Introduction
In the terrestrial magnetosphere there is a transition region called the plasmapause, which separates the inner region populated by dense plasma of ionospheric origin (the plasmasphere) from the outer region of low plasma density (the plasmatrough). The propagation speed of MHD waves is lower in the plasmasphere than in the plasmatrough and theory had predicted that the spatial variation of the wave speed leads to trapping of compressional MHD waves in the plasmasphere. The trapped waves could be the source of magnetic field oscillations commonly observed on the ground and known as Pc3 and Pc4 pulsations. Detection of the trapped waves in the dayside plasmasphere was challenging since ULF waves originate from a variety of processes operating both inside and outside the magnetosphere and not many spacecraft had proper orbits and experiments to directly observe the waves. Observations from the THEMIS spacecraft provided the first convincing evidence of the trapped waves.
Observations
On June 21, 2008, the THEMIS-A spacecraft was moving outward in the dayside magnetosphere and crossed the plasmapause at ~2000 UT while other spacecraft observed ULF waves in the outer magnetosphere and in the solar wind (Figure 1). The plasmapause location was determined using the spacecraft electric field potential and the plasma mass density estimated from the observed frequencies of standing Alfvén waves.
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| Figure 1. The location of satellites for 1810-2110 UT on June 21, 2008, projected on the equatorial plane of the geocentric solar ecliptic coordinates. The spacecraft are THEMIS probes, abbreviated A through E, and the GOES-11 (G11) and -12 (G12) geostationary satellites. The region shaded in blue is a schematic plasmasphere.
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In the 3-hour period for which the orbit is shown, the electric and magnetic field experiments on THEMIS-A detected continuous ULF wave activity. The spectrograms of the measured fields (Figure 2) indicate that there were oscillations in the 10-20 mHz band when the satellite was in the plasmasphere, from ~1820 UT (L = 1.5) to ~2000 UT (L = 4.7). The oscillation is most prominent in the Ey and Bx components (referred to as the poloidal components), but it also accompanies a compressional magnetic field component Bz. Data from other spacecraft indicate ULF waves were present at each spacecraft but their spectral properties differed from those at THEMIS-A. From this comparison we infer that the 10-20 mHz oscillations were localized to the plasmasphere. In addition, the fact that the frequency of the plasmaspheric poloidal oscillation remained constant implies that the oscillation corresponds to an eigenmode of the whole plasmasphere.
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| Figure 2. Dynamic spectrograms of the electric field (Ex, Ey) and magnetic field (Bx, By, Bz) measured by the THEMIS-A spacecraft. The subscripts x, y, and z respectively denote field components radially outward (x) and azimuthally eastward (y) in the plane perpendicular to the ambient magnetic field, and parallel (z) to the ambient magnetic field. The location of the satellite is shown at the bottom using dipole-based coordinates L (equatorial field line radial distance in Earth radii), magnetic latitude MLAT (degrees) and magnetic local time MLT (hour).
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In general, the amplitude of ULF waves varies with both position and time, and it is necessary to remove the temporal variation in order to determine the spatial mode structure of waves observed from a moving spacecraft. We do this by using ULF waves detected on the ground as a reference signal. We chose the H component of the magnetic field observed at Lyford (L = 1.53) as the reference since the ground station was longitudinally close to THEMIS-A and detected ULF waves similar to those at the spacecraft. The results of the satellite-ground spectral analysis are shown in the upper part of Figure 3. The Ey component at THEMIS-A had an amplitude peak at L = 3.5, well within the plasmasphere, and maintained a nearly constant phase of 90° from L = 2 to L = 5.5 (see the left column). A similar feature was observed for Bx (middle column) except the cross phase was ~180°. By contrast, the compressional magnetic field component (Bz) exhibited an amplitude minimum at L = 3.8, which coincided with a phase shift from ~180° to 0 (right column). These amplitude and phase structures are consistent with a numerically simulated plasmaspheric trapped mode shown in the lower part of Figure 3.
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| Figure 3. (upper 3 rows) The radial structure of the 10-20 mHz oscillations observed by THEMIS-A, referenced to the H-component oscillations at the Lyford ground station (L = 1.5, longitudinally close to THEMIS-A). Different shadings are use to indicate the plasmasphere (L = 1.0-4.7) and the plasmapause transition region (L = 4.7-6.6). From top to bottom: the satellite to ground amplitude ratio; the coherence; the cross phase. (lower 2 rows) The amplitude and phase of the same three field components for trapped MHD waves obtained in a numerical simulation.
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Conclusions
Multipoint observations in space and on the ground enabled us to determine the polarization and mode structure of 10-20 mHz ULF waves in the inner magnetosphere and relate the mode structure to the radial profile of the plasma density. The observations match the mode structure of numerically modeled plasmaspheric trapping of MHD waves, providing convincing evidence of mode trapping in the real plasmasphere.
Source
Takahashi, K., J. Bonnell, K.-H. Glassmeier, V. Angelopoulos, H.J. Singer, P. Chi, R.E. Denton, Y. Nishimura, D.-H. Lee, M. Nose, and W.L. Liu (2010), Multpoint observation of fast mode waves trapped in the dayside plasmasphere, J. Geophys. Res., 115, A12247, doi:10.1029/2010JA015956.
Biographical Note
Kazue Takahashi is physicist at the Johns Hopkins University Applied Physics Laboratory. He has used data from various satellites to investigate the propagation modes and generation mechanisms of a variety of magnetospheric ULF waves and to use the waves in magnetospheric diagnostics.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu
