Electric fields in the vicinity of auroral forms from motions of barium vapor releases

In August‐September 1967 eleven barium vapor clouds were released during evening twilight between invariant magnetic latitudes of 67.3° to 68.1° from Andøya, Norwav. Two flights (8 releases) occurred during moderate negative bays in H , whereas the third flight (3 releases) took place during a positive bay in H . Visual auroral displays were observed in the vicinity during all flights. In the negative bay situation, barium ion cloud motions were eastward and closely parallel to auroral arc alignments. Electric fields transverse to the magnetic field with intensities of 10–130 mv/m directed southward were observed. During the positive bay event the barium clouds spanned the breakup transition region, with the two equatorward clouds moving westward while the poleward cloud went east. Observed reversals in direction were closely correlated with magnetic variations. North‐directed electric fields of up to 50 mv/m were found in the positive bay sector. In all events the ion cloud motions revealed that E was perpendicular to the ionospheric current, hence we conclude that the auroral electrojets, both eastward and westward, are essentially Hall currents. The results illustrate that the magnitude of E driving ionospheric currents cannot be deduced solely from ground magnetic observations because of the variable ionospheric electrical conductivity. There is evidence that while E is large near an auroral arc, the field within is very low. Large gradients and/or irregularities in the E field are found to exist most of the time. These are revealed on three different time‐space scales: from differences in velocity for parallel moving clouds, from velocity changes along the path of a given cloud, and in the form of rayed structure within a cloud.