Mirror structures in the magnetosheath: 3D structures on plane waves

Mirror mode oscillations are often observed in the magnetosheath. According to linear, kinetic theory, these waves do not propagate in the plasma rest frame and posses a linear polarisation. However, observations of these waves show them to exhibit a non-planar polarisation. Using data collected by the AMPTE-UKS satellite on 21^st September, 1984, this paper investigates the nature of these waves using the variance analysis technique to determine the polarisation and maximum/minimum variance directions. The results show that the individual oscillations do indeed posses non-planar characteristics and the variation in the maximum/minimum variance directions indicates that these oscillations are in fact 3D structures propagating with the plasma rather than a 2D plane wave.