Browsing by Author "Milling, David K."

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    Ground-based magnetometer determination of in situ Pc4–5 ULF electric field wave spectra as a function of solar wind speed
    (2012) Rae, I. Jonathan; Mann, Ian R.; Murphy, Kyle R.; Ozeke, Louis G.; Milling, David K.; Chan, Anthony A.; Elkington, Scot R.; Honary, Farideh; American Geophysical Union
    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.
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    ULF wave derived radiation belt radial diffusion coefficients
    (2012) Ozeke, Louis G.; Mann, Ian R.; Murphy, Kyle R.; Rae, I. Jonathan; Milling, David K.; Elkington, Scot R.; Chan, Anthony A.; Singer, Howard J.; National Aeronautics and Space Administration; American Geophysical Union
    Waves in the ultra-low-frequency (ULF) band have frequencies which can be drift resonant with electrons in the outer radiation belt, suggesting the potential for strong interactions and enhanced radial diffusion. Previous radial diffusion coefficient models such as those presented by Brautigam and Albert (2000) have typically used semiempirical representations for both the ULF wave’s electric and magnetic field power spectral densities (PSD) in space in the magnetic equatorial plane. In contrast, here we use ground- and space-based observations of ULF wave power to characterize the electric and magnetic diffusion coefficients. Expressions for the electric field power spectral densities are derived from ground-based magnetometer measurements of the magnetic field PSD, and in situ AMPTE and GOES spacecraft measurements are used to derive expressions for the compressional magnetic field PSD as functions of Kp, solar wind speed, and L-shell. Magnetic PSD results measured on the ground are mapped along the field line to give the electric field PSD in the equatorial plane assuming a guided Alfvén wave solution and a thin sheet ionosphere. The ULF wave PSDs are then used to derive a set of new ULF-wave driven diffusion coefficients. These new diffusion coefficients are compared to estimates of the electric and magnetic field diffusion coefficients made by Brautigam and Albert (2000) and Brautigam et al. (2005). Significantly, our results, derived explicitly from ULF wave observations, indicate that electric field diffusion is much more important than magnetic field diffusion in the transport and energization of the radiation belt electrons.