Transverse-scale size of Pc3 ULF waves near the exterior cusp

摘要

The concept of a geomagnetic field line has been widely used in the study ofmagnetospheric physical phenomena. For example, the mode in which ULF wavespropagate relates to the direction of the field, with the Alfven wave mode propagatingalong the field direction. Very little is known about the perpendicular extent of thepropagating wave. In this paper, wave coherency methods are utilized to analyze ULFwaves in the Pc3 band that were simultaneously observed by the Cluster satellites near theexterior cusp and by ground stations at local magnetic noon near the footprint of the cusp.The results show that the coherency of waves observed at the ground on theH component is much larger than that on the D component, which is opposite to that seenby Cluster in space. The coherency between the H component on the ground andthe y component in space was higher than the other combination of pairs, with thecoherency between the satellite and the Daneborg (DNB) station having the maximumvalue. These results suggest that the polarization of the waves are rotated by 90° afterpropagating through the ionosphere, and the magnetic footprint of Cluster is closest to theDNB station at this time. The coherency of the Pc3 waves between the satellites is highlyrelated to the alignment of satellite pairs with respect to the geomagnetic field direction.This alignment may provide a transverse-scale size of the geomagnetic Pc3 ULF wavesnear the exterior cusp at .-900 km with a coherency of 0.65.