Ionospheric plasma bubble encounters or F region bottomside traversals?

In situ AE‐C and AE‐E ion composition measurements, together with Manila and Huancayo ionosonde electron density profiles, are used in an attempt to distinguish between spacecraft encounters with equatorial plasma bubbles which have been pinched off from below, those still in the formation stage, and spacecraft excursions below the steep ionization gradient at the bottom edge of the postsunset F layer. Such excursions can result from quasi‐periodic oscillations of the altitude of the F layer—as deduced from the ion composition measurements during the circular orbit phase of the low‐inclination satellite AE‐E. It is found that depletion features that appear to be due to topside bubbles which have been pinched off from below seldom have ion concentration reductions as high as a factor of 10²; concentration drops associated with bubbles directly connected to bottomside plasma via vertical plasma density contours or those due to bottomside excursions, however, can be nearly a factor of 10 4 . In the former case, O + remains the dominant ion; in the latter case NO + often becomes dominant. These results have important theoretical implications in that they are consistent with placing the bubble generation region on the steep density gradient of the lower ledge of the F region where O + is the dominant ion.