Pi2 pulsations in a small and strongly asymmetric plasmasphere

Abstract

[1] We study a Pi2 pulsation that occurred at $1520 UT on 29 August 2000. This Pi2 event was observed at ground stations from high (geomagnetic latitude $65°) to low latitudes ($17°) near midnight with an identical waveform and oscillated with a frequency of $11 mHz. During the event, a global image of the plasmasphere was obtained from the IMAGE satellite, and the location of the plasmapause was clearly identified. The plasmasphere was small and strongly asymmetric in longitude. The plasmapause was located at L $ 2.4, 3.8, and 3.3 near the duskside, midnight, and the dawnside, respectively. Using a magnetospheric mass density model constructed from the IMAGE satellite data and ground-based data, we examine whether the Pi2 pulsation observed inside the plasmasphere can be explained by a plasmaspheric cavity mode. We find that the frequency of 11 mHz is too low for a cavity mode in the plasmasphere. Thus the plasmaspheric cavity mode is not an appropriate model for our Pi2 event observed at midlatitudes and low latitudes. We discuss what determines its period and waveform inside such a small and asymmetric plasmasphere.Citation:

Review of Pi2 Models

“…For example, Kim et al (2005b) reported a significant mismatch between the frequency of an observed Pi2 (11 mHz) and the PCR frequency (28 mHz) estimated using (1) for a realistic V Aeq profile in the magnetosphere derived by Denton et al (2006). Even with the assumption of a larger radial distance of the effective outer boundary in consideration of a possible extension of the mode beyond the plasmasphere (the PVR effect, Sect.…”

“…2.2), the model frequency was higher than the observed frequency. Although Kim et al (2005b) favored a BBF model for their particular Pi2 event (see Sect. 3.3 for more discussion on the BBF-driven mechanism), caution is in order regarding the mass density estimates employed by the model (see below for additional comments).…”

“…The radial profile of the mass density, which controls MHD wave speed, can differ significantly from that of electrons (Fraser . Constructed using various ground and satellite observations near the time of the Pi2 event, the mass density model adopted by Kim et al (2005b) was far more realistic than statistically derived plasmasphere models. Still, the mass density was estimated from toroidal wave frequencies that were measured away from midnight, which implies a considerable uncertainty in the mass density at the Pi2 generation region.…”

Initial deflection of middle‐latitude Pi2 pulsations in the premidnight sector: Remote detection of oscillatory upward field‐aligned current at substorm onset

“…According to Lester et al [], the SCW model is reasonably successful at explaining middle‐latitude magnetic bay structures and the orientations of the major axes of Pi2 hodograms, although the model oversimplifies substorm onset currents. The SCW model for Pi2 pulsations is presently accepted as useful for determining the approximate locations of the upward/downward field‐aligned currents (FACs) of SCW systems [e.g., Kitamura et al , ; Kim et al , ]. However, some unsolved questions remain regarding the SCW model for Pi2 pulsations.…”

Excitation mechanism of low‐latitude Pi2 pulsations: Cavity mode resonance or BBF‐driven process?

“…Thus these researchers concluded that the Pi2 period is determined by the temporal structure of BBF. Kim et al [2005a] examined a Pi2 pulsation when the plasmasphere becomes small and highly asymmetric. They concluded that this Pi2 pulsation is unlikely to be caused by the cavity mode and is expected to be associated with BBFs.…”