Measurement of the Magnetic Reconnection Rate in the Earth's Magnetotail

<jats:title>Abstract</jats:title><jats:p>In the Earth's magnetotail, magnetic reconnection releases stored magnetic energy and drives magnetospheric convection. The rate at which magnetic flux is transferred from the reconnection inflow to outflow regions is determined by the reconnection electric field <jats:italic>E</jats:italic><jats:sub><jats:italic>r</jats:italic></jats:sub>, which is often referred to as the unnormalized reconnection rate. To better quantify the efficiency of reconnection, this electric field <jats:italic>E</jats:italic><jats:sub><jats:italic>r</jats:italic></jats:sub> is often normalized by the characteristic Alfvén speed and the reconnecting magnetic field. This parameter is generally called the normalized or dimensionless reconnection rate <jats:italic>R</jats:italic>. In this paper, we employ a two‐dimensional fully kinetic simulation to model a magnetotail reconnection event with weak geomagnetic activity (<200 nT of the <jats:italic>AE</jats:italic> index) observed by the Magnetospheric Multiscale (MMS) mission on 11 July 2017. We obtain <jats:italic>R</jats:italic> and <jats:italic>E</jats:italic><jats:sub><jats:italic>r</jats:italic></jats:sub> from direct measurements in the diffusion region and indirect measurements of the rate at the separatrix using a recently proposed remote sensing technique. The measured normalized rate for this MMS event is <jats:italic>R</jats:italic> ∼0.15–0.2, consistent with theoretical and simulation models of fast collisionless reconnection. This corresponds to an unnormalized rate of <jats:italic>E</jats:italic><jats:sub><jats:italic>r</jats:italic></jats:sub> ∼2–3 mV/m. Based on quantitative consistencies between the simulation and the MMS observations, we conclude that our estimates of the reconnection rates are reasonably accurate. Given that past studies have found <jats:italic>E</jats:italic><jats:sub><jats:italic>r</jats:italic></jats:sub> of the order ∼10 mV/m during strong geomagnetic substorms, these results indicate that the local <jats:italic>E</jats:italic><jats:sub><jats:italic>r</jats:italic></jats:sub> in magnetotail reconnection may be an important parameter controlling the amplitude of geomagnetic disturbances.</jats:p>