A satellite study of VLF hiss

Abstract

Broad-band VLF radio noises from about 4 kc/s to above 10 kc/s are frequently observed near the auroral zone with the Injun 3 satellite. These broad-band VLF radio noises are called VLF hiss. In this study we select VLF hiss events for analysis by requiring that the radio noise intensity from 5.5 to 8.8 kc/s exceed 3×10−10 gamma2/cps (about 5 times the receiver noise level). During the 10-month lifetime of Injun 3 approximately 140 events occur that satisfy this criterion. The frequency spectra of the VLF hiss observed by Injun 3 is typically a flat noise spectrum with a distinct lower frequency cutoff. The lower frequency cutoff is often found to have a nearly symmetric latitude variation centered on a region of intense electron precipitation. The range of invariant latitudes (INV) for which VLF hiss typically occurs is about 7° wide and is centered on 77° INV at 12.0 hours magnetic local time (MLT), decreasing to 72° INV at 23.0 hours MLT. On the high-latitude side of the 40-kev trapping boundary, where VLF hiss usually occurs, intense fluxes of soft electrons are often accompanied by VLF hiss. It is found that the correlation between VLF hiss and intense fluxes (j > 2.5 × 107 (cm2 ster see)−1) of electrons (E > 10 kev) is dependent on the exponential folding energy E0. The correlation is very good for E0 from 3 to 4 kev but poor for larger E0 values.