Evidence for interplanetary magnetic field B y controlled large‐scale reconnection at the dayside magnetopause

We report evidence of a long‐lasting reconnection event during which the accelerated plasma flow direction changes in response to an interplanetary magnetic field (IMF) B y reversal, indicating a change in the reconnection site location. The observations were made by Equator‐S on the dawn flank of the magnetopause and consist of a large number of plasma jets detected mostly within magnetospheric flux transfer events. The plasma jets were found in quantitative agreement with the theoretical predictions for reconnection. The reversal of the plasma flow direction in the jets following the reversal of the B y component not only confirms that the dayside reconnection configuration is controlled by the IMF, as opposed to local control, but also stresses the importance of the IMF dawn‐dusk component, in addition to the north–south component, in determining the global configuration of the reconnection.