Spectral Slope Variation at Proton Scales from Fast to Slow Solar Wind

We investigated the behavior of the spectral slope of interplanetary magnetic field fluctuations at proton scales for selected high-resolution time intervals from the WIND and MESSENGER spacecraft at 1 AU and 0.56 AU, respectively. The analysis was performed within the profile of high-speed streams, moving from fast to slow wind regions. The spectral slope showed a large variability between -3.75 and -1.75 and a robust tendency for this parameter to be steeper within the trailing edge, where the speed is higher, and to be flatter within the subsequent slower wind, following a gradual transition between these two states. The value of the spectral index seems to depend firmly on the power associated with the fluctuations within the inertial range; the higher the power, the steeper the slope. Our results support previous analyses suggesting that there must be some response of the dissipation mechanism to the level of the energy transfer rate along the inertial range.