Seasonal, latitudinal and diurnal distributions of whistler‐induced electron precipitation events

Abstract

The seasonal, latitudinal, and diurnal distributions of whistler‐induced electron precipitation events, detected as subionospheric signal perturbations (Trimpi events), have been studied by means of data sets acquired in 1982‐1983 at Palmer and Siple stations, Antarctica. The data sets, substantially larger than any previously examined, confirm previous indications of a broad (∼ 4 hour) maximum in hourly event occurrence rates. The peak was centered ∼ 1‐2 hours after local midnight at Palmer for the roughly north‐south Argentina Omega 12.9‐kHz path, but was shifted several hours later for the 23.4‐kHz NPM path, which has a westerly arrival bearing at Palmer. The previously reported seasonal variation, with peaks at the equinoxes, was confirmed; activity persisted in the austral winter, particularly following a magnetic storm, but in general occurred on fewer days and during shorter daily periods. Comparisons of occurrence data at Siple (L ≃ 4.3) and Palmer (L ≃ 2.4) for particular signal sources showed the number of days of activity to be larger at Palmer by a factor of ∼ 2 or more. Comparisons of five months of simultaneous Palmer and Siple receptions of 21.4‐kHz NSS signals on north‐south paths suggest that most of the events observed at Siple occurred as the result of ionospheric perturbations relatively close to the L shell of Palmer, that is, in the range L = 2 – 3. This result is consistent with the expected variation with latitude of the energy of electrons precipitated due to equatorial gyroresonance at typical whistler frequencies.