Coordinated polar spacecraft, geosynchronous spacecraft, and ground-based observations of magnetopause processes and their coupling to the ionosphere

摘要

In this paper, we present in-situ observations of processes occurring at the magnetopause and vicinity, including surface waves, oscillatory magneto-spheric field lines, and flux transfer events, and coordinated observations at geosynchronous orbit by the GOES spacecraft, and on the ground by CANOPUS and 210° Magnetic Meridian (210 MM) magnetometer arrays. On 7 February 2002, during a high-speed solar wind stream, the Polar spacecraft was skimming the magnetopause in a post-noon meridian plane for ～3 h. During this interval, it made two short excursions and a few partial crossings into the magnetosheath and observed quasi-periodic cold ion bursts in the region adjacent to the magnetopause current layer. The multiple magnetopause crossings, as well as the velocity of the cold ion bursts, indicate that the magnetopause was oscillating with an ～ 6-min period. Simultaneous observations of Pc5 waves at geosynchronous orbit by the GOES spacecraft and on the ground by the CANOPUS magnetometer array reveal that these mag-netospheric pulsations were forced oscillations of magnetic field lines directly driven by the magnetopause oscillations. The magnetospheric pulsations occurred only in a limited longitudinal region in the post-noon dayside sector, and were not a global phenomenon, as one would expect for global field line resonance. Thus, the magnetopause oscillations at the source were also limited to a localized region spanning ～ 4 h in local time. These observations suggest that it is unlikely that the Kelvin-Helmholz instability and/or fluctuations in the solar wind dynamic pressure were the direct driving mechanisms for the observed boundary oscillations. Instead, the likely mechanism for the localized boundary oscillations was pulsed reconnection at the magnetopause occurring along the X-line extending over the same 4-h region. The Pc5 band pressure fluctuations commonly seen in high-speed solar wind streams may modulate the reconnection rate as an indirect cause of the observed Pc5 pulsations. During the same interval, two flux transfer events were also observed in the magnetosphere near the oscillating magnetopause. Their ground signatures were identified in the CANOPUS data. The time delays of the FTE signatures from the Polar spacecraft to the ground stations enable us to estimate that the longitudinal extent of the reconnection X-line at the magnetopause was ～ 43° or ～ 5.2 R_E. The coordinated in-situ and ground-based observations suggest that FTEs are produced by transient reconnection taking place along a single extended X-line at the magnetopause, as suggested in the models by Scholer (1988) and Southwood et al. (1988). The observations from this study suggest that the reconnection occurred in two different forms simultaneously in the same general region at the dayside magnetopause: 1) continuous reconnection with a pulsed reconnection rate, and 2) transient reconnection as flux transfer events.