Why do equatorial ionospheric bubbles stop rising?

The Naval Research Laboratory (NRL) three‐dimensional simulation code SAMI3/ESF is used to study the long time evolution of equatorial spread F (ESF) bubbles. The ESF bubbles are modeled until they stop rising and become “fossils,” with results analyzed to address previously‐untested hypotheses. Specifically, it has been suggested that bubbles stop rising when either the local electron density inside the bubble is equal to that of the nearby background or the flux‐tube‐integrated electron density inside the bubble is equal to that of the nearby background. It is shown that equatorial bubbles stop rising when the magnetic flux‐tube‐integrated ion mass density inside the bubble equals that of the surrounding background ionosphere. In the case of a single‐ion ionosphere this reduces to the condition that the flux‐tube‐integrated electron densities are in balance, consistent with the hypothesis of Mendillo et al. (2005).