At high latitudes during solstice the ray path through the atmosphere to about 400-km altitude near midnight is either so small that production of O+, He+, and H+ continues at all times or so large that production of these species is negligible. The latter condition will produce an ionospheric ''hole'' if the plasma stagnates or circulates in the nightside winter pole. Near equinox, however, the shadow height near midnight may ensure that photoproduction of 0+ is negligible, while the ray path at higher altitudes may be short enough that significant production of He+ and H+ takes place. Observations made by Atmosphere Explorer D and calculations suggest that an ionospheric hole should also be observable at equinox but will not be so pronounced as that observed during winter. However, the dominance of He+ in the hole between 500 and 10000 km is unique to equinoctial conditions. We note that stagnation of the plasma rather than slow passage across the polar cap is required, since at equinox the field tubes must reach the nightside before the shadow height ensures that the photoproduction of O+ ceases. Stagnation times between 3 and 6 hours are required for He+ to be dominant at altitudes around 600 km.