Turbulence in the high-altitude cusp: Cluster FGM and STAFF observations

High-resolution magnetic field data from Cluster Flux Gate Magnetometer (FGM) and the Spatio-Temporal Analysis of Field Fluctuations (STAFF) instruments are used to study turbulent magnetic field fluctuations during the high-altitude cusp crossing on 17th of March 2001. Despite the quiet solar wind conditions, the cusp was filled with magnetic field turbulence whose power correlates with sheared field-aligned plasma number flux. The slopes of the magnetic field wave power spectra are evaluated throughout the crossing and examples are found of magnetic field wave spectra with inertial range and dissipation range. The break between the ranges usually occurred in the vicinity of the local ion cyclotron frequency, indicating that the energy dissipation due to kinetic processes started at this frequency.However, intervals where the break was observed at lower and higher frequencies were also found. Sometimes strong peaks in the wave power close to local ion cyclotron frequency were observed, with secondary peaks at harmonics indicative of resonant processes between protons and the waves. We show that the observed frequency for the onset of the dissipation range can be calculated from the dispersion relation for kinetic Alfven waves (including finite Larmor radius and finite frequency effects), provided the parallel wavelength is in the range of 40-1500 km, which in turn implies very oblique propagation angles.