High resolution OI (630 nm) image measurements of F‐region depletion drifts during the Guará Campaign

Abstract

A high performance, all‐sky, imaging system has provided data on the evolution and drift motions of F‐region depletions above the magnetic dip equator at Alcântara, Brazil, (2.3°S, 44.5°W). Monochromatic images of depletions in the OI(630 nm) nightglow were recorded on eight nights during 1‐16 October, 1994, as part of the Guará campaign. The drift motions of the depletions were typically 80–100 m/s eastward prior to local midnight and reduced to a minimum of ∼30–50 m/s in the morning hours, in accord with previous observations. However, on October 2–3 and 12–13 the depletions were observed to reverse direction for ∼60–90 min, achieving westward speeds of ∼30 m/s before the motion reverted to eastward around 0100 LT and accelerated to 35–45 m/s near dawn. Magnetic activity and other evidence suggests that these reversals in the motion of the airglow depletions probably result from reversals in the F‐region dynamo rather than from shifts in the altitude of the shear in the nighttime F‐region plasma drift.