Ground-based measurements of an arc-associated electric field

Abstract The electrodynamics of a pre-midnight auroral arc, situated close to the equatorward border of the convection reversal, are studied by ground-based measurements. The optical intensity and the location of the arc is determined by a scanning photometer and an all-sky camera. Ionospheric electric fields around the arc are measured by the STARE radar. The EISCAT radar provides measurements of electric fields and conductivities equatorward of the arc. The EISCAT magnetometer chain is used to observe the development of ionospheric currents. An enhanced northward component of the electric field is observed equatorward of the arc. The width of this arc-associated electric field is estimated to be less than, or equal to, 45 km. A quite new result is the simultaneous intensification of the arc-associated electric field and the optical brightening of the auroral arc. The brightening of the arc is related to an intensification of the upward current from the arc, carried by energetic electrons. Previous satellite and rocket measurements have shown that auroral arcs are associated with matched pairs of currents with the upward current flowing from the arc and the downward current, carried by cold ionospheric electrons, flowing on the equatorward (poleward) side of the arc in the evening (morning) sector. The field-aligned currents are connected by a Pedersen current flowing in the direction of the electric field in the ionosphere. If the magnetospheric generator region acts as a current generator, the ionospheric electric field has to modify itself in a way that the current continuity in the ionosphere is preserved. When the ionospheric conductivity is low adjacent to the arc, as the EISCAT measurement indicates, the meridional electric field has to increase in order to produce an enhanced Pedersen current and an enhanced downward current (by the gradient of the Pedersen current) in response to the increased upward current. Thus we suggest that the enhanced electric field adjacent to the arc is a result of the ionosphere-magnetosphere coupling, influenced by the low ionospheric conductivity.