Whistler-mode quasiperiodic emissions and magnetospheric line radiation: Fine inner structure

Abstract

The intensity of magnetospheric whistler-mode waves at frequencies of a few kilohertz sometimes exhibits nearly periodic temporal or frequency modulation. Events exhibiting temporal modulation are typically referred to as quasiperiodic (QP) emissions, while those with frequency modulation are commonly known as magnetospheric line radiation (MLR). Although these events are rather routinely observed both by spacecraft and ground-based instruments, their exact formation mechanism is still not fully understood. We use high-resolution burst mode data, measured by the Van Allen Probes spacecraft in the equatorial region at larger radial distances, by the low-altitude DEMETER spacecraft, and by the Kannuslehto and PWING ground-based instruments, to demonstrate and investigate the fine inner structure of these events. We show that such a fine inner structure is often present for both QP and MLR events. Detailed wave propagation and timing analysis reveals that it corresponds to the wave bouncing between the hemispheres. We discuss the possible implications of these observations for understanding the event formation mechanisms.