Derivation of magnetospheric electric fields from whistler data in a dynamic geomagnetic field

Abstract

The whistler method of determining magnetospheric electric fields has until recently been applied on the assumption of a static dipole geomagnetic field. We consider the effect on the whistler analysis of including both departures of the field from a dipole and temporal variations in the field. Departures from a dipole field appear to have relatively small effects on the analysis or can be relatively easily taken into account when it is necessary. Temporal changes present a more serious problem. In the presence of a changing geomagnetic field the temporal change in equatorial gyrofrequency of a drifting whistler path will consist of two parts: the variation due to radial drift of the path in the presence of the inhomogeneous B field and the variation due to the changes in B with time. Given knowledge of the temporal variation of B, it is possible to infer the total (induced plus potential) electric field associated with the radial drift. A substorm event is analyzed in which cross-L inward drifts near L = 4 occurred as the night side B field (as reflected in the low-latitude H component) exhibited a rapid increase. It was found that on a time scale of 15 min or greater the inferred total east-west electric field differed by ∼20% from the field estimated on the assumption of a static magnetic field. It is possible that effects of B may be relatively more important on a shorter time scale or during disturbances much larger than those thus far investigated. Also, in one quiet period preceding a substorm it appeared that all or a major part of the inferred cross-L motions of the plasma could have been due to fluctuations in the magnetic field. It is stressed that both equatorial and ionospheric measurements of electric fields are needed, since certain types of distortions of the high-altitude magnetosphere are not readily observed at ionospheric heights.