THE INFLUENCE OF THE RING CURRENT ON WHISTLER GROUP DELAY TIME IN THE MAGNETOSPHERE

An approximate analytical model of the magnetic field of the ring current is proposed. This field is assumed to be perpendicular to the magnetic dipole equator with its value depending on the crossing point distance of the field line from the centre of the Earth (r0) and the D(st) index measured at the Earth's surface near the equator. This model of the magnetic field of the ring current is used to obtain an estimate of the perturbation of whistler-mode group velocity and group delay time due to the ring current. An explicit expression for the perturbation of whistler-mode group delay time (DELTA-t(g)) is derived in the form of a simple integral. DELTA-t(g) calculated within our model is shown to agree within an accuracy of about 30% with the results of more rigorous analysis. It is pointed out that the influence of the ring current on whistler group delay time is about an order of magnitude larger at L = 3.5 than at L = 2.4. However, even in the latter case this influence is comparable to other perturbations of whistler group delay time and should be taken into account when interpreting whistler frequency/time profiles and group delay times of whistler-mode signals from ground-based VLF transmitters.