A model for the generation of obliquely propagating ULF waves near the magnetic equator

Abstract

Several spacecraft, notably OGO 3, IMP 6, Hawkeye 1, and GEOS 1 and 2, have detected harmonically structured wave emissions in the ULF range at low magnetic latitudes between L values of 4 and 8. These waves propagate at large angles with respect to the magnetic field, and, in the case of GEOS 1, appear to be associated with ringlike proton distributions. It is demonstrated that the excitation of obliquely propagating fast Alfvén waves by such distributions can account for many of the observed features of harmonically structured ULF emissions. Linear instability can occur at, or slightly below, the proton cyclotron frequency and all its harmonics. The maximum growth rates of some harmonics, notably the fundamental, occur at propagation angles which are oblique rather than strictly perpendicular. Because of magnetic field gradients, waves which propagate approximately azimuthally and within a few degrees of the perpendicular direction are likely to be amplified to the highest levels. Certain features of the ULF data, such as the finite bandwidth of the harmonics, can be explained only by allowing the wave vector to have a finite parallel component.