Observation and theoretical modelling of a region of downward field-aligned flow of O + in the winter dayside polar cap

Abstract

Combined optical and radar measurements of ion drift at high latitudes near the terminator show that large downward field-aligned ion flows occur below the F-peak. At an invariant latitude of 72° and in the local time period from 1100 to 1500, downward velocities of 400 m s −1 have been observed. At the same time, the poleward component of field-perpendicular ion velocity was only 100 m s −1. The high latitude ionospheric model of Quegan et al. (1982), as modified by Allen et al. (1984), predicts downward field-aligned velocities with the same sign morphology as the observations, but with only one fifth of the magnitude. However, the existence of downward neutral winds might lead to non-linear amplification of the downward ion motion. Using the vertical wind measurements of Rees et al. (1984), a possible explanation of the fast ion flow is suggested.