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

Abstract

Measurements of thermal O + ion number fluxes, densities, field-aligned velocities, and convection speeds from the Thermal Ion Dynamics Experiment (TIDE) on POLAR obtained near 5000 km altitude over the Southern hemisphere are examined. We find that the O + parallel velocities, densities, and number fluxes are strongly related to the convection speeds. The polar cap densities decrease rapidly with convection speed, with a linear least square fit formula to bin averaged data giving the relationship log N O + =−0.33∗V conv +0.07 , with a correlation coefficient of r=−0.96. The parallel bulk flow 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 also find that the downward number flux is strongly related to convection speed 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 Fountain paradigm. The density decline with convection speed may result from increased spreading and resulting dilution from the restricted cleft source over the polar cap area with convection speed. The parallel velocities tend to be downward for low convection speeds because at such speeds, the ions fall earthward at shorter anti-sunward distances into the polar cap. At the higher convection speeds, the initially-upward flows are transported further into the polar cap and thus occupy a larger area of the polar cap.