An experimental investigation of thermospheric structure near an auroral arc

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Abstract
Observations of thermospheric parameters, made from the Dynamics Explorer 2 (DE 2) spacecraft during three successive orbital crossings of a quiescent dusk sector auroral arc, have been compared with the predictions of three fine‐grid auroral arc models. DE 2 measured the ion and neutral winds, electron and neutral temperatures, neutral composition, and energetic auroral electron spectra (5 eV to 32 keV) at ∼320 km altitude. These observations were at high spatial and temporal resolution, suitable for comparisons with the models. The observed zonal and meridional neutral winds near the arc were greater than the model predictions, probably because of the presence of stronger electric fields and higher ion and neutral densities during the observations than were used in the models. The measured vertical winds were also larger than the corresponding model values. The DE 2 composition measurements showed a local increase of the N2/O ratio in the arc, which is interpreted to be a result of the upward motion of N2‐rich air. A reduction in the measured neutral temperature of ∼100 K in the arc, relative to temperatures on either side, was found for all three arc crossings. Of the three theoretical models examined, only one shows any tendency for the neutral thermospheric temperature to drop in the auroral arc, and the decrease calculated is significantly less than the observed temperature change. A simple calculation of the adiabatic cooling effect of the observed upward motion yields a temperature drop of ∼160 K, comparable with the observed temperature reduction (∼100 K).