Seasonal and solar wind control of the reconnection line location at the Earth’s dayside magnetopause

Abstract:

Magnetic reconnection at the dayside magnetopause is the most important process by which solar wind energy enters the magnetosphere. Under southward and/or predominantly dawn-dusk IMF conditions, magnetic reconnection takes place along an extended reconnection line at low latitudes, called the X-line. In recent years, some models and observations showed that finite dipole tilt or IMF B_X controls the X-line location by moving it northward or southward from the subsolar point. Although the X-line location is an important parameter in solar wind energy transport, its dependence on dipole tilt and IMF B_X is not yet made clear based on statistical observational studies. We statistically estimated the X-line location by investigating the occurrence pattern of reconnection jets observed at the dayside magnetopause. Used here are plasma and magnetic field data taken in the dayside magnetopause region within the magnetic local time range of 10 to 14 hours from 10 years of observations by the THEMIS spacecraft. Among full magnetopause crossing events, flows with a speed tangential to the magnetopause exceeding 150 km/s, which is of the order of the Alfvén speed in the magnetosheath, are chosen as candidates of the reconnection jets. The Walén relation is used to test whether the flow was generated by reconnection, and a total of 715 jets were identified. We estimated the X-lne location by the direction and position of the identified jets. The average X-line location was estimated by determining a linear discriminant function that minimizes the occurrence probability of the southward jets on the northern side of the estimated X-line or the northward jets on the southern side. It was found that the X-line location shifts about 6 Earth radii from the subsolar point toward the winter hemisphere under the largest dipole tilt. This is also the first study that reveals that the X-line location shifts about 2.5 Earth radii from the subsolar point when the IMF B_X component is large. The results demonstrate that the effect of dipole tilt on the X-line position is larger than that of the IMF B_X component. To summarize, the dipole tilt dependence of the X-line location suggests that efficiency of energy transport into the magnetosphere by dayside magnetic reconnection during southward IMF may decrease when the dipole tilt is large.