Investigations of Plasma Turbulence Using the Cluster Electron Drift Instrument

Defining the nature of any turbulent flow of plasma within the Earth's magnetosphere is of fundamental importance to understanding the distribution and transport of energy within the system. Of greatest interest is determining the coefficient of the power law governing the cascade of energy from one scale size to another. In previous analysis by single spacecraft, measurements with respect to a spatial scale were calculated by converting temporal sampled data to spatial samples utilizing a known relative velocity. This is achieved in select regions of space where either the plasma flow is much greater than the motion of the spacecraft or the opposite, the spacecraft is moving across the plasma with greater velocity than the relative bulk motion of the plasma. Both cases require knowledge of the relative phase velocity of the observer with respect to the medium. While this technique is extremely useful, it excludes analysis in many regions of the magnetosphere. However, with multi-spacecraft measurements such as Cluster, we are able to investigate regions that were previously excluded. For this analysis, we use the Electron Drift Instrument on board each Cluster satellite because it directly measures the electron gyro-orbit drift step vector, from which is then derived the exact electron flow velocity vector perpendicular to the ambient magnetic field. The average covariances of velocities between two spacecraft are calculated as a function of payload separations. From this technique we can derive approximate spatial structure functions for high latitude regions (auroral zone and polar cap). These results are compared with weak Magnetohydrodynamic fluid turbulence theory.