Improved hodograph method applied to ground magnetometer data to determine and error-estimate the field-line eigen-frequency

Details Improved hodograph method applied to ground magnetometer data to determine and error-estimate the field-line eigen-frequency Improved hodograph method applied to ground magnetometer data to determine and error-estimate the field-line eigen-frequency

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsa41b1730k&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SA41B-1730

Physics

[2730] Magnetospheric Physics / Magnetosphere: Inner, [2740] Magnetospheric Physics / Magnetospheric Configuration And Dynamics, [2752] Magnetospheric Physics / Mhd Waves And Instabilities, [2794] Magnetospheric Physics / Instruments And Techniques

Scientific paper

Magnetospheric magnetic field lines eigen-oscillate in the frequency range of ULF waves. A generation mechanism of the eigen-oscillations is the field-line resonance (FLR). The eigen-oscillations are useful for us to estimate the magnetospheric plasma mass density from the ground, because the eigen-frequency (FLR frequency below) decreases with increasing plasma mass along the field line. However, FLR frequencies are often difficult to identify in the ground magnetometer data, because different kinds of waves with large amplitudes are often superposed onto the FLR signal and mask the FLR signal. As countermeasures to this problem, methods called amplitude-ratio method and cross-phase method have been used; these methods take the difference between the data from two magnetometers that are latitudinally separated by an order of 100km. However, a problem here is that the two methods can yield different values of the FLR frequency from the same dataset. Another (related) problem is that the two methods do not provide a statistically accurate way to calculate an estimation error (error bar) of the obtained FLR frequency. Hodograph method solves the former problem, because it merges the amplitude-ratio method and the cross-phase method into one method. Furthermore, it can estimate the resonance width. However, the resonance width is assumed to be a constant of latitude in the hodograph method, while in reality the resonance width can be a function of latitude. To solve this problem, we have newly introduced a latitude dependence of the resonance width into the hodograph method. We have tested the validity of this modification by applying the modified method to simulated datasets, and the result is successful. That is, as a result of applying the improved method to the simulated datasets, we have obtained the latitude dependence of the resonance width close to that assumed in the simulation data. We have also obtained error bars correctly. We have also successfully applied the improved hodograph method to actually observed data; the results are presented at the meeting.

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