Abstract
Cusp auroras poleward of the typical auroral oval are ascribed to high-latitude lobe reconnection when the Interplanetary Magnetic Field (IMF) B
z
is predominantly northward. In this study, we further investigate the ionospheric characteristics of a unique high-latitude cusp region employing multiple satellite observations. A cusp aurora event with wide spatial spread was observed in the postnoon polar cap region. It was found to be associated with northward IMF B
z
and positive B
y
components. The cusp aurora was located from 68° to 86° in magnetic latitude and within 15–17 hr in magnetic local time. This broad coverage in the polar cap indicates direct precipitating particles from the magnetosheath. Particle energy is different between the equatorward and poleward edges of the cusp aurora. The precipitating ions at the equatorward side maintain magnetosheath particle characteristics as expected, while ions with higher energies occurred in the poleward side. Further, the poleward edge of the cusp aurora was nearly situated in the center of a convection shear and was associated with an upward field-aligned current. These observations suggest a lobe cell circulation, hence we attribute the formation of the cusp aurora to the high-latitude lobe reconnection. Simultaneous observations in the southern hemisphere indicate the absence of cusp aurora. The auroral presence only in the northern hemisphere is probably due to the combination of large dipole tilt angle and positive IMF B
z
, which facilitates the lobe reconnection.
