A two-dimensional sheet model for solar filaments (Kippenhahn and Schluter configuration) is considered. We investigate, the quasi-static evolution of gravito-magnetohydrostatic equlibria in exploring the response of massive current sheets to a slow continuous variation of the mass/flux ratio with fixed boundary conditions. A catastrophic behavior of the field topology is found to occur in the sequence following the formation of a cusp point (bifurcation).

We determine the electron distribution function within a hot coronal loop using a hybrid numerical scheme which couples the Spitzer-Härm method at low velocities with the solution to the high velocity form of the Landau-Fokker-Planck equation. From this we calculate the heat flux throughout the loop and compare it with the classical fourier law of Spitzer and Härm(1953).

We discuss a mechanism whereby a current loop embedded in plasmas such as the solar and stellar coronae can dissipate magnetic energy without resistive effects or reconnection. This mechanism arises from the motion of magnetic/current structures driven by “toroidal forces”. Using a simple model loop, we show that it can exhibit a wide range of motion with correspondingly wide range of magnetic energy dissipation rates. For example, a loop with ~20G can attain expansion velocities of ~1200km s–1 under solar coronal conditions, dissipating ~1032erg in a few tens of minutes.

The arguments in favour of that the primary heating of the gas at the impulsive phase of stellar flares is caused by charged particles of higher energies than in the solar flares are given. It is shown that the model of the deep heating by high energy protons (E ≃ 10 MeV) or electrons (E ≃ 100 keV) with taken into account of the radiative transfer in flare region explane the main properties of the optical continuum of the flare.

Two kinds of MHS equilibrium are studied; isothermal and nonisothermal. The onsets of two - ribbon flares and flare - loops are explained by this study.

A detailed analysis of the kink instability in finite length (inertially line-tied), cylindrically symmetric coronal loops is presented. The correct line-tying boundary conditions within the framework of ideal and resistive magnctohydrodynamics are discussed, and the growth rates of unstable modes and corresponding eigenfunctions are calculated. Resistive kink modes are found to be unstable in configurations where the axial magnetic field undergoes an inversion, resistive effects being confined to a small region around the loop vertex.

We investigate numerically the effect of a finite density gradient on the Langmir wavelevel produced by an electron beam in the flaring solar corona. The energy losses and associated plasma heating resulting from the presence of a beam-neutralizing reverse current are taken into account. The density and temperature structure of the background plasma are determined self-consistently, assuming steady state energy balance and hydrostatic equilibrium. The implications of our results for flare acceleration mechanisms are discussed.

We consider the problem of ion-acoustic wave generation, and resultant anomalous Joule heating, by a return current driven unstable by a small-area thick-target electron beam in solar flares. We find that, contrary to the usual assumption, the hard X-ray bremsstrahlung emission may actually be enhanced in comparison to conventional thick-target models. This present paper is a summary of the work of Cromwell, McQuillan and Brown (1988).

A model for the acceleration of electrons in a flaring coronal loop is described. The mechanism is stochastic acceleration by resonant interactions with a spectrum of conipressive magnetosonic waves. Current results of test particle calculations examining the feasibility of this model are presented.

The proposed model of particles transport in the solar atmosphere during flares consists in a low density plasmoid originating deep in the atmosphere and rising under magnetic and buoyancy forces. Confined particles are selectively released during the ascent and their interaction with the solar atmosphere produces X and γ bremsstrahlung. The characteristics of high energy particles released in the interplanetary medium are found to agree with observations.

Recent observations suggest that the energy release in solar flares may occur in many small bursts. If these bursts give rise to plasma heating, a large number of collisionless shocks will be generated. These shocks can individually heat plasma and accelerate particles, but the interaction of particles with many shocks as well, as of shocks with each other can give rise to further heating and acceleration.

The emission mechanism studied here, the synchrotron maser instability is based on a gyroresonance of highly energetic electrons. Compared to the cyclotron maser, this mechanism has the great advantage to be a direct amplification process even if the ratio ωp/ωc is not very small; a situation relevant to stellar flares.The high efficiency of the process can be confirm by the non linear study of its saturation.

The propagation and absorption of the maser radiation emitted during solar flares are examined through linear theory and electromagnetic particle simulations. While linear theory predicts complete absorption of the maser radiation at the second harmonic layer (where the local cyclotron frequency is roughly one-half the frequency of the maser radiation), particle simulations indicate that some radiation escapes clue to reemission of absorbed energy by the ambient plasma. Strong perpendicular heating of the ambient electrons occurs as well.

A simple impulsive event observed at microwave frequencies between 3.2 and 50 GHz is simulated with a homogeneous source model. Intensity and polarization of the 1984 April 26 event are shown in fig. 1a and 1b, respectively. The spectral maximum is close to the observing frequency of 11.8 GHz.

The time profiles of the intensity I and the circular polarization ρc at this frequency as well as the spectrum at different characteristic phases (rise, maximum and decay) are calculated and compared with the observation in order to deduce parameters of the microwave source.

The investigations of plasma motions at the initial phases of solar flares (Antonucci and Dennis, 1983; Doschek, 1983; Watanabe, 1987) suggest evaporation from the chromospheric flaring area. According to de Jager (1983) when seen at the limb the evaporated plasma will look like a “convective plume” and it can be seen separated from heated footpoint areas.

The subject of this work is the study of the possibility of forming hydrodynamic structures o-f thermal and starting plume's kind at the time of evaporation of the upper chromosphere in a flaring area. Also the possibility of increasing an initial magnetic field by a periodically moving vortex in a plume structure is investigated.

Synthetic spectra computed svith semiempirical and theoretical models of the flare atmosphere are discussed. Stark profile of the Dahner lines ( n ≥ 6) and the Balmer continuum have been computed according to Donati-Falchi et al., (1985). A semiempirical model in between the F2 and F3 models proposed by Avrett et al., (1986) might represent a good approximation of the atmosphere of a flare during the impusive phase. Among the theoretical models proposed by Ricchiazzi (1982), the model, indicated as 1073, with very high coronal pressure (Po = 1000 dyne cm–2) probably represents the best approximation of the emission observed during the impulsive phase of a chromospheric flare. Continuum and Balmer lines ought to be measured and interpreted on the same time for a self-consistent diagnostic of the chromospheric modification.

Periodic Doppler motions inside small fluxtubes are determined from the zero-crossing point of the circular Zeeman polarization profile (Stokes-V=0). The gas fully participates on the known solar 5min oscillations without affecting amplitude or frequency. Averaging the oscillations, the gas is at rest in an absolute solar scale. Discrepancies with formerly observed redshifts are explained by the systematic westward displacement of the magnetic structure relative to the Ca+K bright point used for guiding.

We discuss an ordinary differential equation which describes how the pressure in a coronal loop may evolve in time under the influence of a uniform, but time varying heating rate.

It has been proposed that non-thermal ions dominate the energy transfer at the onset of solar flares. Here we examine this hypothesis in the context of flares on dMe stars. If the magnetic field in the stellar corona is significantly larger than that in the solar corona, and if strong fields in the photosphere, analagous to active regions, are absent, then a self-consistent explanation of stellar flares may be formulated.

The formation of the Lyman and Balmer lines of Hydrogen at densities typical in dMe stars is shown to be very dependent on the atomic model used viz. the number of levels and line transitions. The effect of increasing the number of levels and line transitions on the line fluxes and ionization fraction is also discussed.