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The properties of magnetic reconnection in an asymmetric magnetopause current layer are investigated using three‐dimensional particle‐in‐cell simulations and THEMIS subsolar magnetopause crossings. The strong density gradient region on the magnetosphere side of the current layer is found to be unstable to a k⋅B = 0 lower hybrid drift instability type mode with dominant wave number kyρe ∼ 0.5. The accompanying perpendicular electric fields are intense (Ey ∼ 4vAB0 ∼ 100 mV/m) with characteristic frequencies (typically two narrowband signals) between the proton cyclotron and lower hybrid frequencies. In the observations, neither the wave amplitude nor frequency appears to be sensitive to the value of any guide magnetic field or to the degree of asymmetry in Bz/n across the magnetopause. These electric fields remain confined away from the reconnection X line and do not appear to affect the value of the reconnection rate. Key Points The subsolar magnetopause is unstable to a localized LHDI type mode The electric fields are intense (100mV/m) and contain discrete frequencies The electric fields remain confined away from the X line

The properties of an asymmetric magnetopause current layer without a guide magnetic field are investigated using three‐dimensional particle‐in‐cell simulations. The low‐density (magnetosphere) side of the current layer is found to be unstable to an internal mode that ripples the current layer with a wavelength comparable to the ion inertia length based on the high (magnetosheath) density. These modes produce localized perpendicular electric fields with peak magnitude of the order of 100 mV/m, which is more than an order of magnitude greater than in corresponding two‐dimensional treatments. The rippling of the current sheet leads to a modulation in the east‐west direction on time scales of the order of a second of the electron outflows and Poynting flux produced by magnetic reconnection. Key Points Magnetopause current layer is subject to a rippling instability Rippling produces localized electric fields of order 100 mV/m 3‐D magnetic reconnection rate and electron outflows are modulated at 1 Hz