Statistical Relationship Between ULF Wave Power and Magnetopause Location Using THEMIS Satellite Observations: Implications for Fast Mode Wave Propagation

Magnetohydrodynamic fast mode waves (FMW) in the Earth's magnetosphere can impact the radiation belts directly via wave-particle interactions and indirectly via coupling to other wave modes. FMW properties - including frequency, amplitude and spatial distribution - depend in part on the magnetopause location. We use 11 years of Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite measurements to statistically characterize ULF wave magnetic and electric field power in the Pc4-5 range (2-22 MHz) as a function of frequency, magnetopause location, and geomagnetic activity level. We show that power generally decreases with distance from the magnetopause on the dayside, as expected for upstream energy sources. However, the spatial distribution depends on frequency and the electric/magnetic field component of interest, with frequency dependent enhancements in wave power at particular locations. We interpret these results in the context of theory, numerical simulations, and past observations of FMW. Finally, we discuss the implications of our results for wave-particle interactions in the inner magnetosphere.