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- Title
On the Estimation of the Ratio of ULF Wave Electric Fields in Space and the Magnetic Fields at the Ground.
- Authors
Warden, L. J.; Waters, C. L.; Sciffer, M. D.; Hull, A. J.
- Abstract
Three new methods for estimating a ratio of the ultralow frequency (ULF; 1–100 mHz) wave equatorial electric field amplitude in the Earth's magnetosphere to ground magnetic field amplitudes for field line resonances (FLR) are described. These methods use ratios of the time series extrema, ratios of the envelope waveform and the ratio of the spectral amplitude at the FLR frequency. These methods were applied to four ULF resonance intervals; three detected by the Van Allen Probe A spacecraft and one detected by the POLAR spacecraft. The intervals were conjoined with the CARISMA and IMAGE ground magnetometer arrays. The spectral ratio results for the Van Allen Probe intervals were approximately twice to three times the ratios estimated from the two time series based methods. The POLAR interval showed similar values across all three methods. The differences are attributed to broadband frequency signals that modify the time series amplitudes, while the spectral method avoids these off‐resonant frequencies. Based on the results of this study, a spectral based method for calculating the ratio at the FLR frequency is best. Plain Language Summary: A reliable method to remote sense the amplitude of electric fields of natural frequencies in space using ground magnetic fields would remove difficulties in obtaining sufficient spatial information of fields that are thought to energize electrons to high energies. One recently proposed method maps ground magnetic field variations, b, to electric field variations, e, in space via the ratio, e/b. However, there are different ways to calculate e/b and the uncertainties of this metric have not been reported. This paper presents a study of three methods used to calculate e/b and the uncertainty limits; a ratio of the peaks/troughs of the fields, ratios of each point of the envelope waveform of the fields, and a ratio of the spectral amplitude of the frequency where resonance occurs. These methods were applied to electric field data obtained from two satellites with corresponding magnetic field data taken from ground magnetic field detectors at the footprint of the resonant magnetic field line. It was found that the frequency‐based method gave two to three times larger values and smaller uncertainty values compared with the other methods. Based on the results of this study, the frequency‐based method is preferred for estimating e/b. Key Points: Three methods for estimating the ratio of ultralow frequency electric fields in space to magnetic fields on the ground, e/b, are presentedRatio values for 106 field line resonant events were calculate from spacecraft and ground data over September 25, 2012 to February 25, 2014The uncertainties in e/b for using the spectral method were consistently smaller than the other methods
- Subjects
ELECTRIC fields; ELECTRIC dipole transitions; VAN Allen radiation belts; ELECTROMAGNETIC fields; GEOMAGNETISM
- Publication
Journal of Geophysical Research. Space Physics, 2021, Vol 126, Issue 7, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2020JA029052