The roles of the magnetopause and plasmapause in storm-time ULF wave power enhancements

J. K. Sandhu is supported by NERC grants NE/P017185/2 and NE/V002554/2. I. J. Rae is supported by NERC grants NE/P017185/2 and NE/V002554/2, and STFC Consolidate grant ST/V006320/1. F. A. Staples is supported by a Science and Technology Funding Council (STFC) studentship. D. P. Hartley is supported under JHU/APL contract 921647 under NASA Prime contract NAS5-01072. M.-T. Walach is supported by NERC grant NE/T000937/1. T. Elsden is supported by a Leverhulme Trust Early Career Fellowship (ECF-2019-155) and the University of Leicester. K. R. Murphy is partially funded by NASA ROSES Guest Investigator 18-HGIO18_2-0122 and Space Weather Operations to Research 18-HSWO2R18-0010. We thank the EMFISIS instrument team for data provision.Ultra low frequency (ULF) waves play a crucial role in transporting and coupling energy within the magnetosphere. During geomagnetic storms, dayside magnetospheric ULF wave power is highly variable with strong enhancements that are dominated by elevated solar wind driving. However, the radial distribution of ULF wave power is complex - controlled interdependently by external solar wind driving and the internal magnetospheric structuring. We conducted a statistical analysis of observed storm-time ULF wave power from the Van Allen Probes spacecraft within 2012-2016. Focusing on the dayside (06 < magnetic local time ≤ 15), we observe large enhancements across 3 < L < 6 and a steep L dependence during the main phase. We consider how accounting for concurrent magnetopause and plasmapause locations may reduce statistical variability and improve parameterization of spatial trends over and above using the L value. Ordering storm time ULF wave power by L provides the weakest dependences from those considered, whereas ordering by distance from the magnetopause is more effective. We also explore dependences on local plasma density and find that spatially localized ULF wave power enhancements are confined within high density patches in the afternoon sector (likely plasmaspheric plumes). The results have critical implications for empirical models of ULF wave power and radial diffusion coefficients. We highlight the necessity of improved characterization of the highly distorted storm-time cold plasma density distribution, in order to more accurately predict ULF wave power.Publisher PDFPeer reviewe