ION-CYCLOTRON HARMONIC-WAVES IN THE EQUATORIAL MAGNETOSPHERE

Harmonically-structured wave emissions in the ULF and ELF ranges have been detected at low magnetic latitudes by several spacecraft. These waves propagate in directions nearly perpendicular to the magnetic field, and are often associated with particle distributions exhibiting population inversion in velocity space. After a brief description of the wave and particle data, theoretical work relevant to its interpretation is reviewed, with particular reference to the resonant excitation of perpendicular-propagating and obliquely-propagating fast Alfven waves. Linear stability analysis indicates that fast Alfven waves which propagate approximately azimuthally are likely to be amplified to the highest levels, although certain features of the wave data can only be explained by allowing the wavevector to have a finite parallel component. The implications of the observed wave activity for ion heating and precipitation are also discussed.