Electron streams with type III burst characteristics are numerically modeled. The electronplasma wave quasilinear interaction is assumed to be the dominant velocity diffusion process. The quasilinear equations with the addition of spontaneous emission, magnetic and collisional effects are numerically solved as an initial value and a half-space boundary value problem with time, distance and velocity as the independent variables for a solar-type background plasma and a type-IlI-like stream. Background density and temperature coordinate structure, spontaneous emission, magnetic fields, electron-ion collisions, stream reabsorption and wave pileup are shown to affect propagation and are incorporated into a physical description of the stream motion. The calculated electron flux-time profiles at the Earth suggest scatter-free propagation and compare well with type III stream observations.
We examine observational evidence concerning energy release in solar flares. We propose that different processes may be operative on four different time scales: (a) on the sub-second time scale of 'sub-bursts' which are a prominent feature of mm-wave microwave records; (b) on the few-seconds time scale of 'elementary bursts' which are a prominent feature of hard X-ray records; (c) on the few-minutes time scale of the impulsive phase; and (d) on the tens-of-minutes or longer time scale of the gradual phase.We propose that the concentration of magnetic field into 'magnetic knots' at the photosphere has important consequences for the coronal magnetic-field structure such that the magnetic field in this region may be viewed as an array of 'elementary flux tubes'. The release of the free energy of one such tube may produce an elementary burst. The development of magnetic islands during this process may be responsible for the sub-bursts. The impulsive phase may be simply the composite effect of many elementary bursts.We propose that the gradual phase of energy release, with which flares typically begin and with which many flares end, involves a steady process of reconnection, whereas the impulsive phase involves a more rapid stochastic process of reconnection which is a consequence of mode interaction.In the case of two-ribbon flares, the late part of the gradual phase may be attributed to reconnection of a large current sheet which is being produced as a result of filament eruption. A similar process may be operative in smaller flares.
Hard X-ray emission is generally interpreted as bremsstrahlung of 10-100 keV electron beams. This requires a high number of electrons to explain the intense X-ray flux observed. An interesting suggestion was made recently that 100 keV proton beams bombarding the atmosphere would create a hot thermal plasma at the origin of the observed X-ray emission. Such low-energy protons have never been detected. We propose to use impact linear polarization of chromospheric lines as a diagnostic of 100 keV protons. Recent observations of Ha linear polarization in solar flares are presented. A polarization degree as high as 2.5% is observed in some Ha-bright patches. The electric vector is radial. This polarization is interpreted as impact polarization by low-energy protons. From existing observations we cannot discriminate between local and nonlocal origins of the detected population of energetic particles. However, we show that nonlocal proton acceleration leading to chromospheric bombardment is a likely explanation of the observed polarization. Whatever the initial distribution, transport effects in the chromosphere generate an anisotropic velocity distribution for protons.
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