Suppression of Magnetopause Reconnection in the Presence of Cold Magnetospheric Plasma

The temporal variability of magnetopause reconnection is an important aspect of solar wind magnetosphere coupling. Even under stable solar wind driving, reconnection can be triggered, modulated, or suppressed because of magnetic field and plasma conditions near the magnetopause boundary. We analyze a unique event in which a THEMIS satellite crosses the subsolar magnetopause three times within a $\sim$5 min interval in the presence of a cold-ion population on the magnetospheric side of the boundary. During the first crossing, the satellite detects reconnection outflow and a D- shaped ion velocity distribution earthward from the boundary, indicating an active reconnection. The signatures disappear during the second crossing when the magnetospheric cold-ion density increases significantly and reappear during the third crossing when the magnetospheric density drops to a level comparable to that of the first crossing. The solar wind and magnetosheath conditions do not change much during the interval. The magnetospheric population is evidently associated with a plasmaspheric plume with considerable variation in density. According to the theory of mass loading, the presence of such a plume population results in the local Alfvén speed at the second crossing being $\sim$40% smaller compared to the first and third crossings. However, the theory itself does not suggest suppression. We discuss possible suppression mechanisms considering the additional effects of the prevailing solar wind and local magnetopause conditions.