The Motion of the Primary Dayside Reconnection X-Line

The location and stability of the dayside X-line at the magnetopause are two fundamental properties of the process that dominates solar-terrestrial interactions. Various investigations using simulations and theory concluded that the location of the dayside X-line is not stable in physical space but convects away from the subsolar region in the direction of the plasma bulk velocity in the magnetosheath. This study investigates the stationarity, or stability, of the dayside reconnection X-line using MMS observations. The X-line encounters are identified by ion beam switches. For a given beam switch time series, it is assumed that the first or initial ion beam direction observed by the satellites during a magnetopause crossing indicates the direction the X-line convects. The ratio of X-lines moving toward or away from the subsolar region is about equal. No preference of the X-line drift in the direction of the bulk velocity in the magnetosheath could be found. In about half of the cases, the X-line moves against the magnetosheath flow direction and toward the subsolar region. In addition, about 40% of the events show multiple crossings of a single X-line. These multiple crossings in short succession suggest that the X-line oscillates around the primary reconnection location.