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While magnetopause reconnection naturally occurs in three-dimensional environments, our understanding of the process has been mainly confined to the two-dimensional reconnection plane. How reconnection varies in the third direction, that is, the azimuthal direction, and what causes the variation remain open questions. We study the azimuthal variation of reconnection at an underexplored scale, which is below an Earth radius, using THEMIS spacecraft. Our results show that reconnection jets transition from being present to absent within tenths of a RE, corresponding to tens of ion inertial lengths. The absence of jets cannot be explained by an ion or electron diffusion region, or a magnetic flux rope flanked by two reconnection sites. The sharp transition of reconnection is associated with spatially varying waves. No drivers of the transition are identified locally at the magnetopause or in the upstream, implying that reconnection may naturally have a finite extent at certain stages of its development.

Plain Language Summary Magnetic reconnection extracts energy from the solar wind to power plasma convection, auroral displays, and geomagnetic substorms and storms in Earth's magnetosphere. Although reconnection is often pictured as a two-dimensional laminar process, where oppositely directed magnetic field lines in a plasma break and reconnect, the actual reconnection is three-dimensional where it varies along the direction perpendicular to the plane. Since the scale in the third dimension determines how much energy is imparted to the magnetosphere, knowledge of how reconnection varies in this direction and what causes the variation is extremely important. Among existing reports, the utilized spacecraft constellations are often too small or too large, only loosely constraining the range of variability in the third dimension. Here, we use serendipitous constellations with a spatial size that is right at the predicted scale for reconnection to vary. Our results show that reconnection jets transition from being present to absent within tenths of an Earth radius. The sharp transition of reconnection is associated with spatially varying waves. Causes of the transition cannot be identified from the surrounding or upstream electromagnetic field and plasma conditions, implying that reconnection may naturally have a finite extent at certain stages of its development.