Statistical Properties of Upstream Whistler-Mode Waves of the Terrestrial Bow Shock Observed by Geotail

Abstract:

We present observational features of electromagnetic waves detected by Geotail in the frequency range of several Hz in the upstream region of terrestrial bow shock. Two types of upstream whistler-mode waves are observed: left-hand polarized waves in narrow frequency range near 1 Hz and right-hand polarized waves in broad frequency range around the lower hybrid resonance frequency in the spacecraft frame. We performed statistical snalyses on these waves. The former narrowband whistlers and the latter broadband whistlers are observed when the interplanetary magnetic field is nearly parallel and perpendicular to the solar wind flow, respectively. The properties of the narrowband whistlers including the observed frequency, spectral shape, and wave vector direction suggest that they are nearly group-standing, which is a situation the waves are stagnating in the spacecraft frame. On the other hand, the broadband whistlers are not group-standing, and both types of the waves originate from the same wave source. Though most properties of the narrowband whistlers are restricted to be group-standing, the properties of the broadband whistlers can reflect their generation processes. We suggest that the waves are mainly generated through the modified two stream instability caused by reflected ions near perpendicular shock or the shock front dynamics, which can excite whistler mode waves in the frequency range around the lower hybrid resonance frequency with the oblique propagation angle to the ambient magnetic field.