Dynamics of the H+ and O+ polar wind in the transition region as influenced by ionospheric convection and electron heating

Abstract

We have conducted a set of systematic generalized semikinetic simulations 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 in regions where the convection electric field is strong (Ei ≥ 100 mV/m) the steady state polar wind may be characterized as primarily a centrifugally accelerated O+ outflow together with an ambipolar H+ outflow, as a minor component, up to 4 RE geocentric distance. Owing to the increase in the O+ upward flow speed, the increase in the O+ density, and the decrease in the H+ flow speed, H+–O+ collisions are important to extended altitudes during enhanced convection periods. The exobase (defined here as the altitude where the O+ scale height is equal to the mean free path of an H+ ion with a speed three thermal speeds larger than the H+ bulk speed) shifts from 1900 km for Ei = 0 mV/m to 3000 km for Ei = 100 mV/m. For the range of convection electric fields considered here (Ei = 0 mV/m to Ei = 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, respectively. A set of relationships between ion parallel speeds and normalized collisional mean free paths was found which are associated with the maximum upward and downward heat flux, regardless of the value of Ei, for steady state conditions. We find that the heated and centrifugally accelerated O+ ions can obtain upward bulk velocities of 5 km/s above 3 RE geocentric distance for Ei ≥ 80 mV/m. These ions exert a large downward drag on the H+ ions which stretches out the tail on the lower velocity side of the distribution creating large downward heat fluxes. These effects may explain features of the 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 of cleft associated enhanced convection and elevated electron temperatures, followed by convection across the polar cap. These result in O+ ions 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 up to several thousand kilometers altitude which is consistent with DE 1/RIMS observations as presented by Chandler[1995].