Non-linear Least Square 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 and phase difference
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 amplitude
ratio and phase difference calculations. We now introduce a
physics-based automated technique, using non-linear least square 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 (iMAGS)
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.