Signatures of non-ideal plasma evolution during substorms obtained by mining multi-mission magnetometer data

Substorm-type evolution of the Earth's magnetosphere is investigated by mining more than two decades (1995-2017) of spaceborne magnetometer data from multiple missions including the first two years (2016-2017) of the Magnetospheric MultiScale mission. This investigation reveals interesting features of plasma evolution distinct from ideal MHD behavior: X-lines, thin current sheets and regions with the tailward gradient of the equatorial magnetic field Bz. X-lines are found to form mainly beyond 20 Re, but for strong driving, with the solar wind electric field exceeding ~ 5mV/m, they may come closer. For substorms with weaker driving, X-lines may be preceded by redistribution of the magnetic flux in the tailward Bz gradient regions, similar to the magnetic flux release instability discovered earlier in PIC and MHD simulations as a precursor mechanism of the reconnection onset. Current sheets in the growth phase may be as thin as 0.2 Re, comparable to the thermal ions gyroradius, and at the same time, as long as 15Re. Such an aspect ratio is inconsistent with the isotropic force balance for observed magnetic field configurations. These findings can help resolve kinetic mechanisms of substorm dipolarizations and adjust kinetic generalizations of global MHD models of the magnetosphere. They can also guide and complement the micro-scale analysis of non-ideal effects.