Abstract
Abstract. A case study of a poloidal ULF pulsation near the dayside plasmapause is presented based on Cluster observations of magnetic and electric fields. The pulsation is detected close to the magnetic equatorial plane at L shells L=[4.4, 4.6] and oscillates with a frequency of f=23 mHz. Investigating the wave energy flux reveals the standing wave nature of the observed pulsation. An estimation of the azimuthal wave number exposes a narrow azimuthal structure of the wave field with m≈160. Spatial and temporal characteristics of the pulsation are analyzed in detail by representing data in a field line related coordinate system and a range-time-intensity representation. This allows an estimation of both the spatial extension of the wave field in the radial direction and its temporal decay rate. The analysis furthermore indicates that the same field lines are excited to a standing wave oscillation twice. Furthermore an accurate identification of a phase jump of the wave field across L shells is possible. Comparing the radial localization of the detected wave with theoretically expected field line eigenfrequencies reveals that the wave field is confined in the Alfvén resonator at the outer edge of the plasmapause.
Highlights
A wide variety of magnetohydrodynamic waves occur in magnetospheric regions of the Earth excited by different pro-cesses inside the magnetosphere and the solar wind
Toroidal oscillations are believed to be driven by e.g. solar wind impulses (Allan et al, 1986) and Kelvin-Helmholtz instabilities at the magnetopause (Fujita et al, 1996; Engebretson et al, 1998) generating a compressional wave which propagates across magnetic field lines
Due to the process known as field line resonance (Tamao, 1965; Southwood, 1974), the compressional wave can be transformed to an Alfven wave propagating parallel to the magnetic field
Summary
A wide variety of magnetohydrodynamic waves occur in magnetospheric regions of the Earth excited by different pro-. The spatial structure of the resulting toroidally polarized, standing Alfven wave is expected to show an almost Gaussian amplitude variation in radial direction, i.e. across L shells, accompanied with a 180◦ change in the wave phase around the resonant magnetic field line (Southwood, 1974). Both spatial features were detected in ULF pulsations by ground magnetometer and ionospheric observations, respectively The wave polarization changes from poloidal to toroidal An explanation for this spatio-temporal behavior can be given by considering phase mixing effects on field lines excited independently and oscillating with their own eigenfrequencies The investigation of the spatiotemporal structure of the observed wave field is favored by a Cluster configuration with spacecraft separation of the order of 0.2 RE
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