Anisotropy of the Taylor scale and the correlation scale in plasma sheet magnetic field fluctuations as a function of auroral electrojet activity

Abstract

Magnetic field data from the Cluster spacecraft in the magnetospheric plasma sheet are employed to determine the correlation scale and the magnetic Taylor microscale from simultaneous multiple‐point measurements for multiple intervals over a range of mean magnetic field directions for three different levels of geomagnetic activity. We have determined that in the plasma sheet the correlation scale along the mean magnetic field direction decreases from 19,500 ± 2200 to 13,100 ± 700 km as the auroral electrojet activity increases from quiet (<80 nT) to active conditions (>200 nT). The reverse occurs for the correlation scale perpendicular to the magnetic field, which increases from 8200 ± 600 km to 13,000 ± 2100 km as the auroral electrojet activity increases from quiet to active conditions. This variation of the correlation scale with geomagnetic activity may mean either a change in the scale size of the turbulence driver or may mean a change in the predominance of one over another type of turbulence driving mechanism. Unlike the correlation scale, the Taylor scale does not show any clear variation with geomagnetic activity. We find that the Taylor scale is longer parallel to the magnetic field than perpendicular to it for all levels of geomagnetic activity. The correlation and Taylor scales may be used to estimate the effective magnetic Reynolds numbers separately for each angular channel. Reynolds numbers were found to be approximately independent of the angle relative to the mean magnetic field. These results may be useful in magnetohydrodynamic modeling of the magnetosphere and can contribute to our understanding of energetic particle diffusion in the magnetosphere.