Limited cold ion heating in the magnetopause boundary layers

For cold and heavy magnetospheric ion populations that reach the dayside magnetopause, how those populations evolve across magnetopause separatrices into the reconnection exhaust, and how the populations may affect or be affected by reconnection, are still not well understood. Observations from the Magnetospheric Multiscale (MMS) mission from January 2019 are analyzed for a series of magnetopause crossings during a time period with a “string-of-pearls” configuration of the MMS constellation. With inter-spacecraft separations of ~100-300 km, this configuration allows for simultaneous measurements of the cold ion populations in different regions of the magnetopause boundary and current layers. For several magnetopause crossings on 2019-01-25, while magnetospheric heavy ions (He⁺ and O⁺) are not observable, a significant amount of cold (temperatures of ~1’s-10 eV) magnetospheric H⁺ is present in the outer magnetosphere. This cold H⁺ population is accelerated by the E×B drift near the magnetopause, but remains as a cold beam (temperatures of 10’s eV) well into the boundary layers and reconnection exhaust. While wave modes are present that could potentially contribute to ion heating, temperature changes are small and occur primarily at the edge of the boundary layer, and so more likely related to the initial acceleration by the normal electric field than wave-particle interactions. The lack of heating for the magnetopause crossings on 2019-01-25 differs from that observed in previous work where MMS was farther away from the X-line, pointing to the highly spatially structured nature of reconnection sites along the separatrices and the importance of the relative density of the cold ion population reaching the magnetopause.