DYNAMICS OF THE H+ AND O+ POLAR WIND IN THE TRANSITION REGION AS INFLUENCED BY IONOSPHERIC CONVECTION AND ELECTRON HEATING

We have conducted a set of systematic generalized semikinetic simulati ons to study the polar H+/O+ upflows in the ionosphere and transition region as influenced by varying convection. Effects of both frictional ion heating and centrifugal acceleration are included. We find that i n regions where the convection electric field is strong (E(i) greater than or equal to 100 mV/m) the steady state polar wind may be characte rized as primarily a centrifugally accelerated O+ outflow together wit h an ambipolar H+ outflow, as a minor component, up to 4 R(E) geocentr ic distance. Owing to the increase in the O+ upward flow speed, the in crease in the O+ density, and the decrease in the H+ flow speed, H+-O collisions are important to extended altitudes during enhanced convec tion periods. The exobase (defined here as the altitude where the O+ s cale height is equal to the mean free path of an H+ ion with a speed t hree thermal speeds larger than the H+ bulk speed) shifts from 1900 km for E(i) = 0 mV/m to 3000 km for E(i) = 100 mV/m. For the range of co nvection electric fields considered here (E(i) 0 mV/m to E(i) = 100 mV /m), we identify an upper and a lower transition region which coincide roughly with the region of downward and upward H+ heat flux, respecti vely. A set of relationships between ion parallel speeds and normalize d collisional mean free paths was found which are associated with the maximum upward and downward heat flux, regardless of the value of E(i) , for steady state conditions. We find that the heated and centrifugal ly accelerated O+ ions can obtain upward bulk velocities of 5 km/s abo ve 3 R(E) geocentric distance for E(i) greater than or equal to 80 mV/ m. These ions exert a large downward drag on the H+ ions which stretch es out the tail on the lower velocity side of the distribution creatin g large downward heat fluxes. These effects may explain features of th e large downward heat fluxes observed in the H+ distributions to large altitude by the retarding ion mass spectrometer (RIMS) instrument on DE 1. We have also considered impulsive events, consisting of pulses o f cleft associated enhanced convection and elevated electron temperatu res, followed by convection across the polar cap. These result in O+ i ons falling back into the ionosphere on the dayside and nightside [e.g ., Horwitz and Lockwood, 1985]. Downward speeds of 1-2 km/s are seen u p to several thousand kilometers altitude which is consistent with DE 1/RIMS observations as presented by Chandler [1995].