Optical observations of the development of secondary instabilities on the eastern wall of an equatorial plasma bubble

Optical evidence is presented for the growth of a local secondary plasma instability on the eastern wall of an equatorial plasma bubble observed from the Haleakala Volcano, Hawaii (geographic: 20.71 N, 203.83 E; geomagnetic: 21.31 N, 271.45 E). Typically, only the western wall of depletions at tropical latitudes are susceptible to secondary structuring. This event occurs during a minor geomagnetic storm when the plasma bubble is observed to be drifting to the west, rather than with the typical eastward drift velocity. The data are interpreted in terms of the generalized E × B instability with an anomalous wind directed westward and toward the equator. This wind drives the eastern wall of the depletion region unstable. The anomalous wind is associated with an equatorward propagating neutral wind surge caused by an ongoing geomagnetic storm. The observations of the growth of these instabilities in the local tropical ionosphere are in stark contrast to the typical development of secondary instabilities on equatorial plasma bubbles, which grow near the equatorial plane on the western wall of the primary depletion and map along the magnetic field lines to off-equatorial latitudes.