are normally coronal lines, appear to have a much slower time dependence than the radiation responsible for SFD's. X-rays in the 0.5-3 Å range are slightly slower than the radiation responsible for SFD's during the decay stage; 1-8 Å X-ray flares are slower, especially during the decay stages; and 8-20 Å flare radiation enhancements are slower throughout the entire SFD.

These Proceedings result from the papers and discussions of the U.S.-Japan Seminar "Recent Advances in the Understanding of Solar Flares" held in Tokyo October 5-8, 1982. The meeting was sponsored jointly by the Japan Society for the Promotion of Science and the u.S. National Science Foundation. The principal aim of the meeting was to obtain the most up-to-date physical picture of solar flares by bringing together results from the recent observations by the Solar Maximum Mission (SMM) and HINOTORI satellites, together with other satellite data and ground-based data at both optical and radio wavelengths. These data cover the recent maximum of the solar activity cycle. The SMM and HINOTORI introduced a new dimension in flare observations by carrying out the first hard X-ray imaging observations, and the organizers especially felt that an intense discussion of the significance of these results in the context of flare theories would be important. Starting with an introductory presentation of the characteristics of the instruments on board the satellites, the sessions of the first day and the beginning of the second dealt mainly with energy transport in flares and with .the formation process of the hot plasma which is .