On the dependence of storm time ULF wave power on magnetopause location: Impacts for ULF wave radial diffusion

Ultralow frequency (ULF) waves play a crucial role in energetic particle dynamics in the inner magnetosphere. We examine the role of the magnetopause location in controlling the amplitude and penetration of ULF waves within the inner magnetosphere during 63 coronal mass ejection (CME)‐driven and corotating interaction region (CIR)‐driven geomagnetic storms. Significantly, at the time when the magnetopause is most compressed, ULF wave power increases and penetrates to the deepest L shells. Most likely this is explained by proximity to the energy source and accumulation of energy within a smaller volume through solar wind‐magnetopause‐magnetosphere coupling, and changes in the storm time Alfvén continuum resulting from variations in the cold plasma density. The observed ULF wave power is consistently larger than Kp ‐dependent statistical estimates—especially in the heart of the outer radiation belt. This has important implications for radiation belt dynamics, including main phase loss and storm time ULF wave radial diffusion.