Error sources and travel time residuals in plasmaspheric whistler interpretation

In the interpretation of observed whistlers by curve fitting, systematic travel time residuals appeared which were studied by extensive simulations using ray-tracing, numerical integration and curve fitting. The residuals were found to originate from the commonly used approximations in the refractive index and ray path of whistler mode waves, which result in travel time increments or decrements, not accounted for in whistler interpretation. These approximations and the assumed form of the electron density distribution also lead to systematic errors in the diagnostics of plasmaspheric electron density by whistlers. In addition, the effects of other error sources, including random measurement errors, are also reviewed briefly. It is shown that the fine structure of residual curves is connected to propagation conditions. Thus, their study may yield a new research tool for studying whistler trapping, ducting structures and other features of whistler propagation. The application of residual analysis in conjunction with digital matched filtering of whistlers seems to be especially promising for further whistler studies

High latitude observations of whistlers using three spaced goniometer receivers

One-hop whistlers were recorded simultaneously at three high latitude stations in northern Norway during February 1978 using VLF goniometer receivers. Triangulation of the azimuthal bearings of whistlers received during an auroral substorm from 22.00 to 22.37 UT on 12 February located their whistler exit-points about 100 km to the north-east of Tromso, corresponding to an L-value for all the determinations of 6.4 ± 0.2. The frequency-time profiles of the same whistlers were analysed using the curve-fitting procedure of Tarcsai (J. atmos. terr. Phys.37, 1447, 1975) to determine their L-value of ducted propagation. These were found to lie in the L-value range 2.8–4.0, assuming a diffusive equilibrium distribution along the field lines. The L-values determined for the whistlers' exit-points were thus found to be considerably greater than that corresponding to the field line along which they were ducted. This discrepancy explained by the whistlers first following a field-aligned ducted path through the plasmasphere and then, after being reflected by sporadic-E ionisation in the lower ionosphere, following a sub-protonospheric path (Carpenteret al., J. geophys. Res.69, 5009, 1964) to higher latitude. It is shown by curve-fitting to whistler frequency—time profiles obtained by ray-tracing that such a path combination yields whistler spectra consistent with those observed

Whistler mode signals from VLF transmitters, observed at Faraday, Antarctica

The Doppler shift, one-hop group travel time and arrival direction of whistler mode signals from the NAA (24kHz) and NSS (21.4kHz) VLF transmitters in eastern U.S.A. have been measured in the conjugate region, at Faraday Station, Antarctica (65°S 64°W, L=2.3). Two identical narrow-band receivers of the type described by N. R. THOMSON (J. Geophys. Res., 86,4795,1981) were used. The technique enables cross-L plasma drifts and flux tube filling and emptying rates in the inner magnetosphere to be inferred continuously with a time resolution of 15min. Ducted whistler mode signals, of typical strength ∿1 μVm^ from both transmitters were observed every night during February-March 1986,usually with multi-duct structure evident. Such structure was generally similar for the two transmitters, indicating propagation along a common set of ducts. This permitted the determination of the L-values of the ducts without reference to natural whistler data. Typical one hop group travel times were in the range 300-900ms and Doppler shifts in the range -500mHz to +500mHz. Most ducts could be tracked for several hours, and during the night their associated group travel times often exhibited a steady decrease followed by a steady increase, suggesting a change from inward to outward cross-L drifting under the action of east-west electric fields of magnitude ∿0.3mVm^. This drift reversal occurred at ∿02 LT and was accompanied by a rapid change from positive to negative Doppler shifts

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