Thinning and expansion of the substorm plasma sheet: Cluster PEACE timing analysis

Abstract

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 R E 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±37 km s −1 . Conversely we find an average thinning velocity of 43±32 km s −1 from 66 substormCorrespondence to: J. P. Dewhurst (jpd@mssl.ucl.ac.uk) 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.

Simultaneous THEMIS observations in the near‐tail portion of the inner and outer plasma sheet flux tubes at substorm onset

“…Previous analysis showed that the thinning sheet boundary velocity evaluated from the timing at two spacecraft (ISEE-1,2) was consistent with simultaneously observed inward (upward in our case) plasma flow velocity [Forbes et al, 1981]. A recent timing study by Cluster [Dewhurst et al, 2004] gave the typical thinning speeds of 10 -70 km/s and confirm that fast thinnings are the expansion-related (rather than growth phase-related) phenomena. In a few cases the fast thinnings were observed as close to the Earth as 9 Re (Polar observations at 2107 UT on 8 September 2002 presented by Sergeev et al [2005]) or even $7 Re (initial thinning at TC2 before the subsequent plasma sheet expansion occurred 1 min later [Sergeev et al, 2008]).…”

A joint Cluster and ground-based instruments study of two magnetospheric substorm events on 1 September 2002

“…The unit normal at which the Cluster spacecraft cross the PSBL to return to the lobe indicates that it is inclined mainly in the y-z plane, with both y and z-components negative. The boundary is travelling at 38 km/s −1 which is faster than the previous entry but again slower than typical values previously found (Dewhurst et al, 2004).…”

“…The unit normal at which the spacecraft cross the PSBL into the plasma sheet at 19:03 UT indicates that it is inclined mainly in the y − z plane, with a negative y-component. The boundary is travelling at 30 km/s −1 which is slower than typical values (Dewhurst et al, 2004).…”

Magnetotail magnetic flux monitoring based on simultaneous solar wind and magnetotail observations

“…The ratio of the plasma sheet cross‐tail area ( ) to the total cross‐tail area ( ), equal to 2Δ/ π R T , can reach ∼15% in the midtail. Note that the latter value may considerably change in the course of substorms [ Baumjohann et al , ; Sergeev et al , ; Dewhurst et al , ; Petrukovich et al , ] and during a bursty bulk flow (BBF) passage [ Panov et al ., ].In view of the above caveats, the algorithm gives an upper estimate of the total magnetic flux through the given tail cross section. Neglecting the finite width of plasma sheet, where the magnetic field lines are closed and the magnetic field magnitude is lower than in the lobes, is unavoidable at the moment.…”

“…The ratio of the plasma sheet cross-tail area (2R * T Δ) to the total cross-tail area ( R 2 T ), equal to 2Δ∕ R T , can reach ∼15% in the midtail. Note that the latter value may considerably change in the course of substorms [Baumjohann et al, 1992;Sergeev et al, 1993;Dewhurst et al, 2004;Petrukovich et al, 2011] and during a bursty bulk flow (BBF) passage [Panov et al, 2010]. In view of the above caveats, the algorithm (1)-(5) gives an upper estimate of the total magnetic flux through the given tail cross section.…”