Group delay times of whistler-mode signals from VLF transmitters observed at Faraday, Antarctica

The group delay times (tg) of whistler-mode waves generated by the NAA (f= 24.0 kHz) and NSS (f = 21.4 kHz) U.S. Navy transmitters and recorded at Faraday, Antarctica (L= 2.3), after following a ducted field-aligned path are analysed theoretically for different L-shells of propagation using models of electron density, temperature, and ion composition distribution for typical day and night-time conditions. tg is presented as the sum of (1) a group delay time calculated for the simplest model of wave propagation parallel to the magnetic field in a cold, dense plasma with the effects of ions neglected (tgo) and (2) the corrections due to finite electron density, that is, finite ratio of electron plasma frequency to electron gyro frequency (Δtgc), contribution of ions (Δtgr), and non-zero electron temperature (Δtgh). It is pointed out that the correction Δtgc is the dominant one, while the ratioΔtgh/Δtgc is only about 1 % for L close to 2.3. The total correction Δtgs, = Δtgc + Δtgr + Δtgh at L = 2.3is about 10 ms and is to be taken into account when interpreting the measurements of tg. However, on the assumption of strictly longitudinal propagation, the parameter [tgm(NSS) – tgm(NAA)]tgm(NSS) [index m indicates measured parameters] can be used for estimating L without taking into account the corrections Δtgs, if we do not require an accuracy better than ± 0.02.