gyration than with translation of guiding centers along field lines. This condition can easily lead to instability in certain electromagnetic wave modes. The anisotropy of ring-current electrons can lead to instability of a field-guided wave with right-handed polarization, i.e., the so-called "whistler” wave mode. The anisotropy of ring-current ions can lead to instability of the analogous field-guided wave with left-handed polarization. Both instabilities require the wave frequency to be somewhat smaller than the corresponding particle gyrofrequency. Both instabilities cause velocity-space diffusion so as to reduce the anisotropy of the corresponding charged-particle species, and so as to reduce the lifetime of that species against precipitation into the earth’s atmosphere (Kennel and Petschek, 1966; Cornwall, 1966, Cornwall et al., 1970). Moreover, the unstable ion-cyclotron waves generated by the anisotropy of ring-current protons are resonant with relativistic radiation-belt electrons (E [[spi:math]] 2 MeV) and thus account for the observed precipitation of such electrons during the recovery phase of a magnetic storm (Thorne and Kennel, 1971; Vampola, 1971). It happens that the electromagnetic instabilities noted here are not effective at ring-current energies for protons outside the plasmasphere, since the larger phase velocities attained there require a correspondingly larger proton energy for cyclotron resonance. Thus, the precipitation of relativistic electrons is contingent on the spatial co-existence of ring current and plasmasphere, which occurs only during the plasmaspheric expansion characteristics of the recovery phase of a magnetic storm. The electromagnetic proton-cyclotron instability, however, is likely to be suppressed by the presence of substantial numbers of heavy ions such as Ar+ or O+ in the magnetospheric plasma (Cornwall and Schulz, 1971). This means that the major mechanism for the depletion of relativistic electrons from the 49
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