Necessary and sufficient criteria of linear ideal magnetohydrodynamic stability are derived for a massive current sheet - idealizing a solar prominence - supported against gravity by a two-dimensional potential magnetic field.

The properties of the interplanetary plasma and magnetic field near 1 AU is determined by the nature of large-scale solar magnetic field and the associated structure of the heliospheric current sheet (HCS). Magnetic multipoles often present near the solar equator affect the solar wind plasma and magnetic field (IMF) near earth's orbit. The observation of four or more IMF sectors per solar rotation and the north-south asymmetry in the HCS are observational manifestations of the influence of solar magnetic multipoles, especially the quadrupole on the interplanetary medium (Schultz, 1973; Girish and Nayar, 1988). The solar wind plasma is known to be organised around the HCS. In this work, we have investigated the possibility of inferring i) the relative dipolar and quadrupolar heliomagnetic contributions to the HCS geometry from the observation of four sector IMF structure near earth and ii) the properties of the north-south asymmetry in HCS geometry about the heliographic equator from IMF and solar wind observations near 1 AU.

We have considered the problem of quasi-static evolution of a magnetic configuration as it is affected by shear motions at field line footpoints. It is found that, with a certain character of the shear motions at the field line footpoints, topological reconstruction will, by necessity, occur in the magnetic configuration. A pattern of magnetic field lines during the course of configuration restructuring is constructed.

The problem of prominences-corona relationship is relativelly old. Already in 1931 Lockyer [1] showed that there is a close relation between prominences distribution and the form of white-light corona. However, this problem is still debatable and poses a number of controversial questions. One of them is the question of the energy and mass exchange between prominences and the ambient corona. It is generally assumed that the mass balance exists between the corona and prominences, but unambiguous observational proofs for prominences-corona plasma exchange are very rare. There are little data [2-4], as well, that could be used to address the problem of slight plasma flows from prominences to the corona.

High–resolution images of post–flare loop systems in Fe XIV (5303Å) and FeX (6374Å) display transient enhancements at the projected intersections of some loops. The brightness of such enhancements gradually increases to a marked maximum and then fades with a typical lifetime of about thirty minutes. The maximum in the red line lags that of the green line by about ten minutes, while Hα reaches a maximum about ten minutes later, which gives a measure of the cooling time, and hence electron density. The observed phenomenon is interpreted as localized loop coalescence and partial magnetic reconnection, with the possibility of an increase in the current density, and heating of the plasma in the immediate vicinity of the X–point.

A rationale is presented for a conception that appearance of flares in active regions is due to the interaction of large-scale convective elements. Such an interaction gives rise to shear motions in the vicinity of the inverse polarity line of the photospheric magnetic field which generate vortical motions leading to non-equilibrium state of the magnetic configuration. Modern concepts of manifestations of turbulent plasma processes are described in terms of theoretical models for solar flares. Plasma effects arising at propagation of electron beams and thermal fluxes in the solar atmosphere are considered. Their role in the interpretation of hard X-ray and type III radio bursts is pointed out. The role of the turbulent Stark effect for diagnostics of collective plasma processes in solar flares is emphasized.

The radiation properties of electrostatic double layers (DLs) are of potential significance for cosmic magnetic explosions. If the emission is sufficiently intense it can serve as a diagnostic tool for particle acceleration by localized strong potential drops in current-carrying plasmas. Moreover such intense emission may form the explanation of some of the observed intense and narrow-band bursts of radiation from stellar and planetary magnetospheres. Here we study the efficiency of two coherent radiation processes: antenna radiation and a maser process. It is found that both processes can operate in the DL to produce intense and narrow-band emission. Antenna radiation occurs if the dimensions of the double layer are smaller than the wavelength of the emitted radiation. This process is therefore relevant to laboratory rather than to astrophysical plasmas. The maser on the other hand requires an amplification length inside the double layer much larger than the emitted wavelength, and can lead to observable emission in astrophysical circumstances. The growth is exponential and the rate depends only on the electric field energy density of the DL. Since the latter is externally controlled by the electric circuit it is a constant for the emission process so as to constitute a true maser. The maximum brightness temperature is of order 1025K. Masing radiation from electrostatic DLs is therefore a candidate for some of the observed intense narrow-band cosmic radio emission.

