Quasi-periodic whistler mode emission in the plasmasphere as detected by the USAF/DSX spacecraft

We describe the quasi-periodic (QP) whistler-mode emissions found in the plasmasphere as detected by electric and magnetic instrumentation onboard the Demonstration and Science Experiments (DSX) spacecraft in medium Earth. The DSX spacecraft was launched on a SpaceX Falcon Heavy on 25 June 2019 as part of DoDs Space Test Program-2 mission and operations ceased on 31 May 2021. Over the course of the nearly 2-year mission, at least 45 episodes of whistler mode QP emissions were detected by the Broad Band Receiver (BBR) onboard DSX. Episodes were classified as QP emissions if they had clear unambiguous periodic events detected by both the electric antennae and search coil magnetic sensor in the BBR survey data at 30 second temporal resolution. Most of the QP episodes occurred in a frequency range between 1- 4 kHz, in a band previously identified by Van Allen Probes and Cluster investigators. However, episodes were also detected by DSX at higher frequencies - events in these episodes extending all the way to 15 kHz. We present our findings on these unusual high frequency events in the presentation herein. Specifically, these high frequency QP episodes tended to be observed at dawn and dusk when the spacecraft was at relatively high magnetic latitudes and on magnetic L-shells between 3-5. As derived from the onboard VLF sounder, a majority of these high frequency QP episodes also occurred as the spacecraft was exiting or entering the high density inner plasmasphere. Thus, there was a simultaneous strong gradient in density occurring with many of the high frequency QP episodes. Another unusual feature of these episodes is that individual up-drifting events making up the episode were found to sometimes occur concurrently in time: The high frequency portion of one up-drifting event overlapped in time with the low frequency portion of the subsequent event. This behavior of the QP emissions has not been previously emphasized and we consider how this temporal concurrence relates to the suggested QP source processes.