Satellite observations of a new type of discrete VLF emission at L < 4

Abstract

A new type of discrete whistler mode emission has been observed in the magnetosphere at L < 4. The emission elements are confined to a bandwidth of 1–5 kHz, with the lower cutoff frequency of the band varying with L shell, being equal to ∼0.2–0.5fHeq, where fHeq is the equatorial electron gyrofrequency. The discrete and burstlike nature of the emissions is similar to that of chorus emissions typically observed at higher L; however, dispersion of individual elements is often different from typical chorus, and the emissions are observed inside as well as outside the plasmapause. The phenomenon seems to occur mainly in the early morning local time sector (0400–0800 MLT) and is well correlated with geomagnetic activity, occurring mostly when ΣKp > 30. The analysis of data from the low‐altitude ISIS 2 and the high‐altitude DE 1 satellites indicates that the emissions may be generated near the equatorial plane at frequencies of ∼0.2fHeq inside and ∼0.35fHeq outside the plasmapause. The parallel energy of electrons for gyroresonance with the observed waves is found to be ∼20–50 keV in both cases. Observations in the vicinity of low‐altitude crossings of the plasmapause also indicate the presence of a second emission band which is interpreted to result from the reflection of the equatorially generated emissions from the lower ionosphere.