Ground-based identification of magnetospheric field line eigenfrequency as a continuous function of ground latitude

*Hideaki Kawano[1] ,Kiyohumi Yumoto [1],V. A. Pilipenko [2]
Yoshimasa Tanaka [1],Satoko Takasaki [1]
Masahide Iizima [3],Masahiro Seto [4]
Department of Earth and Planetary Sciences, Kyushu Unive[1]
Institute of the Physics of the Earth, Russia[2]
Tohoku University[3]
Tohoku Institute of Technology[4]

We apply for the first time the amplitude-phase gradient method (APGM) (Pilipenko and Fedorov, 1994) to actual ground magnetometer data. The data came from three stations around L=1.36 (latitudinal separation: 1.51 deg and 1.06 deg) in Japan. APGM uses the H-component data from a pair of stations and provides the eigenfrequency and the resonance width as a continuous function of latitude, even outside the latitudes covered by the two stations. Applying APGM to our two station pairs (made from the three stations) yields two latitude profiles for each of the eigenfrequency and the resonance width: The two profiles are very close to each other, demonstrating the consistency and usefulness of APGM. The application result also shows clearly that the eigenfrequency at low latitudes increases with increasing latitude.

Ground observations of magnetospheric field line eigenfrequency are useful in continuously monitoring the magnetospheric plasma density. As a technique to make the most use of the ground magnetometer data in obtaining the eigenfrequency, we apply the amplitude-phase gradient method (APGM) (Pilipenko and Fedorov, 1994) to actual data for the first time: We apply it to data from three stations at L=1.32, 1.36, and 1.39 (latitudinal separation: 1.51 deg and 1.06 deg) almost aligned along a magnetic meridian in Japan. The data were taken when Pc 3 pulsations took place. APGM uses the H-component data from a pair of stations and provides the eigenfrequency as a continuous function of latitude, even outside the latitudes covered by the two stations. Applying APGM to our two station pairs (made from the three stations) yields two latitude profiles of the eigenfrequency: The two profiles are very close to each other, demonstrating the consistency and usefulness of APGM. Also obtained are two latitude profiles of the resonance width, also very close to each other. We also present a new technique to obtain one latitude profile of the eigenfrequency and that of the resonance width from any numbers of stations; we apply it to our three station data, and show its consistency and usefulness. The application result unambiguously shows that the eigenfrequency at low latitudes increases with increasing latitude, which is ascribed to mass loading on field lines of ionospheric heavy ions. The result also indicates a large damping rate of the eigen-oscillations.