In this Communication, we consider a simple model of magnetosphere around a fast rotating Jupiter–like object possessing a spin–aligned dipolar moment μ. In this model, low–energy plasma released by inner sources located beyond the corotation radius diffuses outward through closed lines, forming a thin equatorial disk in which there is a quasistatic balance between the centrifugal force and the magnetic tension. At some critical radius r0, however, the magnetic field is no longer strong enough to hold the matter, and blows open, the matter escaping freely (Fig. 1) (this model has been introduced by Hill and Carbary, 1978).

We show that a sheared 2–D force–free field can evolve in a quasi–static way towards an open configuration, and apply this result to a qualitative theory of two–ribbon solar flares.

Multi–frequency Observations of Solar Microwave bursts recorded during solar maximum period 1980–81 are analysed and compared with x–ray data for studying the nature of microwave emissions from active regions. Most of the microwave burst spectra showed that the spectral index below the peak frequency is always less than 2.

The magneto-ionic conditions of the burst sources and the electron energies as obtained from these multi-frequency observations of the bursts showed that the centimetric and x-ray observations are satisfactorily explained, if the emitting regions are dense, hot and compact associated with strong magnetic fields of a few hundred gauss, suggesting that the thermal gyroresonance process is the most likely emission mechanism involved in the emission of microwave and x-ray radiations from the active regions of sun.

Until recently magnetic reconnection in solar flares was discussed simplistically in terms of either a spontaneous tearing mode instability or a driven Petschek mode. Now the subtle relationship between these two extremes is much better understood. Current sheets may form and reconnection may be initiated in many different ways. There are also a variety of nonlinear pathways from a reconnection instability and several types of driven reconnection.

In solar flares current sheets may be important as new flux emerges from below the photosphere and also as a magnetic arcade closes down after being blown open by an eruptive instability. Numerical simulations of these sheets will be described, including new features such as the presence of a fast shock in Petschek's mechanism and impulsive bursty reconnection due to secondary tearing and coalescence.

Flare stars derive their name from intermittent increases in luminosity which have certain characteristics reminiscent of solar flares (e.g. enhanced strengths of emission lines in the stellar spectrum during the outbursts). When a flare star is observed in a filter which transmits, say, the violet part of the visible spectrum, the increase in luminosity during a flare may range from noise level up to perhaps 100 times the quiescent brightness. During a flare, certain spectral features of the quiescent star (e.g. molecular bands) remain visible, indicating that the flare occupies only a fraction of the visible disk. Thus, analagous to a solar flare, a stellar flare is confined to a single active region. However the total power is large enough to affect the integrated light from the stellar disk. In contrast, the largest solar flare (Etot ≈ 1032 ergs) has a rate of energy release (L ≈ 1029 erg/sec) which is so small that a distant observer would record such a flare as a luminosity increase of less than 10−4Lsun. However, even apart from the flares themselves, it has become apparent in recent years that flare stars in their “quiescent state” provide some extreme contrasts with the sun.

Filaments and flares occur in sheared magnetic structures as a result of radiative cooling and resistive reconnection, respectively. A new integrated theory of these two unstable processes is described, which includes the relevant effects of magnetohydrodynamics and energy transport. The normally dissociated thermal and tearing phenomena are coupled together by a temperature–dependent Coulomb resistivity. As a result, the filamentation and flaring instabilities of a sheared field may coexist, as is familiar from the solar example.

The growth rates and spatial structures of these two modes are detailed here. The much faster radiative instability is shown to provide significant magnetic reconnection, particularly at shorter wavelengths. The long–wavelength reconnection mode is found to be abetted by the resistivity increase caused by the dominance of cooling at the X point, in contrast to its nonradiative behavior. Implications of these results for the development of coronal activity are described.

