Correlations between λ4278 optical emissions and VLF wave events observed at L ∼4 in the Antarctic

Abstract

One‐to‐one correlations have been observed at L∼4 between bursts of ducted VLF noise in the ∼2 to 4‐kHz range and optical emissions at λ4278. The optical observations were made at Siple Station, Antarctica, in the austral winter of 1977; the wave activity was recorded both at Siple and its conjugate station Roberval, Canada. The one‐to‐one correlations were observed during 6 of 32 observing sessions. Most of the events occurred near dawn (prior to interruption of the observations by twilight interference) during or shortly following substorm events (Kp =2.5; Dst=−10–40). In most of the cases the plasmapause was equatorward of Siple, and the equatorial electron densities were low (∼10 cm−3). In one case, July 24, 1977, the equatorial density had recovered to a level of 100 cm−3 at the time of observation. The estimated precipitated energy fluxes ranged from 0.04 to 0.1 ergs cm−2 s−1. The correlated VLF activity usually consisted of clusters of discrete rising tones or chorus ranging in duration from ∼1 s to ∼10 s. In three of the six cases some or all of the chorus bursts were triggered by whistlers. Photometer peaks lagged the wave peaks by 1–2 s. In the case of July 24, 1977, a model of the interaction process was constructed using the lag time and the whistler‐derived cold plasma density. The data were found to be consistent with scattering of electrons into the loss cone over Siple by emissions that were triggered by waves propagating away from the equator after reflection in the ionosphere over Siple. This interpretation differs from that of a previous study of X‐ray bursts induced by similar VLF emissions. This difference is attributed to the occurrence of whistler echoing during the reported X‐ray event. During the same period, VLF hiss was observed in the range 2.5–5.0 kHz. Immediately following each discrete event the hiss and the λ4278 intensities were reduced below their pre‐event levels for about 10 s. Both reductions are tentatively attributed to a temporary depletion of particles near the loss cone. Recovery to pre‐event levels could be explained by refilling of the duct through east‐west gradient drift of energetic electrons.