The day-to-day variability of the equatorial ionization anomaly (EIA), especially the 2-day oscillation of the EIA, has recently been studied in China. We suggest that the main driving source of the 2-day oscillation (or the day-to-day variation) of the EIA is a planetary wave with a 2-day period (or the superposition of planetary waves with different periods). The planetary wave modulates the tidal wind, and this results in the 2-day oscillation of the EIA via the dynamo effect and fountain effect. This suggestion, derived from the observational data, has been confirmed by numerical simulations. ¿ American Geophysical Union 1992