Intense Localized Perpendicular Electric Fields and Modulation of the Reconnection Rate Associated with 3D Dayside Magnetopause Reconnection

Two-dimensional treatments of dayside magnetopause reconnection produce small reconnection (inductive) electric fields E_y ∼ 0.05--0.1 V_{AB_0}. Recent Polar observations, however, have identified short duration ( ∼ 1--10 ms) perpendicular electric fields on the magnetosphere side of the current layer with magnitude up to 100 mV/m or higher. This is 1--2 orders of magnitude larger than the 2D predictions. Particle-in-cell simulations in 3D are used to demonstrate that the 3D dayside magnetopause current layer is unstable to a spectrum of rippling modes that produce intense (up to 100 mV/m), localized E_x and E_y fields on the magnetosphere side. These modes appear on spatial scales extending from ρ _e to (ρ _e ρ _i)^1/2, where ρ _e (ρ _i) is the electron (ion) gyroradius in the dominant B_z magnetic field. Once reconnection is initiated, these modes tend to drift in the dawnward direction at a rate ∼ 0.2--0.3 V_A ( ∼ 120--200 km/s). The passage of these structures over a slowly moving spacecraft would produce an apparent modulation of the E_x and E_y fields at a characteristic rate of ∼ 7 Hz. Evidence is presented from THEMIS dayside magnetopause crossings confirming this prediction.