Solar-cycle variations of polar wind and thermal ion outflows: Akebono observations and implications on magnetosphere-ionosphere coupling

Significant long-term variations are present in magnetospheric plasma composition, which result from solar-cycle variations in the coupling between the magnetosphere and the ionosphere, in particular ionospheric ion outflow to the magnetosphere. We present Akebono observations of solar-activity dependences of the lowest-energy components of ion outflow - H+ and O+ polar wind and thermal ion flows. These observations spanned a solar cycle, and covered a wide range of altitudes (1500-8500 km) and invariant latitudes (>60° ILAT) in the polar ionosphere and a variety of geomagnetic activity conditions. At low altitudes, the averaged H+ and O+ ion velocities increase with altitude at a larger rate at solar minimum than at solar maximum. In contrast, the situation is reversed at high altitudes. This may suggest that the polar wind and related thermal ion outflows correlate differently with solar activity at low and high altitudes, and that the dominant ion acceleration process may be different in different altitude regions. We consider geophysical processes that may contribute to the observed solar activity dependences, including the possible contributions of photoelectrons and elevated electron temperatures to the ambipolar electric field that drives the polar wind.