S. I. Syrovatskii

Model for flare loops, fast motions, and opening of magnetic field in the corona

A model is presented for the penetration into the corona of a new magnetic field of a developing bipolar region and for its interaction with an old large-scale coronal field. An important feature of the model is a reconnection of the old and new fields inside the current sheet arising along the zero line of the total magnetic field calculated in the potential approximation. The magnetic reconnection and accumulation of plasma inside the current sheet can explain the appearance of dense coronal loops and the energy source at their tops. The plasma together with the magnetic lines is flowed into the sheet from both its sides. This fact explains the appearance of coronal cavities above the loops. If the large-scale field gradually decreases with the height, the loop motion is slowed down. The account of the dipolar structure of the magnetic field at large heights explains the possibility of a rapid break of the new field through the corona and the appearance of transients and open field regions - the coronal holes. In this case a fast rising current sheet can be a source of accelerated particles and of type II radio burst, instead of the shock wave considered usually.

On the time evolution of force-free fields

Using the adiabatic approximation, it is shown that the problem of the continuous deformation of a force-free (f.f.) field, in general, has no solution. This means that f.f. fields are nonevolutionary and even small perturbations may produce drastic changes in them. By analogy with a special case of f.f. field, the current-free field, we conclude that perturbations of a f.f. field in general produce pinch (current) sheets.

Distribution of temperature and emission measure in a steadily heated solar atmosphere

The distribution of temperature and emission measure in the stationary heated solar atmosphere was found for the limiting cases of slow and fast heating, when either the gas pressure or the concentration are constant through the layer depth. Results are relevant to the conditions when the energy injected by waves or by non-thermal particles or in some different way quickly transforms into a thermal flux. Under these conditions the temperature distribution with depth is determined by radiation loss and thermal conductivity, and at any values of energy flux and plasma concentration it is characterized by two universal functions. One of them gives the relation between the energy flux and temperature at the region boundary: the other - the temperature run with the depth. This run is such that a considerable part of the energy is radiated by a thin transition region with a very large temperature gradient.The results may be applied for calculation of the temperature and the emission measure both for the high temperature region of a flare, and for the quiet corona. The dimensionless structure of the transition region is the same for any value of the energy flux. These results concerning solar flares can help to explain the identity of optical spectra for flares of different types, the emission in a wide temperature interval from nearly the same region of space and the very small thickness of the region emitting optical lines. The latter is due to the ‘shell’ structure of the flare as opposed to the usually assumed filamentary one.

Thermal trigger for solar flares and coronal loops formation

A longitudinal stability is considered for the quasi-steady current sheet which is uniform along the current. In the MHD approximation, the stability problem is solved for the plane neutral sheet and small disturbances propagating along the current. The current sheet is shown to break-up into the system of cooler and more dense filaments due to radiative cooling. The filaments are parallel to magnetic field lines. This process corresponds to the condensation mode of a thermal instability and can play a trigger role for a solar flare. Moreover, at the nonlinear stage of development, it can lead to the formation of very dense cold filaments surrounded by high-temperature low-density plasma inside the current sheet. Flowing into the filaments, hot plasma is cooled by radiation and compressed. Then the cold dense plasma flows out from the current sheet along the filaments. We think that the process under consideration is responsible for the often observed picture of an arcade of cold loops in the solar corona.

Current sheets as the source of heating for solar active regions

Observational data and theoretical arguments suggest that the heating source for an active region is the quasi-steady dissipation of magnetic field in current sheets. Effects in the solar atmosphere which are due to the presence of current sheets are considered. The most important of them is the heating of the chromosphere by the strong ultraviolet radiation of the current sheet. This can give rise to the brightening of an active region in optical emission. The energy flux from the current sheet in different ranges of the ultraviolet spectrum and the depths (column densities) into the chromosphere where this energy is absorbed are estimated.

On the low-temperature region of chromospheric flares

Most of the energy of particles accelerated in a flare is used for the creation of a high-temperature flare region, the structure of which is determined by the heat conduction (Shmeleva and Syrovatskii, 1973). However, as the temperature drops with the depth in the chromosphere, the heat flux decreases quickly and in the low-temperature part of a flare may appear to be lower than the direct energy flux carried to these depths by the most energetic of the accelerated particles. In the latter case the radiation from the low-temperature region will be determined by the direct input of energy by energetic particles (Hudson, 1972; Brown, 1973a). Here we consider conditions under which one of the above-mentioned types of heating dominates. Correspondingly we may consider two types of flares: penetrating flares, when heating is produced by non-thermal particles, and thermal ones, when the heat conduction dominates. The conditions of occurrence of one of these types depend mainly on the particle energy spectrum: the heat conduction dominates for the soft spectra and for the high enough temperature of the hot (coronal) part of a flare, as is usual for X-ray flares.It is essential that for both the heating mechanisms the radiation from the low-temperature regions gives as a rule only a small part of the total flare radiation.In the case of conductive heating the temperature run in the cold part of a flare depends essentially on the mode of hydrogen ionization in this region. It is shown that the optical depth effects in hydrogen ionization can be neglected for flares in the upper chromosphere with unperturbed temperature T ∼- 10000 K. The absorption of radiation begins to play a role for lower boundary temperatures T = 6000–8000 K.

Possible mechanism of surge formation in the solar atmosphere

The possibility of surge formation as a result of plasma ‘raking-up’, the latter being associated with the growth of the local magnetic field in the solar atmosphere, is considered in this paper. The question is treated numerically in the MHD approximation for the case of the dipolar magnetic field. It is shown that the field growth results in the appearance of relatively dense condensations stretched along the axis of a dipole. Simultaneously the plasma acquires the upward velocity along force lines which leads to the formation of a surge. Some properties of this surge model are discussed.

On the possibility of observations of current sheets in radio band

We discuss the possibility of a discovery of current sheets by using their screening and emissive properties in the radio band. It is shown that the presence of a current sheet in the solar atmosphere leads to a depression of the radio brightness at some wavelengths and to its enhancement at shorter wavelengths. Spectral observations with sufficient angular resolution may give such characteristics of the sheet as its temperature, electron density, thickness and height in the solar atmosphere.

Gasdynamics of Impulsive Heated Solar Plasma

Numerical solutions for problem of hydrodynamic response of the inhomogeneous (exponential) atmosphere on impulsive heating by energetic electrons or by very high-temperature thermal fluxes are discussed.