Statistical analysis of the ionization ledge in the equatorial ionosphere observed from topside sounder satellites

For the purpose of clarifying the occurrence character of the ionization ledge and its relation to the diurnal variation of the eastward electric field, plasma density profiles in the equatorial ionosphere of the American longitude sector obtained from topside sounder on-board the Ohzora (EXOS-C) and the ISIS-2 satellites were analyzed. The ionization ledge appeared in a limited dip latitude region from -13.5° to 19.3°, near the dip equator. The structure of the ionization ledge tends to align along the geomagnetic field line, and to appear within local time sectors from morning (09 LT) to slightly after midnight (02 LT). Comparing with the eastward electric field deduced by analyzing the variations of the geomagnetic field on the ground, the ionization ledge tends to occur with the enhancement of the eastward electric field. The occurrence probability shows a seasonal variation giving maxima in equinox seasons and minima in solstice seasons. Comparing with the seasonal dependence of the upward drift velocity based on the variation of the electric field, the seasonal variation of the occurrence probability is well interpreted as the character of the ionospheric electric field.