Inferring proximity to the reconnection site via structural changes to the magnetopause caused by asymmetric reconnection.

The mechanisms of field line breaking and magnetic energy dissipation that result in magnetic reconnection have yet to be determined by spacecraft observations. Many parameters have been proposed to locate the reconnection site, but they either fail to identify uniquely the reconnection site or have not been tested for asymmetric reconnection. We demonstrate that the change in magnetopause structure caused by reconnection can be used to locate and estimate proximity to the site of reconnection. Cluster observations of quiet magnetopause crossings, for which no evidence of reconnection is found, show no obvious spatial dependence of the DC electric field, while the plasma density and velocity make the transition from magnetosheath to magnetosphere values simultaneously with the tangential magnetic field (BL) reversal. Conversely, in-situ observations of several active crossings, for which signs of reconnection are evident, show that the density transition and BL reversal can occur simultaneously or be offset from one another by over 100 ion skin depths (λi) (assuming a constant magnetopause velocity), the outflow jet can occur anywhere from the BL reversal to several λi earthward of the density gradient, and the DC electric field changes sign on either side of the density gradient. Laboratory experiments and 2D and 3D particle-in-cell simulations of asymmetric reconnection reveal that the relative transition offsets are due to exhaust crossings at different proximities to the X-line. Only within the thin electron current layer surrounding the X-line do the transitions remain concurrent. We present one reconnection event during which the transitions in plasma density, DC electric field, and BL are simultaneous in two of the four Cluster spacecraft and offset in the other two spacecraft. The multiple satellite encounter allows us to examine spatial features in the region surrounding the X-line.