Kg. Mcclements et al., A MODEL FOR THE GENERATION OF OBLIQUELY PROPAGATING ULF WAVES NEAR THE MAGNETIC EQUATOR, J GEO R-S P, 99(A12), 1994, pp. 23685-23693
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 pr
opagate at large angles with respect to the magnetic field, and, in th
e case of GEOS 1, appear to be associated with ringlike proton distrib
utions. It is demonstrated that the excitation of obliquely propagatin
g fast Alfven waves by such distributions can account for many of the
observed features of harmonically structured ULF emissions. Linear ins
tability can occur at, or slightly below, the proton cyclotron frequen
cy 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 gradien
ts, waves which propagate approximately azimuthally and within a few d
egrees of the perpendicular direction are likely to be amplified to th
e highest levels. Certain features of the ULF data, such as the finite
bandwidth of the harmonics, can be explained only by allowing the wav
e vector to have a finite parallel component.