The response of the near Earth magnetotail to substorm activity

M. G. Kivelson (1,2), R. L. McPherron (1,2), S. Thompson (1), K. K. Khurana (1),
J. M. Weygand (1), and A. Balogh (3)

(1) Institute of Geophysics and Planetary Physics, UCLA
(2) Dept. of Earth and Space Sciences, UCLA
(3) The Blackett Laboratory, Imperial College
mkivelson@igpp.ucla.edu /Fax (310) 206-8042


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 (zo in GSM) as in a Harris 
neutral sheet. The latter implies that Bx = BxL tanh((z-zo)/h) where BxL is the 
magnitude of the Bx component in the northern lobe. Correspondingly, the cross 
tail current should be approximated by jy = (BxL/h) sech 2 ((z.zo)/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 satisfactory estimate of the average 
current and field distributions, but during disturbed intervals, the best fit 
fails to represent the data. If, however, the parameters zo and h of the model 
are taken as variable functions of time, the fits become 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 will be used 
to interpret the response of the plasmasheet to substorm related changes of the 
global system.

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Presented at the 34th COSPAR Scientific Assembly, Houston, Texas, U.S.A.  
10-19 October 2002