Abstract.Whistler emissions close to the magnetopause on the magnetospheric side are investigated using the four Cluster spacecraft. The waves are found to be generated in thin (electron-scale) sheets moving with the plasma drift velocity. A feature in the electron data coincides with the waves; hot magnetospheric electrons disappear for a few satellite spins. This produces or enhances a temperature anisotropy, which is found to be responsible for the generation of the whistler mode waves. The high energy electrons are thought to be lost through the magnetopause and we suggest that the field lines, on which the waves are generated, are directly connected to a reconnection diffusion region at the magnetopause.
Abstract. We use whistler waves observed close to the magnetopause as an instrument to investigate the internal structure of the magnetopause-magnetosheath boundary layer. We find that this region is characterized by tube-like structures with dimensions less than or comparable with an ion inertial length in the direction perpendicular to the ambient magnetic field. The tubes are revealed as they constitute regions where whistler waves are generated and propagate. We believe that the region containing tube-like structures extend several Earth radii along the magnetopause in the boundary layer. Within the presumed wave generating regions we find current structures moving at the whistler wave group velocity in the same direction as the waves.
Abstract. Simultanous Freja observations of precipitating keV electrons and electromagnetic emissions around half the local proton gyrofrequency are analyzed. The observed wave fields are used for reconstructing the energy density in wave vector space, the wave distribution function (WDF), and detailed observations of the electron distribution are used to analyze the linear stability of the plasma. We find that a local Landau resonance with precipitating electrons can generate electromagnetic waves having their wave vector (k) pointing obliquely relative to the background magnetic field. However, the observed parallel electron drift energy suggests that the unstable frequencies are located above the peak in the observed power spectrum and that the k spectrum predicted from the linear stability analysis does not overlap with the reconstructed WDF. This apparent inconsistency between the proposed instability mechanism and the available Freja data is resolved by placing the source region within 2400 km above Freja. By using a ray-tracing mapping of an instability at an assumed generation altitude down to Freja altitudes, we show that an electron Landau instability occurring above Freja can be made consistent with the observed wave frequency a.s well as the reconstructed WDF. The agreement that we obtain between data and theory leads to consistent picture of the generation of electromagnetic ion cyclotron ELF waves on auroral field lines.
In this paper we present an investigation of low‐frequency waves observed on auroral field lines below the acceleration region by the Swedish satellite Viking. The measured frequency spectra are peaked at half the local proton gyrofrequency, and the waves are observed in close connection with precipitating electrons. In order to obtain information about the distribution of wave energy in wave vector space, we reconstruct the wave distribution function (WDF) from observed spectral densities. We use a new scheme that allows us to reconstruct simultaneously the WDF over a broad frequency band. The method also makes it possible to take into account available particle observations as well as Doppler shifts caused by the relative motion between the plasma and the satellite. The distribution of energy in wave vector space suggested by the reconstructed WDF is found to be consistent with what is expected from a plasma instability driven by the observed precipitating electrons. Furthermore, by using UV images obtained on Viking, we demonstrate that the wave propagation directions indicated by the reconstructed WDFs are consistent with a simple model of the presumed wave source in the electron precipitation region.
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