Solar wind compressible turbulence near proton scales: Cluster observations

The solar wind compressible turbulence attracts more attention recently due to its possible role in plasma heating. Using the plasma density and magnetic field measured simultaneously by the Cluster Cl probe, we statistically investigate the wave characteristics of the proton-scale compressible turbulence. In most cases, the density and magnetic field strength fluctuations almost show no positive correlation, suggesting the absence of fast mode waves. Moreover, the magnetic compressibility C-b as measured by the level of magnetic fluctuations parallel to the background field, is enhanced continuously around proton scales and is consistent with kinetic Alfven wave (KAW) prediction. Interestingly, kinetic slow mode waves (KSW) are identified in a few cases through large values of C-b and plasma compressibility C-p, together with the the anti-correlation between n and B. We suggest that both KAW and KSW can account for the compressible fluctuations energy, while their relative proportions under certain plasma condition require further studies.