VLF hiss and related plasma observations in the polar magnetosphere

Abstract

This paper presents a study of auroral-zone VLF hiss and low-energy charged-particle observations with the Injun 5 satellite. The results of this study provide a direct verification of the association between auroral-zone VLF hiss and intense fluxes, 104 to 107 electrons (cm² sec ster ev)−1, of low-energy electrons with energies on the order of 100 ev to several kev. On the dayside of the magnetosphere, these low-energy electrons are identified with the dayside polar-cusp region observed at higher latitudes with the Imp 5 satellite. At other local times, through the dawn and dusk regions and into the nightside of the magnetosphere, the VLF hiss and low-energy electron precipitation regions are believed to correspond to the extension of the dayside polar cusp into the distant plasma sheet and downstream magnetosheath on the nightside of the magnetosphere. Intense fluxes of upgoing electrons are often observed in a narrow latitudinal band near the low-energy electron precipitation bands. These upgoing electrons are believed to be associated with another type of VLF emission called a saucer, which is frequently observed with Injun 5. On the basis of present models, the observed VLF hiss intensities cannot be accounted for by incoherent Cerenkov radiation from the observed electron fluxes, thus indicating that a coherent plasma instability mechanism is involved in some, if not all, of the VLF hiss generation. A model for the generation regions of VLF hiss and saucer emissions is discussed.