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

Abstract

[1] 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.

The influence of solar wind variability on magnetospheric ULF wave power

“…This threshold is chosen to separate the solar wind regimes dominated by fast and slow solar wind (Bothmer and Daglis, 2007). To illustrate the increase in ULF PSD power under higher mean solar wind speed known from previous studies (Pahud et al, 2009;Rae et al, 2012), the median values of GOES PSDs for all solar wind conditions whenever the mean solar wind speed exceeds 400 and 500 km s −1 are shown, respectively, by dotted lines and dashed lines. This demonstrates the increase in ULF PSD during geomagnetically disturbed periods.…”

“…This MLT sector is chosen because earlier studies of the solar wind control (e.g. Rae et al, 2012) demonstrated the solar wind control to be most effective in the dayside and morning sectors. Other MLT sectors (analysed in this study but not shown here) demonstrate similar dependencies on solar wind variability.…”

“…In particular, the solar wind speed has been shown to be the crucial parameter in controlling ULF power (Greenstadt et al, 1979;Engebretson et al, 1998;Mathie and Mann, 2001;Mann et al, 2004), especially in the dayside and morning sectors and peaking at magnetic shells L = 6-7 (e.g. Rae et al, 2012). It has also been suggested that ULF pulsations in Pc 4-5 range could be directly driven by periodic solar wind pressure pulses (Kepko et al, 2002) though the physical mechanism of energy transfer across the bow shock and magnetopause remains unclear.…”

“…Time intervals when the GOES spacecraft may have been located near, or outside of, the magnetopause have been excluded from this data set by estimating the magnetopause stand-off distance using simple pressure balance relation and excluding the hourly intervals when the magnetopause is expected to be within 8 R E radial distance. The PSDs were calculated using a 1 h fast Fourier transform, as in Rae et al (2012) and Ozeke et al (2012).…”

Discovery of the action of a geophysical synchrotron in the Earth’s Van Allen radiation belts

“…Typically FLRs have maximum power in the auroral zone around LB7-9 (ref. 18). However, in this case large amplitude monochromatic ULF wave fields peaked in the heart of the outer Van Allen belt.…”