J. N. Tandon

On multimode pulsations of degenerate stellar models

Nonlinear dissipative radial oscillations of partially and completely degenerate stellar models have been studied. The general mathematical framework for the study of interaction between various radial modes of oscillation has been presented. The dynamical tools, e.g., Poincare surface of section and the spectrum of Liapunov exponents, have been used to investigate the nature of pulsation in the phase space. For simplicity, numerical computations have been made to study the interaction of only the first three modes, including the fundamental mode for both partially and completely degenerate stellar masses. For these models, with varying degrees of central degeneracy, the possible occurrence of quasi-periodic motions with as many as three irrationally related frequencies has been established in the dissipative regime. 56 refs.

Viscous interaction at the solar wind-magnetosphere boundary

Abstract The authors consider wave refraction and stability of the magnetopause using Chew et al. (1956) equations to describe the magnetosphere and solar wind plasmas. The results are discussed with a view to account for a possible viscous interaction at the boundary.

Effect of a magnetic field on the adiabatic oscillation of convective stellar models with radiation pressure

Using an unrestricted second order virial tensor analysis, the characteristic frequencies of small adiabatic oscillations of a convective stellar model in the presence of a weak poloidal magnetic field, with significant radiation pressure, have been calculated. In the absence of the magnetic field, the effect of increase in radiation pressure is to decrease the frequencies of radial modes, in spite of the increase in central condensation of the system. The characteristic frequencies of the transverse shear mode, the toroidal mode, and the pulsation modes show a general increase with the increase in the field strength. The marginal stability of the configuration in the presence of the magnetic field has also been discussed.

Stability of a collisionless contact discontinuity

The stability of a contact discontinuity in a collisionless plasma is examined. It is shown that the discontinuity can become unstable when the pressure component normal to the magnetic field is not continuous across the discontinuity. Even when the system is unstable, the growth rates are very small and unimportant in the context of wave propagation, except when the propagation is almost normal to the field. The instability could, however, lead to the slow dissipation of contact discontinuities in the solar wind and in the day side of the cusp region of the solar wind-magnetosphere boundary.

Rapidly rotating stellar models with radiation pressure

A modified perturbation technique, developed earlier by the authors, has been used to study the effect of rapid uniform and differential rotation on the equilibrium structure of early type stars. The models are in convective equilibrium with significant radiation pressure. The effect of both uniform and differential rotation on various structural parameters like mass, radius, central condensation etc., are calculated for different values ofyc (the ratio of central radiation pressure to central gas pressure).

Stability of the magnetopause

The Magnetosphere-Solar Wind boundary is treated as a tangential discontinuity between collisionless plasma described by C.G.L. equations. An estimate is made, analytically, of the minimum shear speed required to render the interface unstable for propagation in any arbitrary direction. It is found that even systems, which are, for all shear speeds, stable towards propagation parallel to the field, are rendered unstable by propagation away from the field, in some velocity domain. Numerical evaluations for parameters characteristic of the Solar wind-Magnetosphere boundary show that the interface could be unstable even under relatively quiet conditions.

Discontinuities and the magnetospheric phenomena

Abstract Wave coupling at contact discontinuities has an important bearing on the transmission of waves from the solar wind into the magnetosphere across the cusp region of the solar wind-magnetosphere boundary and on the propagation of geomagnetic pulsations in the polar exosphere. Keeping this in view, the problems of wave coupling across a contact discontinuity in a collisionless plasma, described by a set of double adiabatic fluid equations derived by Chew , Goldberger and LOW (1956) is examined. The magnetic field is taken normal to the interface and it is shown that total reflection is not possible for any angle of incidence. The Alfven and the magneto-acoustic waves are not coupled. The transmission is most efficient for small density discontinuities. Inhibition of the transmission of the Alfven wave by the sharp density gradients above the F2-peak in the polar exosphere appears to account for the decrease in the pulsation amplitude, on the ground, as the poles are approached from the auroral zone.

Cosmic ray increases during the sunspot minimum of 1953

Geomagnetic sequence associated with a most pronounced u.m.-region of 1953 was analyzed. It was concluded that the trapped particles of the solar ion streams in the radiation belt may be responsible for the increases in cosmic ray intensities associated with quite M-storms. In this way, the radiation belt acts as the secondary source of cosmic ray particles. During the sunspot maximum period, these increases of cosmic ray intensity will get masked on account of very many solar-ion streams of large frozen-in magnetic fields. Hence, observations of such in- . creases in cosmic rays during the high sunspot period should not be expected. (Meteorol. Geoastrophys. Abstr., Feb. 1962)

The convection zone in stars with radiation pressure in the presence of magnetic field

The effect of a uniform magnetic field on the envelope convection zone of an 8.8M⊙ star has been studied. The adiabatic exponents Γi (i=1, 2, 3), adiabatic temperature gradient and specific heat of stellar matter has been computed. It is shown that the magnetic field tends to increase the values of adiabatic temperature gradient and specific heats of stellar matter in the envelope convection zone.

The structure and oscillations of low-mass stars in the presence of a magnetic field

The equilibrium structure and oscillations of a partially degenerate standard model in the presence of a poloidal magnetic field have been studied. The magnetic field in the interior has been matched with an outside dipole field. The effect of magnetic field on the various structural parameters, e.g., mass, central condensation, moment of inertia, and oblateness has been computed for different values of the central degeneracy of the model. We have also studied the effect of magnetic field on radial oscillations of the configuration. A variational formulation is used to compute the changes in the frequency of radial mode of oscillation. It has been shown that the changes in frequency computed for various models using a two-parameter eigenfunction are in fair agreement with the values obtained by using the exact eigenfunction.