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

Abstract

The mechanism for the formation of thin current sheets in magnetotail‐like magnetic fields is investigated by numerical experiments using topology conserving equilibrium sequences. Motivated by the magnetotail magnetic field in the midnight meridian plane, a two‐dimensional model field is computed including a line dipole‐like (inner) part and a tail‐like part. The plasma is modeled as a polytropic gas and the total amount of plasma in each flux tube is fixed during an equilibrium sequence. Topology conservation is ensured by the use of inverse coordinates. The resulting set of nonlinear partial integrodiiferential equations is solved numerically using a continuation method. Starting from a current‐free field, stretching and compressing deformations are applied at the boundaries to mimic the analogue of a quasistatic 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 predetermine the site of thin current sheet formation. Current sheets are only found to form if a transition region between dipolar and tail‐like field exists in the equilibrium, and the current density exhibits two different cross‐tail length scales showing the importance of the transition region between 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 the 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 thin current sheets in the Earth's magnetotail.