Universal Scaling of Thin Current Sheets

Thin current sheets (TCSs) with thicknesses about ion Larmor radii are widespread in space. It is important to describe their equilibrium structure allowing them to store and then explosively release the accumulated free energy. When ions are moving along quasi-adiabatic trajectories while magnetized electrons follow guiding center drift orbits, TCSs can be described within the framework of a hybrid approach. The thickness of the embedded electron sheet remains uncertain because of the scale-free character of electron motion. In this work, we propose a novel analytical model of the multilayer TCS that provides a universal expression describing the inner (embedded) electron sheet in dependence of TCS characteristics. An unusual property of the embedded electron layer revealed in this analysis is the nonlinear profile of the magnetic field in the inner layer: B(z) similar to z(1/3), which conforms excellently with MAVEN observations of 43 TCSs in the Martian magnetotail. Plain Language Summary Current sheets (CSs) are widespread in nature. They accumulate magnetic energy and then quickly explosively release it due to fast magnetic reconnection. We revealed unusual properties of a very narrow electron layer within also rather thin ion current sheet. Electron physics has the primary importance for this problem for two reasons: (1) very thin but very intense election currents provide substantial contribution to CS free energy and (2) magnetized electrons provide the rigidity to magnetic field lines preventing the immediate onset of reconnection. Once electrons are magnetized in CS, their average motion can be described by guiding center drift approximation which is scale free, so there are some difficulties to estimate the thickness of the electron layer. We developed here simple analytical model that allowed to determine (1) spatial scales of the narrow electron CS embedded inside the wider proton current sheet and (2) unusual and never reported before profile of magnetic field (B(z) similar to z in power 1/3) within the electron layer instead of generally assumed linear dependence B(z) similar to z. CS structure generally is maintained by the coupling of ion and electron effects. Theory predictions conform quite well with a numerous MAVEN observations of CS crossings in the Martian magnetotail.