Owen Roberts (Space Research Institute, Graz): Sub-ion scale plasma turbulence in the solar wind and the saturation of intermittency
Abstract: Measurements of magnetic field and density in the solar wind show disordered fluctuations over a large range of scales. Fluctuations, at large scales (much larger than the ion gryo/inertial length), are often observed to have Gaussian statistics but develop heavier tails increasing their kurtosis as the scale decreases. The non-Gaussian fluctuations are characteristic of coherent structures e.g. current sheets/magnetic vortices and are often associated with temperature increases, and could be sites for the dissipation of energy. In the sub ion range (below the ion inertial/gyro lengths) the nature of the fluctuations is less clear. In this range measurements are challenging due to the requirements for high sensitivity and time resolution. For magnetic field measurements at one astronomical unit, the most sensitive magnetometer f>1 Hz is the Cluster Search coil magnetometer. For density measurements, high time resolution data can be obtained by calibrating spacecraft potential data with a sampling rate of 8kHz available from the magnetospheric multiscale mission. The multiple spacecraft of the Cluster and MMS missions also allow fluctuations to be calculated using a time lag (single spacecraft) and spatial lags (pairs of spacecraft) allowing the anisotropy of fluctuations to be investigated. The evolution of the scale-dependent fluctuations from inertial scales to sub ion scales shows an increase at large scales and a flattening and a plateau at sub ion scales. Differences are also seen between time lags (Taylor shifted to a spatial scale), and spatial lags are also observed indicating the anisotropy of the fluctuations.