Optical observations of the growth and day-to-day variability of equatorial plasma bubbles

A new narrow-field ionospheric imaging system, the Portable Ionospheric Camera and Small-Scale Observatory, has been installed at the Cerro Tololo Inter-American Observatory near La Serena, Chile (geographic 30.17 degrees S, 289.19 degrees E; geomagnetic 16.72 degrees S, 0.42 degrees E). We present observations of the naturally occurring nightglow emission at 630.0 nm on three consecutive nights demonstrating the day-to-day variability in the occurrence of equatorial plasma bubbles or depletions. On two nights, large-scale undulations with a wavelength on the order of 300-600 km are observed in the emission regions magnetically connected to the bottomside of the equatorial F layer. We demonstrate that at each crest of these large-scale waves, zero, one, or multiple depletions may grow. Thus, the presence of a large-scale wave on the bottomside alone is not sufficient for irregularity growth. This variability is presumably due to the presence, or lack, of small-scale seed waves or some other mechanism needed to increase the instability growth rate past the critical threshold.