Size, shape, and orientation of magnetosheath mirror mode structures

Abstract

Information about the size, shape, and orientation of mirror mode structures in the magnetosheath is obtained by combining data from the four Cluster spacecraft, resulting in an estimate of the form of the magnetic field magnitude spatial autocorrelation function. It is shown that the longest direction of the structures is typically inclined at around 30° from the local magnetic field direction, consistent with predictions based on linear growth theory; their smallest size is also consistent with these predictions. The ratio of the longest to shortest scales of the structures is typically 2–6, with some evidence for this value to decrease as the structures are carried downstream. This suggests that the mirror modes are generated as long structures, but gradually become isotropic, possibly as part of their nonlinear growth. In common with earlier work, we find that the direction in which the structures are shortest is typically well aligned with the local magnetopause normal, consistent with compression against this boundary. However, the maximum variance direction is not a good proxy for the long axis of the structures, typically lying around 25° from it.