Thinning and expansion of the substorm plasma sheet: Cluster PEACE timing analysis
Abstract. The storage and subsequent removal of magnetic flux in the magnetotail during a geomagnetic substorm has a dramatic effect on the thickness of the cross-tail plasma sheet. The near-Earth plasma sheet is thought to thin during the growth phase and then rapidly expand after onset of the substorm. The direction of propagation, whether earthward or tailward along the GSM-X direction in the near-Earth tail, may suggest the time ordering of current-disruption and near-Earth reconnection, both of which are key to the substorm process. Cluster's Plasma Electron And Current Experiment (PEACE) allows 4-point observations of electrons at the plasma sheet - lobe boundary as this interface passes over the Cluster tetrahedron. The relative timings of the boundary passage at each spacecraft allow a determination of this boundary's speed and direction of motion, assuming this is planar on the scale of the Cluster separation scale. For those boundaries corresponding to the expansion of the plasma sheet, this direction is fundamental to determining the direction of expansion. We present an example of isolated thinning and expansion of the plasma sheet, as well as a multiple thinning-expansion event that occurs during a more active substorm. Data from the 2001 and 2002 tail passes have been analysed and the average plasma sheet – lobe boundary normal vectors and normal component velocities have been calculated. A total of 77 crossings, typically between 10 and 20 RE downtail, correspond to substorm associated expansion of the plasma sheet over the spacecraft. These had normal vectors predominantly in the GSM-YZ plane and provided no clear evidence for the formation of the near-Earth neutral line occurring before current disruption or vice versa. The expansions of the plasma sheet generally exhibit the appropriate GSM-Z direction expected for the given lobe, and tend to have GSM-Y components that support onset occurring near the origin of the GSM-YZ plane. This result is noteworthy in that it indicates a homogeneous plasma sheet expansion. These expansions have an average velocity along their normal of 60±37kms–1. Conversely we find an average thinning velocity of 43±32kms–1 from 66 substorm-associated thinnings. The normal vectors of the thinning plasma sheet vary considerably in the GSM-YZ plane across the entire magnetotail, suggesting that more complex dynamics govern this process.
Key words. Magnetospheric physics (Magnetotail; Plasma sheet; Storms and substorms)bk\rasphone.