Dynamic Features of the Polar Ionosphere
and Its Relation to the External Conditions

*Takumi Abe[1] ,Yoh-ichi Ichikawa [1],Andrew W. Yau [2]
Institute of Space and Astronautical Science[1]
University of Calgary[2]

We present the dynamic features of plasma in the polar cap ionosphere and its relationship to ionospheric, magnetospheric, geomagnetic and interplanetary conditions on the basis of direct observation on Akebono satellite. The Akebono satellite regularly observes the polar wind ion outflow, which is thought to exist on the ordinary ionospheric condition, in the high-latitude polar cap at altitudes from ~1000 km to ~10000 km (the satellite apogee). The observed features of the polar wind are basically consistent with the classical polar wind theory prediction: the ion drift velocity increases with altitude and reaches supersonic values typically above ~5000 km. However, the observed polar wind velocity and ion outflow flux exhibit significant temporal variations inside the polar cap, where the ionospheric conditions are usually considered as less variable. For example, the drift velocities of hydrogen and oxygen ions are sometimes a factor of 2-3 larger than the averaged value. The outflow flux varies with time by more than an order of magnitude. In addition, localized regions of enhanced polar wind density are sometimes present in the polar cap.
These features suggest a possible relationship between the observed polar wind and the conditions in the ionosphere below or the magnetosphere above. These are also considered as evidence that the polar ionospheric plasma is shaken by external conditions. In the polar ionosphere, the plasma density may be influenced by variation in the solar EUV flux, precipitating particle ionization or anti-sunward plasma convection from lower latitudes. Thereby, it is possible to modify the vertical profile of the polar wind outflow. However, the plasma density and flux variations in a large horizontal scale (> 1000 km) suggest that the polar ionosphere is affected by not only ionospheric-originated phenomena but also energy transport from distant regions. For example, field-aligned heat flux from higher altitude can result in an increase in plasma temperature, which has an important influence on the polar wind outflow. Thus, the temporal and spatial variations of the polar wind are strongly related to the physical phenomena occurring at lower and higher altitudes. Moreover, the polar wind outflow can possibly influence the large-scale plasma density distribution in the magnetosphere. These indicate that the polar ionosphere is closely connected with the external regions by the mass and energy transfer processes.