Numerical solutions of the MHD equations are used to investigate the nonlinear behavior of the tearing instability. The mode evolves from a linearly growing excitation, followed by a period of greatly reduced nonlinear growth. Constant-Ψ solutions evolve much more slowly than comparable nonconstant-Ψ modes with orders of magnitude less conversion of the stored magnetic energy. The nonconstant-Ψ computations indicate a reduction by approximately 20% of the energy in the initial shear layer. For long–wavelength solutions, secondary–flow vortices, opposite in direction to the linear vortices, generate a new magnetic island centered at the initial x-point.

Resistive instabilities have often been indicated as the possible cause of rapid release of energy in astrophysical situations. A correct assessment of the validity of this idea requires a detailed analysis of the theory of resistive instabilities in the regimes of astrophysical interest. In particular, effects as the presence of asymmetries due to current gradients, the influence of geometry and of shear flows must be explicitly evaluated. We have started a program of investigation on this subject, whose preliminary results are reported here.

Magnetodynamical processes in RS CVn binaries are discussed in the scheme of Active-Longitude-Belt picture (Uchida and Sakurai, 1983) in which the “photometric wave” is due to a number of spot pairs which emerge, drift across, and submerge in the “active longitude belt” on the K-star. The formation of the corona and the origin of flares in these close binary systems having starspots are interpreted in terms of the reconnections of the magnetic flux tubes of the companion star with the emerging and submerging pairs of spots on the K star. The injection of the hot plasma into the large scale pole-to-spot connections is required to explain the extended corona with large emission measure, and we attribute this to the “sweeping-pinch” mechanism (Uchida and Shibata, 1984) associated with the relaxation of the toroidal component in the twisted magnetic flux tubes which emerge and reconnect with the flux tubes connecting pole and spots.

A magnetodynamic mechanism of jet formation, in which a packet of the toroidal component of the magnetic field Bϕ plays a role, is proposed. Such a packet of toroidal field, produced by the rotational motion in the β=Pg/Pm≫1 region, relaxes itself in the β≪1 region when brought up into such a region, for example, by the process of flux emergence due to magnetic buoyancy. In the β≪1 region, a progressive pinch is caused by this relaxation and the mass is swept out by the pinch near the axis and also by the j x B force in the twisted field region surrounding the axis.

In early research on solar flares, attention was focused on the impulsíve or flash phase, and it was assumed implicitly that virtually all the energy of a flare is released during that short phase. In recent years, however, it has been realized that the long-lived soft X-ray emission which follows the impulsive phase may require a separate energy-release process, which has been termed the “gradual phase” (Kane 1974). The fact that the impulsive phase is often preceded by soft X-ray emission has also led to the suggestion that there may be a third phase of energy release, which might be termed the “onset phase” (Sturrock 1980). It has long been realized that filament eruptions frequently precede flares, and it has been suggested (Kiepenheuer 1964) that the two should be regarded as parts of the same process. For these and other reasons, it is appropriate to question how many phases of energy release are involved in flares and what are their characteristics.

Recent observations of the fast time variability in the hard X-ray emission from solar flares are interpreted. The fast spikes are assumed to be superimposed on the thermal X-ray emission. The rise and fall of a spike are caused by disruptions in the plasma. The rise time represents the impulsive heating time and the decay or fall time represents a quick cooling of the plasma due to the accelerating growth rate of the m=1 tearing mode. The estimated characteristic time durations of the spike are found to be in good agreement with the observed ones.

Observations of the neutral sheet in the Earth's magnetotail are presented to show different magnetic signatures of the neutral sheet which have been used to infer (1) wave profiles on the neutral sheet surface, (2) magnetic islands embedded in the neutral sheet, and (3) localized turbulent magnetic field regions. The occurrence of these features even at magnetospheric quiet conditions suggests that the above features are intrinsic to the current sheet and may possibly play a role in its stability. There are indications that these features are common to other current sheets in space.

