Whistler Propagation and Geomagnetic Activity

The relationship between whistler propagation and geomagnetic activity is examined. The most reliable data are available for winter nights at middle and high latitudes. Under these conditions, whistler activity reaches a maximum at a certain value, Kopt, of the planetary magnetic index Kp. Kopt varies progressively with ‘invariant latitude’ (derived from McIlwain's L parameter), being about 4 at invariant latitude 45°, and 1 at 68°. For Kp>Kopt, whistler activity rapidly decreases. Possible mechanisms producing this effect are discussed. At low latitudes, enhanced whistler activity follows severe geomagnetic storms by 1–2 days. This delayed effect is not evident at higher latitudes.The advantage of using invariant latitude rather than geomagnetic latitude in studies of this kind is stressed. The parameters ‘whistler rate’ and ‘percentage occurrence’ are found to be equally good indicators of whistler propagation conditions.