The HEX experiment: Determination of the neutral wind field from 120 to 185 km altitude near a stable premidnight auroral arc by triangulating the drift of rocket‐deployed chemical trails

Abstract

On 25 March 2003, Horizontal E‐Region Experiment (HEX) released trimethyl aluminum trails from two rockets launched northward from Poker Flat Research Range near Fairbanks to map the vertical wind field near a stable premidnight auroral arc system. They deployed three trails of trimethyl aluminum chemical “puffs,” whose subsequent motion traced the prevailing wind field. This motion was determined using triangulation from four ground observation sites. Position and speed accuracies were estimated to be ∼1 km and a few meters per second, respectively. The first rocket followed a novel flat trajectory; it released a nearly horizontal trail of length 200 km, at an average altitude of ∼145 km. The second rocket was launched 19 min later and released two trails between 125 and 175 km altitude along a conventional steep trajectory. All puffs between 130 and 175 km altitude drifted geomagnetic westward, almost exactly parallel to the aurora. From prior observations and modeling, we had expected to observe convective upwelling near the arcs. We did not; vertical winds were essentially downward throughout the horizontal trail, with speeds between 0 and 20 m s−1. Although an abatement of downward flow was observed ∼40 km equatorward of the arcs, these data alone do not establish a causal relationship between the abatement and the arcs. Vertical speeds of ≤20 m s−1 are relatively modest. However, because the observed wind field would entrain air parcels in flow parallel to the arc system, even vertical speeds around 15 m s−1 could displace individual air parcels by several scale heights if they occurred all along the arcs.