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

Abstract

The group delay times ( t g ) 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. t g 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 ( t go ) and (2) the corrections due to finite electron density, that is, finite ratio of electron plasma frequency to electron gyro frequency ( Δt gc ), contribution of ions ( Δt gr ), and non-zero electron temperature ( Δt gh ). It is pointed out that the correction Δ t gc is the dominant one, while the ratio Δt gh/ Δt gc is only about 1 % for L close to 2.3. The total correction Δt gs , = Δt gc + Δt gr + Δt gh at L = 2.3is about 10 ms and is to be taken into account when interpreting the measurements of t g . However, on the assumption of strictly longitudinal propagation, the parameter [ t gm (NSS) – t gm (NAA)] t gm (NSS) [index m indicates measured parameters] can be used for estimating L without taking into account the corrections Δt gs , if we do not require an accuracy better than ± 0.02.