Generation of high-density plasma in the polar cap observed by the Akebono satellite Original Research Article

The polar cap ionosphere is known as a comparatively low-density (<103 cm−3 above 3000 km) plasma region and less active than the cusp or auroral region in terms of particle precipitation. Because of the low density, very limited information on the thermal plasma in the high-altitude polar cap is available in the literature. On very rare occasions, the Akebono satellite encounters regions of unusually high-density plasma above 4000 km altitude, in which both the electron temperature (<3000 K) and parallel ion drift velocity (<1 km s−1) are distinctively low. The ion drift meter observations on the DMSP satellite show that the plasma convection in the polar cap is predominantly directed from the dayside to nightside on such occasions, suggesting that anti-sunward convection is a necessary condition. The low electron temperature and ion velocity accompanied by the high plasma density may suggest that a small-amplitude ambipolar electric field is also another necessary condition for increasing the plasma density.