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Using data from the EISCAT (European Incoherent Scatter)VHF radar and DMSP (Defense Meteorological Satellite Program) spacecraftpasses, we study the motion of the dayside open-closed field line boundaryduring two substorm cycles. The satellite data show that the motions of ionand electron temperature boundaries in EISCAT data, as reported by Moen et al. (2004),are not localised around the radar; rather, they reflect motions of theopen-closed field line boundary at all MLT throughout the dayside auroralionosphere. The boundary is shown to erode equatorward when the IMF pointssouthward, consistent with the effect of magnetopause reconnection. Duringthe substorm expansion and recovery phases, the dayside boundary returnspoleward, whether the IMF points northward or southward. However, thepoleward retreat was much faster during the substorm for which the IMF hadreturned to northward than for the substorm for which the IMF remainedsouthward - even though the former substorm is much the weaker of the two.These poleward retreats are consistent with the destruction of open flux atthe tail current sheet. Application of a new analysis of the peak ionenergies at the equatorward edge of the cleft/cusp/mantle dispersion seen bythe DMSP satellites identifies the dayside reconnection merging gap toextend in MLT from about 9.5 to 15.5 h for most of the interval. Analysisof the boundary motion, and of the convection velocities seen near theboundary by EISCAT, allows calculation of the reconnection rate (mapped downto the ionosphere) from the flow component normal to the boundary in its ownrest frame. This reconnection rate is not, in general, significantlydifferent from zero before 06:45 UT (MLT<9.5 h) - indicating that the Xline footprint expands over the EISCAT field-of-view to earlier MLT onlyoccasionally and briefly. Between 06:45 UT and 12:45 UT (9.5<MLT<15.5 h)reconnection is continuously observed by EISCAT, confirming the (large)MLT extent of the reconnection footprint deduced from the DMSP passes. Aswell as direct control by the IMF on longer timescales, the derivedreconnection rate variation shows considerable pulsing on timescales of2-20 min during periods of steady southward IMF

Motion of the dayside polar cap boundary during substorm cycles: I. Observations of pulses in the magnetopause reconnection rate