Longitudinal variations of polar cusp hiss observed by ISIS satellites Original Research Article

Spectra of Polar cusp hiss observed by ISIS satellites are irregular in frequency, intensity and space compared with stable broad-band auroral hiss. The polar cusp hiss often occurs at geomagnetic invariant latitudes from 74° to 84° and at geomagnetic local times from 10 to 14 hours. Irregular spectra of the polar cusp hiss are due to ionospheric irregularities caused by cusp electron precipitation. The whistler-mode Cerenkov radiation from electrons below 1 keV is discussed for generating the polar cusp hiss, but this mechanism can not work in the polar cusp ionosphere for VLF frequencies much lower than a local electron gyrofrequency. The whistler-mode VLF Cerenkov radiation is generated from the electrons below 1 keV at high altitude cusp magnetosphere, where a local electron gyrofrequency is in VLF band. The whistler-mode cusp hiss generated at high altitudes propagates along field-aligned cusp plasma down to a strong geomagnetic field region where the cusp plasma structure fades out. Below this region, the whistler-mode hiss spreads over the polar cusp ionosphere where low energy electron precipitation produces ionospheric irregularities.