Energy distribution and lifetime of magnetospherically reflecting whistlers in the plasmasphere

Abstract

Using ray tracing and Landau damping calculations based on recent data on suprathermal particle fluxes from the HYDRA instrument aboard the POLAR satellite, we estimate the energy distribution and the lifetimes of 0.2–10 kHz whistler mode waves in the plasmasphere. The rays are injected at 1000 km altitude and latitudes of 25°, 35°, 45°, and 55° to represent whistler wave energy originating in lightning discharges occurring in middle to low‐latitude thunderstorms. The lifetime is defined as the time at which the wave power is reduced by 10 dB. Results indicate that the lifetime of whistler waves at lower frequencies is dramatically larger than those at higher frequencies and that rays injected at lower latitudes generally persist longer than those injected at higher latitudes in agreement with previous studies. An important characteristic of magnetospherically reflected (MR) whistlers is the strong tendency for whistler wave components at each frequency to eventually migrate to and settle into a multiply reflecting pattern at a specific (determined only by the wave frequency) L‐shell, at which the wave energy would persist indefinitely in the absence of Landau damping and other losses. Consideration of this behavior together with the typical power spectrum of a single, vertical, cloud‐to‐ground lightning stroke, allows the estimation of the relative MR whistler wave energy in the inner magnetosphere as a function of L‐shell. Results indicate that MR whistler energy deposition is maximized at the location of the slot region, suggesting that such MR whistlers launched by lightning discharges may be responsible for the enhanced diffusion rates and may play a more significant role than previously assumed in the formation and maintenance of the slot region between the inner and outer radiation belts.