WINDS IN THE IONOSPHERE - A REVIEW

Winds in the upper atmosphere, and their effect on the ionosphere, are reviewed with an emphasis on information useful to ionospheric studie s. The winds are driven by pressure gradients from solar and auroral h earing, with some forcing by tidal energy from below. Simple calculati ons which balance the pressure gradient by ion drag and Coriolis force s are generally unreliable, so large-scale numerical models of the cou pled atmosphere and ionosphere are required. The accuracy of these glo bal models is limited by uncertainties in the energy inputs at high la titudes and at the lower boundary (about 90 km). The best current wind data come from incoherent scatter radar or airglow installations, at a few sites and for only a Few nights per month. Satellite data are al so available for several years, and results to 1989 are incorporated i n the global HWM90 model. This seems acceptable for determining mean w inds at night, less good during the day, and least good in the souther n hemisphere where few data were available. Plots are given to show th e mean winds at different latitudes and longitudes, for use in ionosph eric calculations. Meridional winds alter the height of the mid-latitu de F layer, causing large changes in the effective loss rare. This is the major cause of observed seasonal changes, of differences between t he hemispheres, and of changes ar different longitudes. An increased k nowledge of the winds is essential for further progress in F region st udies, Ionospheric data provide the most promising route, using routin ely scaled parameters. The simplest calculations compare observed peak heights, obtained from M(3000)F2, with the value h(o) predicted by si mplified ''servo'' equations. Errors occurring for some hours after su nrise can be overcome using model results to define h(o) this allows r apid and accurate wind calculations at dip latitudes of 23-62 degrees. Winds can also be obtained from full model calculations, designed to match observed values of peak height or density.