An unusual enhancement of low‐frequency plasmaspheric hiss in the outer plasmasphere associated with substorm‐injected electrons

Abstract

Both plasmaspheric hiss and chorus waves were observed simultaneously by the two Van Allen Probes in association with substorm‐injected energetic electrons. Probe A, located inside the plasmasphere in the postdawn sector, observed intense plasmaspheric hiss, whereas Probe B observed chorus waves outside the plasmasphere just before dawn. Dispersed injections of energetic electrons were observed in the dayside outer plasmasphere associated with significant intensification of plasmaspheric hiss at frequencies down to ~20 Hz, much lower than typical hiss wave frequencies of 100–2000 Hz. In the outer plasmasphere, the upper energy of injected electrons agrees well with the minimum cyclotron resonant energy calculated for the lower cutoff frequency of the observed hiss, and computed convective linear growth rates indicate instability at the observed low frequencies. This suggests that the unusual low‐frequency plasmaspheric hiss is likely to be amplified in the outer plasmasphere due to the injected energetic electrons.