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Plasma bubbles in the equatorial F region are thought of as nighttime phenomena because they develop at night and are assumed to vanish after sunrise. However, bubbles occasionally persist throughout the night and into the day. This study investigates the origin of daytime irregularities and their evolution using data from the first Republic of China satellite. Our results show that daytime irregularities occur in the longitudes where bubbles have developed on previous nights. A newly reported feature is the observation of the temporal variation of the locations of daytime irregularities; daytime irregularities are concentrated near the magnetic equator early in the morning, but the location gradually shifts to higher latitudes with time. This phenomenon is explained in terms of the latitudinal redistribution of fossil bubbles by the ionospheric fountain effect.

Plain Language Summary Severe electron density disturbances are created at night in the equatorial ionosphere by the upward transport of the low-density plasma from the bottomside of the ionosphere. These phenomena are called bubbles because the plasma density at the locations of the disturbances is smaller than that of the ambient plasma. Bubbles are thought of as nighttime phenomena because bubbles develop after sunset and decay away after sunrise. However, our study using satellite data shows that some parts of these nighttime bubbles survive through the day and become the major sources of daytime electron density irregularities. The new finding of our study is that the locations of daytime irregularities move from the magnetic equator to higher latitudes over time. The redistribution of fossil bubbles is driven by the same ionospheric electrodynamics that creates the ionospheric equatorial arcs.