Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsa51d1967k&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SA51D-1967
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
Standing Alfven waves can be excited on field lines in the magnetosphere by the field-line resonance (FLR) mechanism, and thus excited field-line oscillations can be monitored by using ground magnetometers. The frequency of the standing wave (FLR frequency below), observed on the ground, is useful for us to estimate the magnetospheric plasma mass density from the ground, because the FLR frequency decreases with increasing plasma mass along the field line. In the direction of latitude (on the ground) or L (in space), FLR at a specific frequency is active in a finite width, which is called the resonance width. The resonance width is larger if a larger portion of the wave energy in the magnetosphere is absorbed by the ionosphere. 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 the amplitude-ratio method and the 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, and provide the FLR frequency at the midpoint between the two magnetometer sites. However, a problem here is that the two methods can yield different values of the FLR frequency from the same dataset. The hodograph method solves this problem, because it merges the amplitude-ratio method and the cross-phase method into one method. Furthermore, it yields the FLR frequency at any latitude from the same dataset (although the estimation error increases with increasing distance from the observation sites). The hodograph method also provides the resonance width, but it is assumed to be a constant of latitude in the 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 confirmed the validity of this modification by applying the modified method to simulated datasets. We have also applied it to actually observed data; we show cases in which the resonance width systematically changes as a function of latitude.
Kawano Hideaki
Mann Ian R.
Pilipenko Viacheslav
Saita S.
Yumoto Kiyo
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