Observations and numerical simulations of the relationship between ULF wave power, radial Alfvén speed profile, and magnetopause location

Magnetospheric Ultra Low Frequency (ULF) waves in the Pc4-5 frequency range (2-22 mHz) affect the radiation belts via wave-particle interactions and coupling to other wave modes. ULF wave properties - including fast mode waves (FMW) - depend on both the radial Alfvén speed profile and magnetopause location. We characterize ULF wave properties using 12 years of Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite measurements. We bin these observations according to magnetopause location and type of Alfvén speed profile, examining how ULF wave properties vary in each case. The results are further compared to a series of numerical simulations corresponding to different magnetopause locations and Alfvén speed profiles. We use both the observations and simulations to determine how different ULF wave modes - including FMW - depend on both the radial Alfvén speed profile and magnetopause location. We show that the spatial distribution of wave energy at a given frequency depends on both parameters, but the dependence is stronger at certain frequencies.