MHD simulations of magnetotail dynamics

Resistive MHD simulations of large-scale magnetotail dynamics demonstrate that the same unstable mode causes plasmoid formation and ejection into the far tail and dipolarization and the formation of the substorm current wedge in the inner tail, consistent with the neutral line model of substorms. However, they have also modified some aspects of the model and added details that could not easily be inferred without the self-consistent approach. We review recent results that include the externally driven formation of a thin current sheet in the near tail, which eases the onset of instability and leads to a faster dynamic evolution. In contrast to earlier expectations, the field-aligned current generation and diversion takes place in the inner tail earthward of the reconnection site, resulting from shear and diversion of the earthward flow caused by reconnection farther out. Dipolarization starts most pronounced in the tail-dipole transition region, propagating both tailward and flankward. Strong electric fields and plasma heating also are most prominent in the inner tail. Three-dimensional simulations without mirror symmetries have generalized the picture of plasmoid formation and ejection, demonstrating a tangled geometry of helical flux ropes with different connections that change increasingly from the Earth to the magnetosheath. The interconnection with the magnetosheath may also play a role in generating plasmoid flux ropes with strong core fields. Mass, energy, and momentum gain of plasmoids results mainly from the accumulation of already accelerated plasma rather than from a sling shot effect acting on the entire plasmoid.