It is well known that particles streaming along the ambient magnetic field in space plasmas may generate waves with frequencies of the order of the local ion gyrofrequency (ion waves), (Kindel and Kennel 1971). In this study we analyze the dispersion relation of these waves numerically and discuss mechanisms for damping and instability. All numerical results in this report are obtained with the computer code WHAMP (Rönnmark 1982a,b), which solves the dispersion relation of linear waves in a homogeneous plasma for a complex frequency as a function of a real wavevector. As a specific example we consider the S3-3 satellite observations of banded electrostatic ion waves, associated with streaming particles (Kintner et. al. 1979, Cattell 1981).

A review of several microinstabilities that have been suggested as possible anomalous transport mechanisms in current sheets is presented. The specific application is to a ‘field reversed plasma’ which is relevant to the so-called ‘diffusion region’ of a reconnection process. The linear and nonlinear properties of the modes are discussed, and each mode is assessed as to its importance in reconnection processes based upon these properties. It is concluded that the two most relevant instabilities are the ion acoustic instability and the lower-hybrid-drift instability. However, each instability has limitations as far as reconnection is concerned, and more research is needed in this area.

The wealth of detailed observations on transport in turbulent plasmas that has become available over the last decade from laboratory experiments, in situ space craft observations and computer simulation demonstrates that in contrast to classical transport, the various steps in the analysis of anomalous transport, involving microscopic processes and the global dynamics, are closely coupled. It also points to many new, exciting, and truly anomalous phenomena. These statements apply especially to the highly dynamic processes connected with field aligned currents. The usually vast difference in time or length scales must allow for considerable simplications in the analysis, depending on the system at hand, but requires careful consideration of the microscopic or macroscopic physics that of necessity is to be treated in a simplified manner. This point is demonstrated by using a marginal stability approach in a numerical model for field aligned currents and electromagnetic coupling in an extended system. Its aim is to see how energy is supplied to a localized dissipation region from a distant generator and how this dissipation in turn affects the global electro-dynamic structure. In addition to the earth's auroral flux tubes, for which this model was primarily designed, these questions are of importance to other extended current systems in astrophysics. Microscopic processes supporting enhanced dissipation and leading to other truly anomalous processes such as acceleration of selected particle groups are then discussed in relation to the global problem.

Cosmic rays do not stream freely through the galaxy, contrary to earlier expectations. Streaming cosmic rays are slowed down by the emission of resonant Alfven waves that scatter the cosmic rays. The theory of self-confinement explains the isotropy of the bulk of the cosmic rays but not of cosmic rays above 103 Gev; it has been a stimulus to the theory for cosmic-ray acceleration at supernova shocks; and, on inclusion of diffusion in a galactic wind, it may explain the uniform cosmic-ray density out to 18 kpc in our galaxy. Rapidly streaming electrons in clusters of galaxies, in supernova remnants, and near solar flares are accomodated by the theory when it is expanded to include the effects of hot plasmas and other wave modes. A “resonance gap” may prevent the turning backwards of streaming particles and thus allow streaming near the particle speed.

Nonlinear effects, such as soliton collapse, will result in evolution of hydromagnetic waves excited by a field-aligned charged particle beam. If the time scale for such evolution is comparable to, or shorter than, linear time scales such as those for wave growth or pitch angle isotropization, then nonlinear effects may limit the instability. For conditions appropriate to relativistic electron streaming in a radio galaxy, the nonlinear time scale may be comparable to the linear time scales.

Parker (1966, 1969, 1979) has shown that the magnetic buoyancy of a uniform horizontal magnetic field will destabilize the Galactic gas layer. Perturbations of the form shown in Fig. 1 will grow in time with the magnetic loops ballooning up into the Galactic halo, and the interstellar gas draining down the field lines to collect in the mid-plane. Parker also showed that if the dynamical effect of the cosmic ray component of the interstellar medium is included, using an isotropic cosmic ray pressure, then the instability is enhanced.

Among MHD waves, Alfvén waves have been proved to be the best candidates to reach the solar corona and, eventually, to be responsible for the heating of this outer part of the solar atmosphere. The problem arises, however, about the mechanism able to transform the energy stored in the waves into heat.