The response of the near earth magnetotail to substorm activity

The large scale structure of the current sheet in the terrestrial magnetotail is often represented as the superposition of a constant northward-oriented magnetic field component (Bz) and a component along the Earth-Sun direction (Bx) that varies with distance from the center of the sheet (z0 in GSM) as in a Hams neutral sheet. The latter implies that Bx = BLx tanh((z − z0)/h) where BLx is the magnitude of the Bx component in the northern lobe. Correspondingly, the cross-tail current should be approximated by Jy = (BLx/h) sech2((z − z0)/h). Using data from the fluxgate magnetometer (FGM) on the Cluster II spacecraft tetrad, we have used measured fields and currents to ask if this model represents the large-scale properties of the system. During very quiet crossings of the plasmasheet, we find that the model gives a reasonable estimate of the trend of the average current and field distributions, but during disturbed intervals, the best fit fails to represent the data. If, however, the parameters z0 and h of the model are taken as variable functions of time, the fits can be reasonably good. The temporal variation of the fit parameter h that characterizes the thickness of the current sheet can be interpreted in terms of thinning during the growth phase of a substorm and thickening following the expansion phase. Ground signatures that give insight into the local time of substorm onset can be used to interpret the response of the plasmasheet to substorm related changes of the global system. During a substorm, the field magnitude in the central plasmasheet fluctuates at the period of Pi2 pulsations.