Instability phenomena in detached plasma regions

Abstract

Wave propagation characteristics, such as plasma frequency and Alfvén velocity, are significantly modified by changes in the cold plasma density. Consequently, regions of so-called ‘detached plasma’ which drift through the plasma trough beyond the plasmapause are highly correlated with waves which develop in a wide range of frequencies and which are observed by spacecraft experiments. In particular, for low frequency ELF signals, the association is sufficiently close to suggest that the waves are locally generated within the detached plasma. Hence, the observations can be used to test theoretical models of wave generation, especially by examining the dependence of the observed frequencies and amplitudes on position in the dipole field ( L-shell) and on the measured particle distributions. ULF waves are frequently observed in detached plasma regions. At times it is possible to use the OGO-5 plasma measurements to determine one component of the electric field of these ULF waves, and thereby to infer that the waves are standing mode oscillations of field lines.