Compared to the dayside, dynamics on the flanks of the magnetopause are poorly understood. To help bridge this knowledge gap we analyzed Cluster plasma and field measurements acquired during a 90‐min period on 20 November 2003 when Cluster crossed the magnetopause four times in the vicinity of the sash. MHD simulations provide a context for Cluster observations. Crossings were between the magnetosheath and an S‐shaped plasma sheet, rather than to the open‐field lobes of the magnetotail. Cluster encountered two regions of MHD‐breaking differences between perpendicular ion velocities andE × B convection. Ion adiabatic expansion parameter (δi) calculations show that ion gyrotropy was not broken during an episode of strong Alfvén wave activity in the magnetosheath. However, gyrotropy was broken (δi> 1) during the fourth magnetopause crossing. In the magnetosheath, ion guiding‐center motion was maintained but inertial effects associated with temporally varying electric fields are probable sources of velocity differences. Regarding the magnetopause crossing, the generalized Ohm's law limits possible sources for breaking ion gyrotropy to inertial forces and/or electron pressure gradients associated with a nearby reconnection event. We suggest that Cluster witnessed effects of a temporally varying and spatially limited, flow‐through reconnection event between open mantle field lines from the two polar caps adding new closed flux to the LLBL at the sash. Future modeling of flank dynamics must consider inertial forces as significant drivers at the magnetopause and in the adjacent magnetosheath.
