Night-time radial plasma drifts and coupling fluxes at L = 2.3 from whistler mode measurements

Abstract

A method recently reported for measuring radial drifts in the equatorial plane and ionosphere-magnetosphere coupling fluxes from the Doppler shifts and group delays on whistler mode signals is applied to VLF transmissions from station NLK on 18.6 kHz. Data from 22 nights, primarily during the months November to February, are analysed. When averaged over a time of about 90 min, drifts found are accurate to ±20 ms −1, corresponding to an equatorial electric field accuracy of ±0.05 mVm −1 and fluxes, to ±1.5 × 10 12 el m −2 s −1 (two hemisphere total). Given currently accepted values of coupling fluxes, the flux accuracy is of marginal value on individual nights, but useful information on average behaviour may be obtained. We find fluxes generally contribute less than 20% to the measured Doppler shift, most of which is therefore produced by cross- L drifts. To an accuracy of about 20% then, Doppler data alone may give information on these drifts. Doppler shift data previously accumulated over a number of years and relating to signals in ducts near L = 2.3 are re-examined. Dominating the nightly behaviour is an inward drift which reaches a maximum of ∼ 100 ms −1 as the duct ends cross the dusk terminator and an outward drift at dawn of the same magnitude which is initiated when the duct end crosses the terminator in the E or lower F-region. In some months, separate effects can be seen corresponding to sunrise at each end of the duct. During the night, there are clear differences between December and June solstice drift behaviour which are as yet unexplained. Two-hemisphere-total coupling fluxes found from the 22 nights of data are ∼ 1−2 × 10 12 elm −2s −1. At the time of the evening maximum of inward drift the flow is to the ionosphere, but later in the night upfluxes were measured.