Stationarity of the Reconnection X-Line at Earth's Magnetopause for Southward IMF

When the interplanetary magnetic field (IMF) is southward and has a substantial Y component, reconnection at the magnetopause occurs at low latitudes. Under these conditions, the maximum magnetic shear model for the reconnection X-line at the magnetopause predicts a continuous X-line stretching from the dawn to dusk terminators. During the solstices, the X-line is not at the subsolar point and may be located in a region where the magnetosheath bulk flow is super-Alfvenic. For a fixed IMF direction, the maximum shear model also predicts a stationary X-line. In response to IMF clock angle changes on the timescale of minutes, the X-line moves on the same timescale. The stationarity of the reconnection X-line is testable observationally under certain, restrictive conditions. This stationarity is tested using observations from the Magnetospheric Multiscale mission. For two events, the distance from the Magnetospheric Multiscale spacecraft to the X-line is constant over several minutes (within relatively large error bars) and the X-line is also near the location predicted by the maximum magnetic shear model. Thus, the reconnection X-line at the magnetopause appears to be quasi-stationary for constant IMF clock angle. These observations also place constraints on the formation and motion of multiple X-lines at the magnetopause.