On the quasistatic development of thin current sheets in magnetotail-like magnetic fields

The mechanism for the formation of thin current sheets in magnetotail-like magnetic fields is investigated by numerical experiments using topology con serving equilibrium sequences. Motivated by the magnetotail magnetic field in the midnight meridian plane, a two-dimensional model field is computed i ncluding a line dipole-like (inner) part and a tail-like part. The plasma i s modeled as a polytropic gas and the total amount of plasma in each Aux tu be is fixed during an equilibrium sequence. Topology conservation is ensure d by the use of inverse coordinates. The resulting set of nonlinear partial integrodifferential equations is solved numerically using a continuation m ethod. Starting from a current-free field, stretching and compressing defor mations are applied at the boundaries to mimic the analogue of a quasistati c substorm growth phase in our model. To investigate whether the process of thin current sheet formation is robust and to understand better the basic properties of that process, the boundary deformations are chosen such that there is no preferred location of compression of the field which would pred etermine the site of thin current sheet formation. Current sheets are only found to form if a transition region between dipolar and tail-like field ex ists in the equilibrium, and the current density exhibits two different cro ss-tail length scales showing the importance of the transition region betwe en the dipolar and the tail-like field. However, the thin current component is much smaller than in other models of thin current sheet formation in th e magnetotail. This result can be attributed to the chosen type of boundary deformations. The results are consistent with the gradient-of-flux-volume mechanism which has been proposed as an explanation for the formation of th in current sheets ire the Earth's magnetotail.