A PURELY GROWING ELECTROMAGNETIC MODE OPERATIVE IN THE GEOMAGNETIC TAIL

This paper discusses a purely growing mode driven by a cross-field current. The study was motivated by a recent article by Chang et al. (1990). The present discussion pays special attention to two aspects. One is to generalize the analysis by Chang et al. (1990) so that the unmagnetized-ion approximation used by these authors is removed, and the other is to apply the theory to several regions in the magnetotail where the value of the plasma beta is in general very high. The present analysis is restricted to waves propagating along the ambient magnetic field. The high ion beta limit is discussed by considering two different situations. The first is to fix the strength of the ambient magnetic field but to increase the plasma temperature, and the second is to maintain the plasma temperature but to decrease the ambient magnetic field strength. It is found that in the former case the mode stabilizes when beta(i) --> infinity, but in the latter case the instability persists even if beta(i) --> infinity (although the growth rate is significantly reduced). The present theory is applied to three regions in the magnetotail. These are: (1) the inner edge region, (2) the midtail region, and (3) the neutral sheet of a distant magnetotail. It is found that, among these three regions, for a given value of upsilon-0/alpha(i), where upsilon-0 is the net cross-field drift speed between the electrons and the ions and alpha(i) is the ion thermal speed, the growth rate in the neutral sheet is found to be the largest.