Investigation of propagation properties of quasi-periodic (QP) VLF emissions observed by the DEMETER spacecraft.

Quasi-periodic (QP) emissions are whistler mode electromagnetic waves which are observed in the inner magnetosphere, either inside the plasmasphere or near the plasmapause. They exhibit a periodic time modulation of the wave intensity with the modulation periods from several seconds to minutes. The analysis of wave propagation properties of nearly 200 QP events measured by the low-altitude DEMETER spacecraft (altitude 700 km) shows that QP emissions generally propagate nearly field-aligned at larger geomagnetic latitudes (> 55 degrees) and they become more oblique at mid-latitudes (latitudes of 45 degrees). However, we have observed several QP events with unexpected variations of both wave and Poynting vectors directions over a short time/spatial interval at larger latitudes. These seem to contradict the general statistical results. We perform a detailed analysis of three such events. All these events were observed during quiet geomagnetic conditions (Kp < 2). A ray-tracing calculation is used to determine the wave paths and the locations of the generation regions. We also discuss the influence of the plasmapause on the wave propagation.