The distribution of plasma density is one of the least studied physical parameters in the magnetosphere. Improving the understanding of the plasma density distribution and its variations will provide valuable information for space weather modeling and prediction. With adequate ground observations, magnetoseismology can be used to probe the magnetospheric structure in a much less costly way compared to satellite experiment. The mass density derived from magnetoseismic methods is also unique and can complement other observations that measure charge density.
1. Normal-Mode Magnetoseismology
The normal mode frequencies are dependent on the Alfvén velocity along the field line, which is a function of magnetic field magnitude and mass density. Because magnetospheric field is relatively well known, the normal mode frequencies can be used to derive the mass density along the field line. Although it is possible to measure the resonant frequencies of a field line by a single ground station, the result can sometimes be spurious because of the interference from the source wave. When two stations that are located on the same longitude but closely separated in latitude are available, the gradient technique can be used to detect the resonant frequencies of the field line situated midway between the two stations because the wave amplitude and phase change dramatically near the resonance point. This method has been found very effective even when the resonance is less than moderate. The spectrogram figure below shows an example of resonant frequency observations during a strong magnetic storm by two low-latitude stations. The substantial rise in resonant frequency in the spectrogram implies the strong depletion of the plasmasphere at that time.
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