Morphology of the equatorial anomaly and equatorial plasma bubbles using image subspace analysis of Global Ultraviolet Imager data

The equatorial anomaly ( EA) is host to the highest ionospheric densities on Earth. Disturbances within the EA result in plasma density depletions and large density gradients. This paper presents observations of global quiet time morphology of the EA as measured by images of nighttime ionospheric 135.6 nm radiation taken by the Global Ultraviolet Imager (GUVI) on NASA's Thermosphere, Ionosphere, and Mesosphere, Energetics and Dynamics ( TIMED) spacecraft. It also presents equatorial plasma bubble (EPB) morphology as determined by detection of intensity depletions in GUVI images. The technique used for analysis is unique in that it allows for simultaneous characterization of the EA and detection of EPBs. This paper also presents extensive observations of EA and EPB morphology and shows that EA morphology can be well characterized by data taken from the 2030-2130 MLT range. Further, this paper identifies crest symmetry in intensity and latitude as an indicator of both EA and EPB morphology. For all longitudes, the crest-to-trough ratio (CTR) is shown to be well correlated with the EPB rate. While the CTR may drop with solar flux, EPB levels do not. Thus the absolute CTR is less an indicator than the change in the CTR as a function of longitude for a given season and solar flux. One significant exception to this correlation is observed in the Pacific sector during the June solstice. In this case the EPB rate is high despite a low CTR.