AMPTE-UKS observations of low frequency waves in the ion foreshock

Abstract

Ion and magnetic field measurements from the AMPTE-UKS spacecraft are analyzed to identify the hydromagnetic waves observed below the proton cyclotron frequency in the Earth's foreshock. The AMPTE-UKS SW mode is unique in providing high resolution in the solar wind phase space distribution. From the ion density, velocity and field fluctuations we evaluate, e.g., the Alfvén ratio, the cross-helicity and the parallel compressibility and compare them to the values of these transport ratios in a linear kinetic approximation. The waves are large amplitude, transverse and elliptically polarized and the majority propagate at small angles to the ambiental magnetic field. From the values of the transport ratios and polarization properties we identified both magnetosonic and Alfvénic fluctuations.