Analysis of subprotonospheric whistlers observed by DEMETER: A case study

Subprotonospheric (SP) whistlers consist of a series of low‐dispersion components that result from repeated reflections between the base of the ionosphere and altitudes up to ∼1000 km. We have used wave‐normal angles and plasma characteristics measured by the DEMETER microsatellite as an input for a three‐dimensional ray tracing technique. For several SP whistlers we have also succeeded in finding the causative lightning located at relatively large distances from the satellite footprint along the geomagnetic field line. We show that the reflections and formation of the SP whistlers take place owing to an oblique propagation, with respect to the magnetic field, in the waveguide formed by a profile of the increasing lower hybrid resonance frequency in the upper ionosphere and the base of the ionosphere. We have observed propagation across the magnetic meridian planes. We conclude that the individual components of the SP whistler propagate along different raypaths. The reflected components enter the ionosphere at relatively large distances from the satellite footprint and experience a spread of wave‐normal angles during this entry. Depending on the initial wave‐normal angle, these waves undergo a different number of reflections before reaching the satellite, thus arriving with different time delays. However, the first component observed of a SP whistler is formed by waves entering the ionosphere at relatively small distances from the satellite footprint and at relatively small wave‐normal angles. These waves do not reflect above the satellite but propagate to the opposite hemisphere.