Thinning and stretching of the plasma sheet

With Cluster observations in the magnetotail we study dynamics of plasma sheet thinning and stretching during 39 intervals associated with substorm growth phases. The cross-tail current density and normal magnetic field generally scale as Bn similar to TpNp1/2/J(0), but with frequent transient variations. Typical pre-onset values are B-z similar to 1-2 nT, J(0) similar to 4-8 nA/m(2), thickness (Harris estimate) > 3000 km. A current density increase in each particular event is not accompanied with a corresponding number density increase. About 30% of the events are characterized by a large (> 5 nT) field component parallel to the current (in most of cases equal to By), implying adiabatic particle dynamics even with small Bz. Most local onsets, associated with the ends of thin sheet intervals, were accompanied with tailward plasma flows. In some cases embedded current sheet structure was detected and, therefore, estimation of thickness requires caution.