A review of imaging low-latitude ionospheric irregularity processes

A review of the imaging of low-latitude irregularity processes conducted over the past 30 years is presented. The signature in optical data of the Rayleigh-Taylor instability (RTI) process is the development of a region of depleted emission that typically shows east-west dimensions of 50 to several hundred kilometers. In the meridional direction the depletions can at times extend over two-thousand kilometers poleward from the genesis region at the magnetic equator. A fairly consistent view of this phenomenon has arisen based on observations around the world. However, several properties show considerable seasonal, longitudinal, and solar cycle dependence. Although significant work and progress has been made on understanding these irregularities, several aspects relating to the day-to-day variability of their occurrence, the latitudinal dependence of their drift velocity, and conjugate nature of their effects still remain to be explored. The answers to these questions should be attainable in the near future as more coordinated observations are carried out.