Post midnight VLF chorus events, a substorm signature observed at the ground nearL= 4

Clouds of energetic electrons, injected sporadically into the nightside magnetosphere during substorm expansion phase onsets, can generate VLF whistler mode noise through the gyroresonance instability, which may then be observed on the ground or in space. Although these substorm-related chorus events (SCEs) have been reported occasionally in the literature, there seems to have been no systematic study, probably because of the lack, until now, of a well-adapted experimental technique. The VLF/ELF Logger Experiment (VELOX) instrument, located at Halley, Antarctica (76°S, 26°W, L = 4.3), is, however, particularly well suited to a systematic study of this aspect of the substorm phenomenon. The data exist almost continuously from January 1992 onward, at 1-s time resolution in eight quasi-logarithmically spaced frequency bands covering the range 0.25–10 kHz. For this paper, 327 days of continuous data from 1992 have been analyzed. The 243 SCEs identified were observed on about 50% of days, almost exclusively in the 2300–0600 MLT local time range, and were characterized by limited duration (typically, ∼10 min at 1.5 kHz) and upward frequency drift of a band of usually weak and relatively unstructured chorus at a rate of ∼200 Hz min−1 between 0.5 and 5 kHz (corresponding to parallel electron energies in the range ∼10–100 keV). This drift is consistent with the combined eastward and inward motion of the resonant electrons due to azimuthal gradient-curvature drift and radial E × B drift under the action of substorm-enhanced westward electric fields of order l mVm−1 near the equatorial plane. The limited MLT viewing window of the station implies an overall detection efficiency for SCEs of ∼20%. The inferred annual mean substorm rate, 1366 ± 188 year−1, and inter substorm interval, 5.5 ± 0.8 hours, are similar to the values derived using other techniques. However, the distribution of intervals between successive SCEs is different from that for substorm-related particle injections at geostationary orbit; in particular, the models around 1 hour rather than 2–3 hours. The SCE as seen by a VELOX-type VLF receiver with a wide field of view is an important alternative ground-observable substorm signature, complementary to those (such as bays and Pi 2 pulsations) indicated by magnetometers.

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