Ground‐based magnetometer determination of in situ Pc4–5 ULF electric field wave spectra as a function of solar wind speed

We present a statistical characterization of ground‐based ultra‐low‐frequency (∼1–15 mHz) magnetic wave power spectral densities (PSDs) as a function of latitude (corresponding to dipole L‐shells from L∼2.5–8), local time, and solar wind speed. We show a clear latitudinal dependence on the PSD profiles, with PSDs increasing monotonically from low‐ to auroral zone latitudes, where PSDs are peaked before decay in amplitude at higher latitudes. In general, ULF wave powers are highest on the nightside, followed by the local morning, noon, and finally dusk sectors, and are well‐characterized and well‐ordered by solar wind speed at all MLTs spanning L∼2.5–8. A distinct peak in PSD in the 2–8 mHz frequency range above a background power law is evident at most stations studied in this paper, demonstrating a significant non power law like component in the ULF wave power spectrum, in particular at high solar wind speeds. We conclude that field line resonance (FLR) behavior in the magnetosphere is most likely responsible for the peak in PSD, and that such peaks should be included in any radiation belt radial diffusion model addressing radiation belt dynamics. Furthermore, we utilize a model in order to map the ground‐based magnetic ULF wave power measurements into electric fields in the equatorial plane of an assumed dipole magnetic field, and find excellent agreement with the in situ CRRES electric fields shown by Brautigam et al. [2005], clearly demonstrating the utility of ground‐based measurements in providing reliable estimates of ULF electric field PSD for nowcast input into radiation belt radial diffusion models.