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Due to differences in solar illumination, a geomagnetic field line may have one footpoint in a dark ionosphere while the other ionosphere is in daylight. This may happen near the terminator under solstice conditions. In this situation, a resonant wave mode may appear which has a node in the electric field in the sunlit (high conductance) ionosphere and an antinode in the dark (low conductance) ionosphere. Thus, the length of the field line is one quarter of the wavelength of the wave, in contrast with half-wave field line resonances in which both ionospheres are nodes in the electric field. These quarter waves have resonant frequencies that are roughly a factor of 2 lower than the half-wave frequency on the field line. We have simulated these resonances using a fully three-dimensional model of ULF waves in a dipolar magnetosphere. The ionospheric conductance is modeled as a function of the solar zenith angle, and so this model can describe the change in the wave resonance frequency as the ground magnetometer station varies in local time. The results show that the quarter-wave resonances can be excited by a shock-like impulse at the dayside magnetosphere and exhibit many of the properties of the observed waves. In particular, the simulations support the notion that a conductance ratio between day and night footpoints of the field line must be greater than about 5 for the quarter waves to exist.
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Nonlinear Least Squares Fitting Technique for the Determination of Field Line Resonance Frequency in Ground Magnetometer Data: Application to Remote Sensing of Plasmaspheric Mass Density
Abstract The accurate determination of the field line resonance (FLR) frequency of a resonating geomagnetic field line is necessary to remotely monitor the plasmaspheric mass density during geomagnetic storms and quiet times alike. Under certain assumptions the plasmaspheric mass density at the equator is inversely proportional to the square of the FLR frequency. The most common techniques to determine the FLR frequency from ground magnetometer measurements are the amplitude ratio (AR) and phase difference (PD) techniques, both based on geomagnetic field observations at two latitudinally separated ground stations along the same magnetic meridian. Previously developed automated techniques have used statistical methods to pinpoint the FLR frequency using the AR and PD calculations. We now introduce a physics‐based automated technique, using nonlinear least squares fitting of the ground magnetometer data to the analytical resonant wave equations, that reproduces the wave characteristics on the ground, and from those determine the FLR frequency. One of the advantages of the new technique is the estimation of physics‐based errors of the FLR frequency, and as a result of the equatorial plasmaspheric mass density. We present analytical results of the new technique, and test it using data from the Inner‐Magnetospheric Array for Geospace Science ground magnetometer chain along the coast of Chile and the east coast of the United States. We compare the results with the results of previously published statistical automated techniques.
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- Award ID(s):
- 1848730
- PAR ID:
- 10444626
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 126
- Issue:
- 2
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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