Electrostatic Ion Cyclotron Waves in the Auroral Ionosphere

Electrostatic waves near harmonics of the ion cyclotron frequency are often seen in the auroral ionosphere.¹ These waves propagate almost perpendicular to the geomagnetic field but are sometimes observed in conjunction with parallel bipolar field structures. This presentation is based on some unpublished FAST satellite data in a downward current region showing electron and ion distribution functions measured over intervals during which waves are observed (over shorter times) near ion cyclotron harmonics. Model particle distributions closely related to the measured ones are used in a linear stability analysis to study instabilities that are capable of saturating into the observed wave spectra and particle distributions. Typically, the measured distributions are highly anisotropic and have drifts. Two kinds of instabilities are found: current-driven instabilities, which represent a generalization of Kindel-Kennel² instabilities, and loss-cone-driven instabilities. (It is also known that shear in the velocity distribution can drive cyclotron harmonic waves in the upward current region.³) An interesting new method for studying stability of quasilongitudinal wave in a magnetoplasma is also presented. Research supported by NSF, NASA, and DOE. ¹ R. E. Ergun, et al., Geophys. Res. Lett., 25, 2025 (1998); F. J. Crary, et al., Geophys. Res. Lett., 28, 3059 (2001). ² J. M. Kindel and C. F. Kennel, J. Geophys. Res., 76, 3055 (1971). ³ V. V. Gavrishchaka et al., Phys. Rev. Lett., 85, 4285 (2000).