Magnetospheric seismology using multiharmonic toroidal waves observed at geosynchronous orbit

摘要

The frequencies of the harmonics of standing Alfvén waves depend on the field line distribution of mass that sustains the waves. This fact is the basis of inversion techniques to estimate the plasma mass density variation along the magnetic field from the frequencies of observed ultra-low-frequency waves (normal mode magnetospheric seismology). However, the technique has not been used extensively because it is difficult to accurately determine the frequencies. Wave observations from geostationary satellites provide the best chance of a successful implementation of the technique because the satellites accumulate the spectral information of the harmonics with minimal variations in frequency. We have statistically studied the frequency of multiharmonic toroidal waves detected by the GOES 5 geostationary satellite. We quantify the relationships among the harmonics using the frequency ratio f n /f 3, where f n is the frequency of the nth harmonic (n = 1 for the fundamental mode). The third harmonic is chosen for the reference frequency because it is the most ubiquitous mode at GOES 5. Using all data acquired in 1986, we find that f 1/f 3 changes from 0.25 in the early morning to 0.22 in the afternoon, whereas the other frequency ratios are more nearly constant: f 2/f 3, = 0.634–0.646, f 4/f 3 = 1.36–1.37, and f 5/f 3 = 1.73–1.74. The frequencies are in general inconsistent with the power law density model. When the frequencies are used in the density modeling technique developed by Denton et al. (2001, 2004), we find an equatorial mass density enhancement in the afternoon local time sector but not on the dawnside.