Inductive Electric Fields and Electron Vorticity as Signatures of Magnetic Reconnection

Abstract:

We illustrate the constraint that the type of inductive electric field required for magnetic reconnection places upon electron flow, and how in some instances the combined measurement of the inductive electric field and electron vorticity suffices to identify global reconnection. The required inductive non-ideal electric field E_N satisfies the necessary and sufficient condition for reconnection: M =(∇×E_N)×B≠0, where B is the reconnecting magnetic field. Placing this constraint upon ∇×E_N in the generalized Ohm’s law, we find that an electron vorticity ω_e develops parallel to B on ion scales, and perpendicular to B on electron scales. We expect the parallel ω_e to have opposite sign on either side of the electron scale diffusion region, and the perpendicular ω_e to be primarily azimuthal. Local reconnection satisfies the notion of local diffusion without any observable topological changes, while global reconnection satisfies our customary notion of reconnection, which necessarily involves topological changes. Under certain circumstances, combined detection of M and the required electron vorticity profile determines whether topological changes are global and thus conforms to our customary notion of reconnection. It is feasible to calculate both ω_e and M from MMS data and from reconnection simulations, the measurement uncertainties and signal to noise ratio presenting the primary obstacles to detecting M. We have found that M does form a detectable signature in PIC simulations of reconnection and in data from the Magnetospheric Multi-Scale mission and the signal does overcome the noise in these instances. In the simulations, M extends in thin layers along the separatrices at locations coinciding with the expected electron vorticity features on ion and electron scales. In two MMS burst data intervals through the electron diffusion region, we find that M exceeds measurement uncertainties and again coexists with electron vortex structures. In addition to revealing the significance of angular momentum to the physics of reconnection, we propose that the relationship between M and ω_e also embody necessary and sufficient signatures of global magnetic reconnection when the data conveys sufficient spatial coverage and signal strength.