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Abstract

When the Interplanetary Magnetic Field (IMF) is northward for a period of time, O+ from the high‐latitude ionosphere escapes along reconnected magnetic field lines into the dayside magnetopause boundary layer. Dual‐lobe reconnection closes these field lines, which traps O+ and mass‐loads the boundary layer. This O+ is an additional source of magnetospheric plasma that interacts with magnetosheath plasma through magnetic reconnection. This mass‐loading and interaction is illustrated through analysis of a magnetopause crossing by the Magnetospheric Multiscale (MMS) spacecraft. While in the O+‐rich boundary layer, the IMF turns southward. As the MMS spacecraft cross the high‐shear magnetopause, reconnection signatures are observed. While the reconnection rate is likely reduced by the mass‐loading, reconnection is not suppressed at the magnetopause. The high‐latitude dayside ionosphere is therefore a source of magnetospheric ions that contributes often to transient reduction in the reconnection rate at the dayside magnetopause.