Turbulence in the Earth's cusp region: k-filtering analysis

On April 13 2002, four Cluster spacecraft with separation up to 127 km measured similar turbulence in the Earth's mid-altitude cusp region during southward IMF Bz. Using the k-filtering technique, we obtain the propagation angle and dispersion relation of the turbulence. The propagation angle is quite large and ranges from 80° to 92°, meaning that the turbulence propagates perpendicularly to the background magnetic field. By comparing the dispersion relationship with the linear solution of the Vlasov kinetic theory obtained by WHAMP, we conclude that the turbulence is Kinetic Alfvén mode (KAWs). The regions of KAWs are located within an inhomogeneous high density plasma background, suggesting that it is the ion drift, not field aligned ion beam that plays an important role in the wave generation. The magnetic flied fluctuation spectrum resembles the classical Kolmogorov power law under the proton gyrofrequency ωci, but breaks and steepens near ωci, indicating that the KAW may be powered by a turbulent cascade transverse to the magnetic field from large MHD scales to proton gyroradius scales.