Trimpi events and other amplitude perturbations observed during 1991 at Kerguelen ( L = 3.7) on the subionospheric North-west Cape (NWC) signal

Abstract

In the period March–November 1991, which was characterized by strong magnetic activity, two kinds of complementary VLF data were recorded at Kerguelen: (1) the amplitude of the subionospheric 22.3 kHz signal from the North-west Cape (NWC) transmitter in Australia, and (2) the integrated intensity in various VLF bands, and broadband waveforms (0.4–10 kHz). Comprehensive analysis of the data allowed one to identify and characterize four types of well-structured perturbations in the NWC signal amplitude, i.e.: classical Trimpi events associated with whistlers; hiss-induced electron precipitation events; inverted dome-shaped events identifiable with dome-shaped events recently observed at Durban by Friedel et al. (1993); and quasi-monochromatic oscillations, with a period of ∼7–15 s observed when Kerguelen is poleward of the plasmapause projection in the pre-dawn sector, similar to the zigzag effect described by Carpenter et al. (1985a). The purpose of this paper is to report on the results of investigations devoted to these various types of event; each of them is illustrated by examples. The main new results relative to classical Trimpi events concern: the dependence on severe magnetic storms (out of 2300 events identified in June and July 1991—period of maximum occurrence—48% were observed on 6 days only in the aftermath of three large magnetic storms); the correlation between amplitude perturbation sign and NWC signal amplitude connected with sunset and sunrise on the east-to-west NWC-KER path; the nocturnal variation of recovery times for plasmaspheric whistler-associated Trimpi events (the mean value increases from 30 to 42 s at night); the detection in the midnight-dawn sector of short-duration Trimpi events (recovery times in the range ∼4–15 s) probably triggered by whistler waves propagating beyond the plasmapause when the latter is equatorward of, or close to, Kerguelen, 24–48 h following the onset of intense magnetic storms. Whereas it is possible to interpret most of the characteristics and properties of classical Trimpi events in terms of electron precipitation induced by magnetospheric VLF waves—a process being favoured by the Occurrence of severe magnetic storms with probable injection of energetic particles—the dome and zigzag effects remain unexplained as yet.