On the theory of the polarity-independent component of the semiannual variation.

Abstract

The polarity-independent component of the semiannual variation has been ascribed to the seasonal variation of the threshold of the Kelvin-Helmholtz instability (ROLLER and STOLOV, 1970) in the dawn-dusk sector of the magnetosphere-solar wind boundary which is regarded as a tangential discontinuity separating incompressable magnetized fluids. In this paper, we incorporate these features by using the collisionless, double adiabatic fluid equations (cf. KALRA et al., 1976) to describe the solar wind and magnetospheric plasma.Perturbations of the form exp i(wt+k. r) (where w is the frequency and k is the wave vector) are introduced in the system and the dispersion relation connecting the frequency with kt, the component of the wave vector along the interface, is obtained (cf. KALRA et al., 1976).The dispersion relation is solved numerically using relevant solar wind parameters (DOBROWOLNY and MORENA 1975, GOSLING et al., 1976), to determine the stability (growth rate r(=lm(w))<0 corresponds to an instability). The magnetic field and solar wind flow are assumed to be directed radially and the post shock values are obtained from the curves of SPREITER and ALKSNE (1969). We find that the dawn and dusk region of the magnetopause is unstable irrespective of the season, showing that the variation in the threshold may not be an appropriate source of the magnetic activity. In Fig. 1, the growth rate, chosen as a measure of induced activity, is depicted for three sample solar wind speeds. The amplitude of the semiannual oscillation decreases with the increase in the solar wind velocity. Hence, from GOSLING et al. (1976), the amplitude of the semiannual variation is expected to increase with increase in solar activity provided the density does not also increase from low to high solar activity.