Abstract
Abstract. Oscillating magnetic field lines are frequently observed by spacecraft in the terrestrial and other planetary magnetospheres. The CLUSTER mission is a very suitable tool to further study these Alfvén waves as the four CLUSTER spacecraft provide for an opportunity to separate spatial and temporal structures in the terrestrial magnetosphere. Using a large scaled configuration formed by the four spacecraft we are able to detect a poloidal Ultra-Low-Frequency (ULF) pulsation of the magnetic and electric field in order to analyze its temporal and spatial structures. For this purpose the measurements are transformed into a specific field line related coordinate system to investigate their specific amplitude pattern depending on the path of the CLUSTER spacecraft across oscillating field lines. These measurements are then compared with modeled spacecraft observations across a localized poloidal wave resonator in the dayside plasmasphere. A detailed investigation of theoretically expected poloidal eigenfrequencies allows us to specify the observed 16 mHz pulsation as a third harmonic oscillation. Based on this we perform a case study providing a clear identification of wave properties such as an spatial scale structure of about 0.67 RE, the azimuthal wave number m≈30, temporal evolution, and energy transport in the detected ULF pulsations.
Highlights
To processes such as magnetic reconnection or collisionless shock waves the process of resonant mode coupling or magnetospheric field line resonance is a physical process whose knowledge we owe ground-based and in-situ observation of space plasmas using spacecraft observations
The theoretical basis for resonant coupling processes was established by Tamao (1965), Chen and Hasegawa (1974), and Southwood (1974) who were the first to provide a theoretical framework to understand the resonant coupling between compressional waves with small azimuthal wave number m and Alfven waves in an inhomogeneous plasma
Considering a fast mode type perturbation with its source-free current in an inhomogeneous medium with spatially changing Alfven velocity current continuity requires the fast mode carried current to be partially closed via field-aligned currents driving Alfvenic perturbations carried by a curl-free polarization current system
Summary
The physical resonant mode coupling can be understood considering electric current continuity. Each of these additional currents influences wave dispersion and propagation velocity of ULF waves observed in the magnetosphere Based on this Leonovich and Mazur (1990, 1993, 1995) and Klimushkin (1998a) have derived a rather general theoretical framework to describe the dispersion characteristics of ULF waves in more complex background plasma situations. With the present study we intend to initiate a series of studies on poloidal ULF waves in the outer region of the plasmasphere/inner region of the magnetosphere In this first report we shall demonstrate the complex spatio-temporal structure of wave fields as seen by the four CLUSTER spacecraft, introduce a new way of representing the observed wave field, and use the theoretical framework of Leonovich and Mazur (1990, 1993, 1995) and Klimushkin (1998a) to model the observed wave fields. Fluxgate magnetometer (FGM) measurements (Balogh et al, 2001) and electric field observations of the EFW instrument (Gustafsson et al, 2001) onboard CLUSTER are analyzed
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