Using two ground stations to identify magnetospheric field line eigenfrequency as a continuous function of ground latitude

Ground observations of magnetospheric field line eigenfrequency are useful in continuously monitoring the magnetospheric plasma density. The amplitude-phase gradient method (APGM) [Pilipenko and Fedorov, 1994] is a technique which is applied to H-component magnetometer data from two ground stations aligned along a meridian; as a result, it yields the eigenfrequency and the resonance width as continuous functions of latitude. This feature (yielding the two quantities at any latitudes from two stations' data) is unique to APGM: Classic gradient methods yield the two quantities only at the midpoint of the two stations. This paper presents the first time APGM is applied to actual data: We apply it to two pairs of stations, independently, to test if the two results agree. The two pairs are made from three stations (one overlapped) almost aligned along a meridian, at L = 1.32, 1.36, and 1.39 (latitudinal separation: 1.51° and 1.06°). The data were taken when Pc3 pulsations took place. The resultant two latitude profiles of the eigenfrequency (resonance width) are very close to each other, demonstrating the consistency and usefulness of APGM. We also present a new technique to obtain one latitude profile of the eigenfrequency and that of the resonance width from any number of stations; we apply it to our three stations' 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 eigenoscillations.