A study of whistlers correlated with bursts of electron precipitation near L = 2

Abstract

The previously reported Trimpi effect involves observation of amplitude perturbations in subionospherically propagating fixed‐frequency VLF signals. The perturbations are coincident in time with magnetospherically propagating whistlers, and it has been inferred that the whistlers drive energetic electron precipitation of E > 100 keV in a manner such that a localized increase in ionization occurs near 80‐km altitude in the nighttime earth‐ionosphere waveguide. In the present work, VLF broadband, analog chart and DF data from Palmer Station, Antarctica (L ∼ 2.3), recorded in 1978 and 1979 are used to investigate additional features of the Trimpi process. Attention was concentrated on perturbations of the signals NSS (21.4 kHz) and NLK (18.6 kHz) that have arrival bearings at Palmer of 352° and 318°, respectively. In September–October 1978, amplitude perturbations on NSS and NLK at Palmer were as frequent as similar events previously reported from a station at L = 4. A large equinoctial peak in activity was found, as in the previous study. The perturbations on NSS and NLK were usually increases in the 10%–30% range; decreases on NSS were observed on one unusually disturbed day when the plasmapause was equatorward of Palmer. Case studies were performed on five days of high activity. Essentially, all of the 93 events involved exhibited a close time correlation between amplitude perturbation and received whistler. In some cases only a fraction of the observed whistlers appeared to correlate with fixed‐frequency signal changes. In one such case, correlated whistlers were found through DF analysis and frequency‐time pattern comparisons to contain a component not present in most other whistlers. Above a field strength of 13 µV/m, the amplitude of this component was found to be linearly related to NSS perturbation amplitude, while for cases of less than 13 µV/m, no NSS change was detected. It is concluded that the southern hemisphere region within which perturbations occurred for the five case studies extends 200 km east and west of Palmer and ∼400 km northward over the L range 2.4 to 2.1 (at 100‐km altitude). Within this area it is estimated that individual ionospheric perturbations were of order 100 km in the east‐west direction and that the perturbed regions were centered within ∼200 km of the affected great circle paths.