Ray-tracing study of the propagation in the magnetosphere of whistler-mode VLF emissions with frequency above one half the gyrofrequency

Abstract

High-frequency whistler-mode VLF emissions are excited at the equator of the magnetosphere, at frequency above one half the electron gyrofrequency and with wave normals close to the oblique resonance cone for whistler propagation. This finding has been evidenced by our measurements of wave normal directions of those waves at the equatorial and off-equatorial regions of the outer magnetosphere. The present paper deals with the propagation characteristics of high-frequency whistler-mode waves in the whole magnetosphere, by means of three-dimensional ray-tracing computations. The waves are traced, starting at the equatorial source, and the wave frequency is fixed as 0.6 times the electron gyrofrequency at the injection point. However, the initial L value is widely varied (not only outside but also inside the plasmasphere), and the initial wave normal direction is varied, but close to the oblique resonance cone. It is found that high-frequency whistler-mode VLF emissions, initially quasi-electrostatic at the equatorial source region, excited outside the plasmasphere, can propagate to a lower altitude at L ≳ 5, and then take the paths of multiple magnetospheric reflections around the equator within the plasmasphere. The characteristics of ray paths ( L value and longitude) and of the corresponding wave normal behaviour have been studied in detail, as a function of the initial azimuthal direction. Finally, the importance of these VLF emissions is discussed.