Analysis of the effect of a ring current on whistlers

Results of a theoretical study to determine the effect of a ring current on the properties of whistlers are presented. The results are obtained by numerical integration of the nose whistler time delay integral that expresses the time required for a VLF radio signal to propagate between conjugate points as a function of the gyrofrequency, plasma frequency, and observing frequency integrated along the connecting field line. It is shown that the presence of a ring current having properties approximating those indicated by magnetometer data from Explorer 6 and Pioneer 5 has the effect of greatly increasing the dispersion and time delay of high-latitude whistlers. The effects are particularly marked at auroral latitudes, and virtually disappear at geomagnetic latitudes less than about 50°. The effects are very similar, however, to those associated with a change in the latitude of the field line. It is concluded, therefore, that it would be very difficult to deduce the properties of a possible ring current from the usual ground-based whistler data, in which the propagation field line is unknown because of lateral spreading under the ionosphere. It is pointed out that this uncertainty in the latitude of the field line might be removed if the data were acquired above the reflecting level of the ionosphere.