Fast time resolved spectral analysis of VLF banded emissions

Abstract

Several classes of VLF banded emissions acquired by the short antenna Ogo 5 electric field detector are investigated by using high time resolution spectral techniques. The fastest temporal resolution of statistically independent spectra is 50 msec; individual spectra are taken every 12.5 msec. VLF banded rising chorus spectra possessed single and/or multiple frequency modulated bursts; discrete single modes with unchanging center frequencies are often observed to be part of the banded chorus spectrum. Inverted or falling banded chorus spectra also contained frequency modulated bursts and single modes, but with greater structural complexity than rising chorus. The spectrum of a banded ‘hisslike’ emission was dominated by multiple discrete single modes with constant center frequencies. A narrowband emission with frequencies slightly above one-half the local electron cyclotron frequency consisted of single modes and groups of unresolved modes that were not frequency modulated. Finally an emission at 3/2 the local electron cyclotron frequency exhibited an almost uniform spectral density across its 200-Hz bandwidth. Since this preliminary study contains only a limited quantity of data, the reported spectral structures probably neither exhaust nor are necessarily typical of all classes of banded VLF emissions.