Extremely low ionospheric peak altitudes in the polar hole region

Vertical electron density N-e profiles, deduced from newly available ISIS 2 digital ionospheric topside sounder data, are used to investigate the altitude structure of localized ionization depletions within the winter, nighttime polar cap ionosphere during solar minimum. The region investigated, called the polar hole-auroral cavity region, is located just after midnight magnetic local time near the poleward side of the auroral oval. Earlier investigations had at times found very low N-e values in this region (down to 200 cm(-3) near 300 km). In the present study, latitudinal N-e depletions were found, but such low N-e values (approximate to 100 cm(-3)) were only found near the ISIS 2 altitude of 1400 km. The peak ionospheric concentration below ISIS 2 remained fairly constant (similar to 10(5) cm(-3)) across the N-e-depleted region, but the altitude of the peak dropped dramatically to the vicinity of 100 km. The observations, in combination with other data, indicate that the absence of an F layer peak may be a frequent occurrence at high latitudes and stress the importance of a knowledge of the vertical N-e distribution in high-latitude investigations of horizontal N-e structures.