The flare images show a compact, nearly cylindrical arcade centered along the filament channel and composed of 4 prominent loops. The loop systems of Fe XII (Te = 1.7 106K) and Fe XIV (Te = 2.3 106K) are smaller than, and lie nested within, the arcades of Fe XVI (Te = 3 106K) and Ca XVII (Te = 5 106K). The loop arcades at different temperatures, as well as different loops within a given arcade, show different expansion rates. These vary from 6 – 8 km s–1 for Fe XIV shortly after flare maximum to 0.5 – 1 km s–1 late in the decay phase. For hotter ions, such as Ca XVII, the velocities are approximately constant at 2 – 3 km s–1 daring the whole of the observations.

Using coordinated X-ray and Hα observations of five solar flares, we investigate the dynamics of chromospheric condensations formed during chromospheric evaporation. We show that the peak downflow velocity of condensations predicted by simple hydrodynamic compression of the chromosphere is in good agreement with empirical downflow velocities implied by impulsive phase Hα redwing Doppler shifts. This agreement indicates that the Hα wing redshift provides a useful diagnostic of the pressure excess in the evaporating region and, hence, the energy flux driving chromospheric evaporation.

A comparative study of H-alpha intensity and area development of several flares is presented in an attempt to investigate characteristic features of various types of optical solar flares.

Hα observations with high temporal resolution allow a detailed analysis of the chromospheric response to thre primary energy release. The timing; relationship between Hα and hard X-rays gives information about the energy transport mechanism. Sites of massive injection of nonthermal electrons are identified by characteristic features in the the line profile (Canfield et al. 1984;. Canfield and Gayley 1987).

In this paper are described the simultaneously observational results of filtergrams, Dopplergrams and vector magnetograms for a solar flare in the active region NOAA/USAF -1862 on 7 Oct. 1987 with the solar telescope-magnctograph of Beijing Astronomical Observatory. From the analyses of these data, some conclusions were obtained.

By means of Solar Magnetic Field Telescope, the fine structure of video magnetic fields (5324A) and sight-of-line velocity fields (4861A) of flare active region on July 23 (E53, S19) and Aug. 8(W25, S28) in 1987 has been obtained. The main characters are following:

1. The flares occur in places where there are compressive and osmotic motion between opposite magnetic poles. In some causes, the osmotic opposite magnetic pole dissipates after the flares.

Simultaneous observations of a surge in Hα- and C IV are analysed in terms of Doppler velocities and “microturbulence”. The behaviour of both quantities suggests strong velocity shears with small spatial scales.

Considering the flaring arch to be the basic element of all flares, we study here the structure of compact flares and double ribbon flares. It is observed that, in the case of compact flares, the flaring arch diverges from a pivot point (rigid rotation), while in the case of double ribbon flares the flaring arches are distributed to either side of the polarity reversal line.

We studied the unique EUV spectra of the 21 Jan. 1974 flare observed by the NRL Normal-Incidence Slit Spectrometer (SO 82B) onboard the Skylab. The results show that the pre-impulsive transition region plasmas exhibited enhanced turbulence and heating before the acceleration of energetic particles. The absence of blue-shifted components in the Fe XXI line profiles shows that the chromospheric evaporation is not important in this flare.

We have made extensive observations of the time-variability of solar active regions in the far-UV using the ultraviolet spectrometer on SMM. We describe the three different modes of solar variability (impulsive events, bursts and oscillations) that are evident in our data and discuss their physical implications.

The coronal mass ejection (CME) data and the data for coronal holes for the period 1979-1982 are compared locationwise. Out of 79 CMEs whose locations and spans are known, 48 (61%) CMEs are associated with coronal holes. We make a tentative suggestion that probably the mass ejected during solar flares and active prominences may move along the open magnetic field of the coronal holes and appear as CMEs.

A clear association is demonstrated between the dips in the total solar irradiance and flare occurrence. It is found that both the irradiance dips and flares are related to emerging new activity.

We discuss observations of a solar flare located close to the limb (N15, W75) at 07 00 UT on 26 September 1963 from the Crimean Astrophysical observatory.

Stark broadening and hence electron density have been obtained from measured time profile and half-width of the Baliner series lines from Hα to H M.

The emission mechanism has been deduced from a study of the continuum intensity as a function of wavelength.

Spectroscopic highly resolved solar granulation observations lead to intense line asymmetries for rising and sinking elements. In order to average several granules it is better to use equivalent widths than continuum intensities.

Based on X-ray (1-8 Å) flux data for 1977-1987, the integral spectra of solar flare energy were computed. The energy spectra were approximated by a power law N(E) α; E~n, with N being the number of flares with energy in excess of E. It is shown that the spectral index (β) varies systematically with the 11-year cycle phase.

Magnetic fields in the solar corona are braught into an endless evolution by the never-ceasing motions of the subphotospheric plasma in which the feet of their lines are anchored. It is generally thought that this evolution is essentially quasi-static, the field passing through a sequence of force-free equilibrium states. Sporadically, however, the equilibrium is broken in a region of limited extent, and during a relatively short interval of time a catastrophic highly dynamic evolution takes place, giving rise to such wellknown phenomena as flares or coronal transients. Understanding the factors which determine if a magnetohydrostatic coronal equilibrium is maintained or, on the contrary, destroyed, when boundary conditions change at the photospheric level, then appears as a central theoretical problem of solar physics. In this Communication, we report some recent results which shed some new light onto this old problem.

In this Communication, we consider in the half-space {z > 0} a fully 3-D force-free magnetic field B˷ embedded in a perfectly conducting plasma and its quasi-static evolution driven by motions imposed to the feet of its lines on the boundary {z = 0}. Assuming that the field lines of B˷ have a simple topology - i.e. that it is possible to choose a (non-unique) set of nested magnetic surfaces which are either “arcade-like” or “tube-like” - we first establish a global representation of B˷ in terms of two Euler potentials u and v (with v multivalued) and derive new formulae giving in particular the relative helicity end the energy of B˷ as functions of the values of u and v (and of their derivatives) on {z = 0}. We thus establish analytically some general qualitative features of the behaviour of B˷.

In order to understand how prominences are supported in the solar corona, we have recently developed several 2D models describing x-invariant equilibrium configurations of a plasma occupying a half-space {z > 0} and submitted to a magnetic field then of the form

(1)

We describe a mechanism for coronal heating. The basic idea is that since the photospheric flux is observed to consist of a complex pattern of positive and negative polarity regions, the topology of the coronal magnetic field (in particular the connectivity) must be discontinuous over a complex network of surfaces and magnetic null points in the corona. Consequently, photospheric motions of the field line footpoints, even if arbitrarily smooth, result in discontinuous stressing of the field. This produces coronal current sheets, reconnection at the null points, and rapid heating.

A two dimensional MHD code is used to study the nonlinear evolution of the Parker instability in isolated horizontal magnetic flux imbedded in (or below) the solar photosphere. It is found that the magnetic loop expands self-similarly in the nonlinear stage. Numerical results explain many features observed in emerging flux regions.