Three‐dimensional ray‐tracing for very low latitude whistlers, taking into account the latitudinal and longitudinal gradients of the ionosphere

Propagation mechanism of very low latitude (geomagnetic latitude less than 20°) whistlers is poorly understood, and this paper examines the propagation characteristics of nonducted propagation at very low latitudes by using three‐dimensional ray‐tracing for realistic ionosphere/magnetosphere models with latitudinal and longitudinal gradients and International Geomagnetic Reference Field (IGRF) magnetic model. By assuming small possible tilts (in the latitudinal and longitudinal direction) of the initial wave normal angle in the input Southern Hemisphere as in our previous paper [Ohta et al., 1997], we have found that it is possible for us to detect, at a very low latitude position in the Northern Hemisphere, some whistler rays being able to penetrate through the ionosphere and some others that are subject to total reflection and come back to the magnetosphere. By making full use of systematic analyses of ionospheric parameters (local time (LT) dependence of latitudinal and longitudinal gradients), we conclude that the LT dependence of occurrence rate of whistlers at very low latitudes can be accounted for by means of such LT dependence of the gradient (especially latitudinal), lending to a further support to our previous nonducted propagation mechanism.