MESSENGER Observations of Flux Transfer Events during the Impact of the 23 November 2011 Coronal Mass Ejection onto Mercury's Magnetosphere

Magnetic reconnection at the dayside magnetopause is the main process by which magnetic flux is transported from the dayside to the nightside magnetosphere at Mercury. At Earth, flux transfer events (FTEs) are formed by simultaneous or sequential reconnection at multiple X-lines on the dayside magnetopause. This reconnection results in a helical magnetic flux rope coupling the interplanetary medium to the intrinsic planetary magnetic field. As these flux ropes are transported toward the nightside, they slide along the surface of the magnetopause and compress the surrounding magnetic field lines, a phenomenon identified by observations of travelling compression regions (TCRs). MESSENGER measurements show that FTEs at Mercury have durations of only ~ 1 - 3 s in the spacecraft frame. They also occur frequently, often in 'showers' lasting at least tens of minutes, during which adjacent FTEs are separated by only ~ 10 s. However, relatively little is known about the formation of these FTE showers at Mercury and how the process may resemble or differ that at Earth. Toward this end, we analyze the MESSENGER FTE observations collected during the impact onto Mercury of the coronal mass ejection of 23 November 2011. Large numbers of FTE-type flux ropes and TCRs were recorded as MESSENGER skimmed the magnetopause from the cusp to the equator on that day. Minimum variance analysis (MVA) and force-free magnetic field models have been fit to the MESSENGER magnetic field measurements to infer the orientation, size, and, ultimately, point of origin of FTEs during this and other intervals of extreme solar wind pressure.