Dynamic spectral characteristics of chorus at a middle-latitude station

Abstract

A study of broadband VLF data recorded during 1966 at Lauder, New Zealand (52.0°S invariant latitude), has shown that the dynamic spectra of chorus at any given time are related to the local magnetic conditions three to four hours earlier. (The K index from Macquarie, 12° poleward of Lauder, was used to describe local magnetic activity.) The mean rate of change of frequency increases markedly from 1 to 30 kHz/sec as the local K index increases from 0 to 8. The increase is due to changes in relative occurrence of four distinct types of midlatitude chorus. Each type occurs only for a limited range of magnetic conditions. The bandwidth of the chorus tones also increases with magnetic activity, the mean lower frequency of the tones decreasing from 2½ to 1 kHz, and the mean upper frequency of the tones increasing from 4½ to 6½ kHz, as the local K index increases from 0 to 8. The mean center frequency is 3.3 kHz, independent of magnetic activity.