ABSTRACT: Magnetic reconnection is the process whereby the change in the magnetic field lines' connectivity allows for a rapid release of magnetic energy into the thermal and kinetic energy of the surrounding plasma. The magnitude of the reconnection electric field parallel to the reconnection x-line (where magnetic field lines break and rejoin) not only determines how fast reconnection processes magnetic flux, but can also be crucial for generating super-thermal particles. Observations and numerical simulations have revealed that collisionless magnetic reconnection in the steady-state tends to proceed with a normalized reconnection rate of an order of 0.1 in disparate systems. However, the explanation of fast reconnection remains an open question. In this talk, I will present a series of theory, modeling, and MMS (Magnetospheric Multiscale mission) observational studies on this issue.

We propose that this value 0.1 is essentially an upper bound value constrained by the force-balance at the upstream and downstream regions, independent of the dissipation-scale physics, independent of the mechanism that localizes the x-line. The prediction from this model compares favorably to particle-in-cell simulations of magnetic reconnection in both the non-relativistic and extremely relativistic limits, from symmetric to asymmetric reconnection.  Lately, we have included thermal pressure effects in our model to predict the rate in the high-beta limit.  We also extend our study from 2D to the 3D system, studying the impact of a short x-line extent in the out-of-plane direction. Finally, we show that the maximum plausible reconnection rate could determine some of the 3D nature of magnetic reconnection, particularly the orientation of the x-line. These results could be interesting to researchers who study solar, magnetospheric, astrophysical, and laboratory plasmas.