DAMPING AND SPECTRAL FORMATION OF UPSTREAM WHISTLERS

Previous studies have indicated that damping rates of upstream whistle rs strongly depend on the details of the electron distribution functio n. Moreover, detailed analysis of Doppler shift and the whistler dispe rsion relation indicate that upstream whistlers propagate obliquely in a finite band of frequencies. In this paper we present results of a k inetic calculation of damping lengths of wideband whistlers using the sum of seven drifting bi-Maxwellian electron distributions as a best f it to the ISEE 1 electron data. For two cases, when upstream whistlers are observed, convective damping lengths derived from ISEE magnetic f ield and ephemeris data are compared with theoretical results. We find that the calculated convective damping lengths are consistent with th e data and that upstream whistlers remain marginally stable. We also s how that the slope of plasma frame spectra of upstream whistlers, obta ined by direct fitting of the observed spectra, is between 5 and 7. Th e overall spectral, wave, and particle characteristics, proximity to t he shock, as well as propagation and damping properties indicate that these waves cannot be generated locally. Instead, the observed upstrea m whistlers arise in the shock ramp, most likely by a variety of cross -field drift and/or anisotropy driven instabilities.