VLF Observations of Auroral Beams as Sources of a Class of Emissions

Abstract

AN interesting very low frequency (VLF) phenomenon which I shall call auroral V-emissions (AVEs) has been known for a number of years, although few reports have been published. The phenomenon has also been known informally as “short saucers” and “splashes”. The dynamic spectra of a collection of AVEs are shown in Fig. 1a. The AVEs shown here have a fairly low frequency minimum, near 0.5 kHz, and have time scales of the order of 10 s. The broader form at the upper part of the record may perhaps be an example of an AVE, but in this paper it will not be considered as an AVE. The name auroral V-emission is based on the region of observation and the V-shaped or hyperbolic spectral appearance. The hyperbolic shape is often quite symmetrical, particularly for the AVEs with shorter time scales. AVEs have only been observed in satellites, usually near the auroral zone. Noise bands with distinct intensity minima at harmonics of the local proton frequency are often observed at the centre of an AVE, especially for observations at relatively high altitude (3,000 km) and for AVEs with a low frequency minimum. The phenomena reported here have a much shorter time scale and a more well defined spectral appearance than the “V-type VLF hiss”^1,2.