Abstract
The present paper deals with spatial distributions of the fluctuations observed in front of the bow shock by the THEMIS B spacecraft and their compressibility. Alfvénic mode are the most frequent fluctuations observed because of their incompressive nature (since they are associated with σB) and survive much longer (are difficult to be damped).
While a nature and intensity of compressive fluctuations (systematically vary with the stream structure) are strongly damped in the solar wind shortly after they are generated. The preliminary study reveals (1) different behaviors of the compressive and Alfvén-like components magnetic field fluctuations leading to compressibility in front of the bow shock, (2) a strong dependence of the fluctuation compressibility on the Alfvénic Mach number of the incoming solar wind, and (3) similar behavior of fluctuations of the compressive component of the magnetic field and ion density.