ESA Science & Technology11-Jul-2005 09:58:32

Science Results

Thinning of the tail current sheet

Ion and magnetic field measurements on 12 August 2001. From top to bottom: X component of the plasma flow (Vx) and the magnetic field (Bx), thickness of the current sheet (L) and density of current (J) (from Nakamura et al., Geophys. Res. Lett., 2002b).

The Cluster quartet of spacecraft allows, for the first time, to unambiguously determine the characteristics of the tail current sheet in the Earth's magnetosphere. At times the neutral sheet current has the structure of a so-called Harris sheet. In a Harris sheet the magnetic field is represented by BX(z)=BLtanh{(z-zo)/L} where BL is the lobe field outside the current sheet, zo is the location of the neutral sheet and L is the half-thickness of the current sheet. Simultaneous measurements from three spacecraft allow to estimate the three parameters and to compare the estimated model BX at the location of the fourth spacecraft with the actual data to check the validity of the estimation.

The magnetic field data shown in the figure clearly shows that the structure of the current sheet undergoes a major change during the fast flow event shown in the uppermost panel. The bottom two panels show the result of the Harris sheet estimation for several sequences before and during the flow interval together with the averaged data. It can be seen how the current sheet structure changes during the flow event. Before the onset of the flow, the spatial scale, L, of the current sheet is 5000 km. After southward excursion of the current sheet, during the maximum of the flow event, the scale reduces to 500 km, which is comparable to the ion inertial length. The maximum current density predicted from the Harris sheet model increases from 5-10 to 20-40 nAm-2. Published in Nakamura et al., Geophys. Res. Lett., 2002b.

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