  The magnetotail current sheet configuration determines magnetic reconnection
properties that control the substorm onset, one of the most energetic phenomena
in the Earth's magnetosphere. The quiet-time current sheet is often
approximated as a two-dimensional (2D) magnetic field configuration balanced by
isotropic plasma pressure gradients. However, reconnection onset is preceded by
the current sheet thinning and the formation of a nearly one-dimensional (1D)
magnetic field configuration. In this study, using particle-in-cell
simulations, we investigate the force balance of such thin current sheets when
they are driven by plasma inflow. We demonstrate that the magnetic field
configuration transitions from 2D to 1D thanks to the formation of plasma
pressure nongyrotropy and reveal its origin in the nongyrotropic terms of the
ion distributions. We show that substorm onset may be controlled by the
instability and dynamics of such nongyrotropic current sheets, having
properties much different from the most commonly investigated 2D isotropic
configuration.
