Seasonal and Solar Wind Control of the Reconnection Line Location on the Earth's Dayside Magnetopause

Abstract

We investigate the average location of magnetic reconnection on the Earth's dayside magnetopause, based on spatial distributions of northward and southward reconnection jets observed by the THEMIS spacecraft at the near‐noon (10–14 magnetic local time) magnetopause. A total of 711 reconnection jets were identified by applying the Walén relation, the tangential stress balance relation to be satisfied for a reconnected (rotational discontinuity) magnetopause, to magnetopause crossings identified from 10 years of THEMIS observations. The directions and positions of jets indicate that during southward interplanetary magnetic field (IMF) conditions, the dayside X‐line location shifts from the subsolar point toward the winter hemisphere by about 6 Earth radii under the largest tilt of the geomagnetic dipole axis. The X‐line location also shifts northward (southward) by at most 2.5 Earth radii when the IMF is predominantly radial and its x component is positive (negative). The dipole tilt effect on the shift of the X‐line location becomes smaller for higher solar wind Alfvén Mach numbers. The dipole tilt effect being larger than the IMF Bx effect suggests that the X‐line location has a seasonal dependence. Since models and theory show that the reconnection rate away from the subsolar magnetopause is lower than that at the subsolar magnetopause, the dipole tilt dependence of the X‐line location suggests that the efficiency of solar wind energy transfer into the magnetosphere may decrease under larger dipole tilt; this may partially account for seasonal variations of geomagnetic activity, which is known to decrease under larger dipole tilts.

Properties of Magnetic Reconnection and FTEs on the Dayside Magnetopause With and Without Positive IMF Bx Component During Southward IMF

“…The north‐south asymmetries introduced by the sunward tilt of the IMF suggest that the X line resides north of the equatorial plane for a longer period of time than in the strictly southward IMF run. The shift of the X line toward the Northern Hemisphere with positive IMF B x component has been observed before in a statistical THEMIS study (Hoshi et al, ) and global MHD simulations (Hoilijoki et al, ; Peng et al, ). Often the shift of the X line is explained using the maximum magnetic shear model (Trattner et al, , ), but in this case without an out‐of‐plane magnetic field B y component the shear is the same everywhere along the magnetopause and, therefore, cannot explain the shift.…”

Dipole Tilt Effect on Magnetopause Reconnection and the Steady‐State Magnetosphere‐Ionosphere System: Global MHD Simulations

“…The inclusion of an IMF B x component also contributes to this shift, as demonstrated in previous simulations (Peng et al, 2010). However, a recent survey of THEMIS data revealed that the seasonal (tilt angle) control of the X line location tends to dominate that of B x (Hoshi et al, 2018). This seasonality may contribute to semiannual variations in geomagnetic activity (Russell & McPherron, 1973).…”

The Location of Magnetic Reconnection at Earth’s Magnetopause

“…In the original cusp study (Trattner et al 2007), cusp events observed during a dominant IMF B Y component also revealed a strong dependence of the dayside reconnection location on the seasonal tilt of the dipole axis (see also Hoshi et al 2018). This seasonal dependency for the dayside reconnection location at the Earth's magnetopause is shown in Fig.…”