The location of the spectral break in compressible fluctuations in the solar wind.

We use density deduced from spacecraft potential to study the power spectral density (PSD) of fluctuations in the solar wind. Typically plasma measurements do not have high enough time resolutions to resolve ion kinetic scales. However, calibrated spacecraft potential allows much higher time resolutions to resolve the spectral break between ion inertial and kinetic ranges. Fast Survey mode data from Magnetospheric MultiScale data are used when the spacecraft were in the pristine solar wind. We find that the density spectra' morphology differs from the magnetic field fluctuations, with a flattening often occurring between inertial and kinetic ranges. We find that the spectral break of the trace magnetic field fluctuations occurs near the expected frequency for cyclotron resonance or magnetic reconnection. Meanwhile, the spectral break at the start of the ion kinetic range for density fluctuations is often at a higher frequency when compared to the magnetic field. We discuss possible interpretations for these observations. Two plausible scenarios are presented; 1. the compressive fluctuations consist of a slow wave cascade at large scales before kinetic Alfven waves become dominant at smaller scales 2. charge separation begins to occur at these scales, and the Hall electric field starts to play a role.