Ionospheric signature of plasma sheet thinning prior to a substorm

A case study of the behaviour of the poleward edge of the main F-region trough in the premidnight sector associated with a substorm is presented. It is found that the poleward edge moves rapidly equatorwards prior to the onset of the substorm as determined from ground magnetometer signatures. It is well known that before magnetic midnight the poleward edge is produced by soft precipitation from the inner plasma sheet. The data are therefore interpreted as the ground signature of the process of plasma sheet thinning and dipolarisation of the near-Earth geomagnetic field on the nightside, as described by the Near Earth Neutral Line model of substorm behaviour. The competing Plasma sheet Boundary Layer model does not appear to be consistent with our data in this case.