Proton acceleration by short-wavelength Alfven (A) waves resonant at the first harmonic of the proton gyrofrequency is reconsidered, taking into account nonlinear wave-wave interactions, collisionless wave losses, and wave escape losses in the geometry of a model coronal loop. It is shown that for the A wave levels required for acceleration in the transrelativistic regime in the 1982 June 3 flare and for acceleration in the nonrelativistic regime in the 1980 June 7 flare, the nonlinear wave interaction of scattering on the polarization clouds of ions will be important. This interaction rapidly isotropizes the A waves which divide their energy with fast magnetosonic (M) waves with a negligible change in their frequency spectrum. Because of electron Landau damping and escape losses, the M waves are confined to two narrow cones about the magnetic field and the total (A+M) wave distribution is still highly anisotropic. The total (A+M) wave spectrum has the same acceleration efficiency as a pure A wave spectrum. There are two principal problems with models of this type. The first is that a large wave energy density is required in a fairly narrow range in k-space. The second is that the protons are effectively bottled up. This makes very impulsive behavior as in the 7 June 1980 flare difficult to explain because proton precipitation is relatively slow.

Special type III radio bursts with significantly lower drifts have been attributed to proton beams at the flare site. These low energy beams (some MeV) are nevertheless super-Alfvénic. Recent reports deal with the stability of such beams against resonant scattering by waves below the proton gyrofrequency. Here a different mechanism is proposed, namely nonresonant Alfvén instabilities, which may have higher growth rates and can thus dominate. A selfconsistent multispecies plasma description of such instabilities, developed for solar wind and cometary plasmas, is applied to slowly drifting type III radio bursts. The instability growth rate is fairly insensitive to the beam velocity but decreases with the ratio of beam to ambient plasma densities. Even low beam densities can trigger these nonresonant and hence generic instabilities. Moreover, growth rates are sizeable fractions of the wave frequency, whereas resonant instabilities would give much lower values at low beam densities.

I review here results of works that we have carried out over the past few years in connection with various manifestations of the impulsive phase of solar flares. The primary goal of this work has been the determination of the characteristics of the accelerated electrons. We take into account various possible interactions that electrons suffer as they traverse the flare plasma and evaluate the expected radiation via several mechanisms. The comparison of these with the observed x-ray, gamma-ray and radio emissions allows us to set constraints on the characteristics of the accelerated electrons and on some properties of the flare plasma. I will describe some of the results we have derived from comparisons with SMM and other observations of flares during the past solar cycle. In particular, I will show that we can set limits on the range and distribution of the accelerated electron spectrum and pitch angle distribution, and on the column depth and field convergence rate of the flaring loop. More complete data and high spatial resolution and polarization observations could be very useful for furthering this kind of work.

The electron density and temperature measurement of solar plasmas from EUV and X-ray emission lines have been briefly discussed. Diagnostic EUV line ratios from MgVI ion and X-ray line ratios from MgVIII and SiX ions have been presented. A brief account of the general methodology for diagnostics of inhomogeneous structures has also been given.

A statistical analysis of solar flare X-rays and interplanetary particle fluxes, measured onboard VENERA-13, 14 Spacecraft, was performed. The correlation of fluences for different manifestations of solar flares is strong, especially for fast electrons and hard and soft X-ray emissions. Frequency dependence on fluence value ϵi for practically all Kinds of solar flare emission can be described by power law ν (ϵ > ϵO) ∼ ϵ−0.45±0.15 which does not change significantly with solar activity. For different Hα flare importances the values of ϵi were obtained. It is proposed that appearance of certain energy flare frequency is strongly dependent on some scale factor.

We discuss the possibility that the plasma trapped inside the solar magnetic loop region may be the principal arbitrator in deciding whether a given active region will release particles into the interplanetary medium or will remain a solely electromagnetic event. We analyze the particle abundance patterns and show that the particle selection occurs in a cool region much below the corona. We discuss the particle acceleration in time varying magnetic field and show that the betatron process can be very effective under solar conditions. We then show that the electron density in the flare region may be the critical parameter in deciding the evolution of the active region. Finally we show that direct observations of flares in the interplanetary medium agrees well with this suggestion.

