Excitation of high-β plasma instabilities at the geostationary orbit: Theory and observations

Abstract

The general theory of low frequency plasma instabilities is developed for a plasma consisting of a cold component and a hot high β component. In the limit k ⊥ r H ⪡1, two groups of instabilities can generate ULF oscillations at the geostationary orbit: for the first the magnetic field line curvature can be neglected ( βk 2 ⊥r 2 H⪢ a R ) in the dispersion relation, for the second it must be retained ( βk 2 ⊥r 2 H≲ a R ). The case of straight field line geometry yi two independent modes: the compressional Alfvén wave and the drift mirror mode. The instabilities that can excite them were treated by Pokhotelov et al. (1985) for a bi-maxwellian distribution function. When the existence of a loss cone is also taken into account, a significantly lower threshold is obtained for both the drift anisotropy instability (corresponding to the compressional Alfvén wave) and the drift mirror instability. The case of curved field lines yields a strong coupling between the compressional Alfvén wave and the drift mirror mode. The theoretical predictions are compared in detail with the Pc5 events of 27 October 1978 and 1 November 1978 observed simultaneously by the geostationary satellite Geos 2 and by the STARE radar system. In both cases the observed anisotropy was too low for the drift mirror mode as well as for the drift anisotropy instability. The calculation of the growth rate for the coupled mode, in which the field line curvature is taken into account, shows that the oscillations generated by this mechanism are growing and propagate in the East—West direction in agreement with the STARE measurements. Thus the coupling between the compressional Alfvén waves and the drift mirror mode can explain the observed low frequency particle intensity oscillations.