Modulation of plasmaspheric hiss intensity by thermal plasma density structure

Plasmaspheric hiss is an electromagnetic whistler-mode emission confined to high density regions such as the plasmasphere and plasmaspheric plumes. We present a dayside THEMIS event showing that the hiss emission intensity is strongly modulated by thermal plasma density variations. The calculated local growth rate is small (less than 5 dB/R-E) and increases as the plasma density increases, which partly accounts for the modulation. Ray tracing shows that chorus rays, which eventually evolve into plasmaspheric hiss, are focused towards field lines of higher density and defocused away from field lines of lower density. We suggest that the observed modulation is caused by a combined effect of local cyclotron resonant amplification together with ray focusing during propagation.