Ground and satellite observations of an auroral event at the cusp/cleft equatorward boundary

Detailed observations by ground‐based and satellite instruments of a transient auroral event in the ∼10–11 MLT sector during negative IMF BZ are reported. The optical phenomenon belongs to a category of dayside auroral event sequences that is considered a candidate signature of impulsive magnetopause reconnection. The present event is characterized by the brightening of a discrete arc (at least 600 km in east‐west extent) at the cusp/cleft equatorward boundary, subsequently moving poleward and increasing in latitudinal extent. This was followed 6 min later by enhanced 630.0‐nm auroral emission ∼3 degrees (300 km) further north, at the cleft/mantle boundary. The satellite (DMSP F7) passed through the event at ∼1000 MLT, in its early phase, revealing a discrete electron precipitation structure with the spectral peak at ∼1 keV, immediately equatorward of a zone of cleft/low‐latitude boundary layer particles. The multiple structures of field‐aligned current near the cleft equatorward boundary included an upward directed (out of the ionosphere) component at the latitude of the discrete electron precipitation structure and a downward current and accelerated ions (1–3 keV) further north. A continuously decreasing electron energy flux was observed along the satellite track from the cleft equatorward edge to the polar cap, as well as clear energy versus latitude dispersion of the precipitating ions. These low‐altitude observations are found to be consistent with essential features of unsteady Petschek‐type reconnection at the magnetopause and modeled mapping of flux transfer events to the ionosphere.