Convection of ion cyclotron waves to ion-heating regions (original) (raw)

1991, Journal of Geophysical Research

Low-frequency waves associated with ion conics have been observed in the ;:entral plasma sheet, in a region where there are no obvious sources of free energy that could destabiliz•,these waves locally. We consider ion cyclotron waves generated in the equatorial plane b•. a i•rotot•' temperature anisotropy and use computed growth rates to create a model wave distri.bution. U•ing raj•t•'?• ing and conservation of the wave distribution function along phase space rays, w.e. th, eri map the; Wave intensities from the equatorial plane to the top of the ion-heating region. We find that •he spectral density at a geocentric distance of 2.8 RE will be about 10 times higher than that in the equatorial region. Thus, convection from the equatorial plane could explain the observed spectral density of 10-(; V 2 m-2 Hz-1 and the associated oxygen ion heating. iNTRODUCTION It is well known that ions in the ionosphere and magnetosphere can be heated perpendicularly to the geomagnetic field. These ions may then move adiabatically up the field lines of the inhomogeneous terrestrial magnetic field and form so-called conics in velocity space. Several alternative ion energization mechanisms have been suggested (see reviews by Klumpar [1986], Lysak [1986], and Chang et al., [1988], and references therein). However, the problem of ion conic generation is still the subject of vigorous debate. At least at altitudes above a few thousand kilometers, ion conics are often observed on the same field lines as relatively intense, broadband waves. Several studies indicate that the waves observed around the ion gyrofrequency may, via resonant cyclotron heating, generate the observed ion conics [Chang et al., 1986; Andrd et al., 1988, 1990; Crew et al., 1990]. Waves at roughly half the ion gyrofrequency may via double-cyclotron absorption contribute to the ion heating ITemerin and Roth, 1986; Ball, 1989; Ball and Andrd, 1991a]. Emissions at even lower frequencies may also cause some ion energization [Lundin and Hultqvist, 1989; Lundin et al., 1990; Ball and Andrd, 1991b]. Broadband waves and ion conics are often observed above the auroral zone and in the polar cusp/cleft region. Here local energy sources such as sharp gradients and drifting particles can possibly generate some of the waves. However, broadband waves associated with ion conics are also common in the central plasma sheet [Chang et ai., 1986]. In this region, equatorward of the auroral zone, there are no obvious local energy sources that can power the broadba• waves. This led Johnson et [1989] to suggest that the waves are generated by anisotropic ion distributions in the equatorial plane and that they then propagate down the field lines. Closely related ideas were considered recently •lso by Horne and Thorne [1990], who used ray tracing to study the propagation of ion cyclotron waves in the plasmapause region. They emphasized ion cyclotron damping at the second harmonic of the oxygen gyrofrequency. The path-integrated absorption they computed was used to estimate the ion heating qualitatively, but their method did not allow quantitative comparisons based on observed spectral densities.