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A remote sensing technique to infer the local reconnection electric field based on in situ multipoint spacecraft observation at the reconnection separatrix is proposed. In this technique, the increment of the reconnected magnetic flux is estimated by integrating the in-plane magnetic field during the sequential observation of the separatrix boundary by multipoint measurements. We tested this technique by applying it to virtual observations in a two-dimensional fully kinetic particle-in-cell simulation of magnetic reconnection without a guide field and confirmed that the estimated reconnection electric field indeed agrees well with the exact value computed at the X-line. We then applied this technique to an event observed by the Magnetospheric Multiscale mission when crossing an energetic plasma sheet boundary layer during an intense substorm. The estimated reconnection electric field for this event is nearly 1 order of magnitude higher than a typical value of magnetotail reconnection.

Plain Language Summary Magnetic reconnection is an important phenomenon in space plasmas that explosively releases the accumulated magnetic energy. In the Earth's magnetotail, it is known that the released energy by this process leads to aurora substorms. The rate of reconnection, which defines how efficiently the magnetic flux is transferred at the central reconnection region called the diffusion region, is a key parameter to explore such reconnection physics. However, it is very challenging to directly measure this parameter by observing the small-scale diffusion region in situ. Thus, in this paper, we propose a new remote sensing technique that infers the reconnection rate along the boundary of the whole reconnection region called the separatrix from in situ measurements. In this technique, by observing how rapidly the magnetic flux passes through the separatrix, the rate can be remotely inferred even outside the diffusion region. We confirmed the adequacy of this technique by testing it in a kinetic simulation, and then applied it to a latest observation event by the MMS spacecraft during an intense substorm. The result shows a remarkably high rate. This indicates the positive correlation between the reconnection rate and the substorm strength and strongly motivates future survey using the technique proposed in this paper.