Local time extent of magnetopause reconnection using space–ground coordination

Magnetic reconnection can vary considerably in spatial extent. At the Earth's magnetopause, the extent generally corresponds to the extent in local time. The extent has been probed by multiple spacecraft crossing the magnetopause, but the estimates have large uncertainties because of the assumption of spatially continuous reconnection activity between spacecraft and the lack of information beyond areas of spacecraft coverage. The limitations can be overcome by using radars examining ionospheric flows moving anti-sunward across the open-closed field line boundary. We therefore infer the extents of reconnection using coordinated observations of multiple spacecraft and radars for three conjunction events. We find that when reconnection jets occur at only one spacecraft, only the ionosphere conjugate to this spacecraft shows a channel of fast anti-sunward flow. When reconnection jets occur at two spacecraft and the spacecraft are separated by 10 Re) are possible forms of active reconnection at the magnetopause. Interestingly, the extended reconnection develops from a localized patch via spreading across local time. Potential effects of IMF B-x and B-y on the reconnection extent are discussed.