Singular plasma disturbances in the low-latitude F region

We describe here a new phenomenon characterized by unusual patterns of ion drifts inside ion density depletion regions observed by the AE-E satellite in the low-latitude F region. In about 30 depletions, vertical ion drift re lative to the background was upward on the western sides, downward on the e astern sides, and zero near the middle where the density depletion was grea test. These drift characteristics are distinct from those observed in plasm a bubble depletions. The structures reported here were observed on circular orbits below 300 km altitude and had density depletions of up to 2 orders of magnitude or more below the ambient ion density. The upward and downward drift excursions were up to 200 m/s relative to the background. Almost all these structures were observed over oceans or near coasts and largely betw een +/-10 degrees and +/-30 degrees dip latitude. The structures were obser ved mostly as isolated, single depletion regions with the majority of them about 250 km wide in the east-west direction. They occurred during quiet ma gnetic conditions with near-equal occurrence frequencies in the premidnight and postmidnight periods. The characteristic density and drift signatures indicate westward propagating disturbances in which the bottomside F layer is first lifted and then returned back to its original position, leaving th e ionosphere undisturbed after the disturbance passes by. The estimated spe ed of these disturbances is of the order of 200 m/s. These unique solitary plasma disturbances, which we designate as singular plasma disturbances, ar e associated with a propagating source of ExB drift, not driven by neutral perturbations at the altitude of observation.