The solar wind power spectrum observed from Cluster: insights in three dimensions and at sub ion scales
In this talk I will summarise some recent insights in determining the shape and nature of the magnetic field power spectrum in the solar wind. At scales where the fluid description of a plasma is valid the power spectrum resembles that of a neutral fluid however as the scale of the fluctuations present in the magnetic field approaches similar sizes to the characteristic scales of protons (either the gyro-radius or, the inertial length) a spectral break is seen and the spectrum steepens.
The Cluster spacecraft are an ideal tool to investigate solar wind turbulence for two reasons (1) the multiple measurement points give information in three dimensions allowing the spectrum (2) the high sensitivity of the search coil magnetometer allow sub ion scale turbulence to be measured. This talk will present results from Cluster regarding the variation of the spectrum in three dimensions which reveal the power spectrum is anisotropic with respect to the magnetic field and also agyrotopic. Results will also be presented which reveal the shape of the spectrum at electron scales is governed by coherent events in the magnetic field. Either parallel whistler waves, or coherent structures. The source of the whistler waves in the intervals concerned is unclear, and a number of possibilities will be discussed. Future work made possible with MMS will also be discussed.