It has been shown that high energy particle spectra, particle dynamics, and radiation in a flare loop are determined by wave-particle interactions. The electron-whistler interaction occurs under conditions of strong pitch angle diffusion that makes the particle distribution function isotropic. The flare loop electrons retain information about the particle source spectrum. The interaction of energetic ions with Alfven waves is characterized by strong, moderate, and weak diffusion. The time delays in hard X-ray and gamma-ray emission during one-step acceleration processes might be understood in terms of a trap-plus-turbulent propagation model. The density of precipitating particles is less than or equal to the trapping one. Radiation signatures of flare loop electrons are discussed.

The strong interaction of cometary plasma with the interplanetary medium results the disconnection events and other features observed in plasma tail and the coma of comets. The understanding of these features provide direct probe of the heliospheric magnetic field. However the interpretation of these events is not without ambiguity, as revealed in the recent work with comet Halley. In this paper a review of the “Post-Halley era” developments of the cometary plasma physics is presented.

An important stage in the evolution of solar flares is the generation of a number of dynamic phenomena, greatly varying in size and velocity, but actually representing plasma ejections. They are responsible for the transport of mass and energy and are usually considered to be result of the development of local magnetic features accompanied by a magnetic energy increase in the solar atmosphere. One of these phenomena is the surge prominence. In this contribution, we propose a possible explanation for the nature of the braking force acting in the decelerated ascending phase of surge dynamics. The possibility of structuralization of the chromospheric plasma during surge formation and the plasma action on the external, colimating magnetic field is taken into account.

A study of the simultaneous observations of solar flares in optical, radio and X-ray bands is important in understanding the process of energy release and particle acceleration in the explosive phenomena of solar flares. In order to determine the characteristics of such energetic electrons in solar flares, a total number of 50 two-ribbon flares have been carefully selected during the period 1979-89 having simultaneous observational data in the optical, X-ray and radio bands.

Beam-Foil spectroscopy(BFS) has proved to be a valuable technique for the determination of radiative lifetimes of excited atomic levels leading to the evaluation of the transition probabilities. The time- resolved nature of the decay process in a collisionless environment is a unique characterstic of the beam-foil light source. The relevance of BFS to astrophysics comes from the importance of radiative transition probabilities in the quantitative analysis of optical spectra. Stellar abundances are obtained from the intensity of a spectral line which essentially is a product of the abundance of the element in the source and the probability of the transition. Thus the evaluation of accurate values of transition probabilities contribute significantly to stellar abundance analysis.

Emission spectroscopic methods are very useful in determining the plasma parameters such as electron density, electron temperature, chemical abundances and energy levels of atoms and ions. A knowledge of the above mentioned parameters and collision cross sections provides an insight into various plasma processes on the Sun. As one passes from photosphere to chromosphere and corona the temperature as well as the electron density changes drastically (Te ~4500 − 2×1060K; n e ~ 108 − 1013 cm−3) (1). Hence the solar spectrum.excited by different mechanisms and different equilibrium conditions, extends from vacuum ultraviolet to visible and infrared regions. For example the spectrum in the region between 3000−1300Å is produced by the upper photosphere and lower chromosphere. In this region the temperature is in the range of 6000-10000 K. This region is characterised by several emission and absorption Lines superimposed over continuum. Below 1600Å consists of emissions from highly ionised atomic species originating from chromosphere and corona (2).A correct interpretation of the spectral features is possible only after understanding the influence of various factors on spectral line shapes and intensities. They are 1) damping by collisions with neutral atoms; 2) collisions by charged particles leading to linear and quadratic Stark effects on atomic lines of hydrogen and helium; 3)thermal Doppler broadening 4) Doppler broadening or shift due to microturbulent velocity field.

The variation of theoretical line intensity ratios with electron density for Mg VI ion are presented. This study indicates that the line intensity ratios for Mg VI can be used to infer electron density for solar plasma.