On the Causality of Chorus Wave Acceleration: A Remarkable Correlation Between Storm-time ULF wave Power Profiles and the POES VLF Chorus Wave Proxy

Abstract:

VLF chorus waves are one of the most popular candidate mechanisms for explaining the efficient acceleration of electrons in the Earth’s radiation belts. To overcome the scarcity of global scale in-situ chorus wave measurements, a number of studies have derived an assumed indirect measure of VLF activity from electron precipitation measurements by the POES satellites in low-Earth orbit. Such studies have then compared the VLF wave proxy to the response in terms of Van Allen belt electron acceleration versus non-acceleration events, and used the resulting correlation to argue for an essential role for VLF chorus acceleration (Li et al., 2015). Here we compare the global spatial and temporal evolution of ULF wave power measured with multiple ground magnetometer arrays to the L-shell and temporal characteristics of the POES VLF proxy for the same 33 storms studied by Li et al. (2015). Remarkably, we find that the Pc5 ULF power distributions for every single storm have an almost identical spatial (L-shell) and temporal evolution at the chorus wave proxy, especially for L<=6. Since it is energetically unfeasible for chorus waves to produce the associated ULF wave power, then either the ULF waves play a direct role in generating an electron precipitation signal corresponding to the chorus proxy, or there is a common driver producing both chorus waves and ULF waves simultaneously and in the same place. In either case, the assumed essential role for VLF waves in separating acceleration and non-acceleration events is challenged, not least since an equally valid interpretation would identify the ULF waves as the causative agent. An alternative explanation for the correspondence between the ULF waves and the chorus proxy is contamination of the proxy by ULF wave driven precipitation. Future simulations will address whether the observed ULF wave amplitudes are themselves able to produce the observed Van Allen belt response, thereby directly testing the alternative hypothesis that it in fact ULF waves and not VLF waves which are the causative agent of Van Allen belt electron acceleration for these storms.