Relationship of O+ field-aligned flows and densities to convection speed in the polar cap at 5000 km altitude

Measurements of thermal O+ ion number fluxes, densities, field-aligned velo cities, and convection speeds from the Thermal Ion Dynamics Experiment (TID E) on POLAR obtained near 5000 km altitude over the Southern hemisphere are examined. We find that the O+ parallel velocities, densities, and number f luxes are strongly related to the convection speeds. The polar cap densitie s decrease rapidly with convection speed, with a linear least square fit fo rmula to bin averaged data giving the relationship log(NO+) = -0.33 * V-con v + 0.07, with a correlation coefficient of r = -0.96, The parallel bulk fl ow velocities are on average, slightly downward (0-2 km/s) for V-conv < 2.5 km/s, but tend to be upward (0-4 km/s) for average V-conv > 2 5 km/s. We a lso find that the downward number flux is strongly related to convection sp eed by log(Flux) = -0.54V(conv) + 5.14, with a correlation coefficient of r = -0.98, We interpret these relationships in terms of the Cleft Ion Founta in paradigm. The density decline with convection speed may result from incr eased spreading and resulting dilution from the restricted cleft source ove r the polar cap area with convection speed. The parallel velocities tend to be downward for low convection speeds because at such speeds, the ions fal l earthward at shorter anti-sunward distances into the polar cap. At the hi gher convection speeds, the initially-upward flows are transported further into the polar cap and thus occupy a larger area of the polar cap. (C) 2000 Elsevier Science Ltd. All rights reserved.