Equatorial plasma bubble zonal velocity using 630.0 nm airglow observations and plasma drift modeling over Ascension Island

We present OI (630.0nm) airglow image data from Ascension Island (geographic: 7.9S, 14.4W; dip latitude: 16S) in the southern Atlantic Ocean taken with the Utah State University all-sky CCD camera during 20 March to 7 April 1997 in order to study plasma bubbles occurring in the low-latitude nighttime ionosphere. The initial plasma bubble onset occurs in the early evening hours at ∼19:15-20:00 LST and is followed by eastward propagation with an average speed of ∼90-120m/s prior to local midnight, rapidly decreasing around the midnight and postmidnight periods. The Ascension results are compared with similar observations from Christmas Island in order to examine the longitudinal variations of EPB development and propagation. The observed EPB velocities from Ascension Island are also compared with the results of a plasma drift model. In a case study during the night of 4-5 April, the velocity reveals unusual latitudinal shear, up to 0.12m/s/km, with a reversal to westward flow at low latitudes while eastward flow is maintained at higher latitudes. Consequently, the bubble rotates counterclockwise and tilts eastward, significantly away from alignment with the geomagnetic field lines. The westward reversal of the drift motion near the geomagnetic equator is most likely the result of a reversal in the F region dynamo or from a large increase in the altitude of the shear node in the F region plasma drift at the geomagnetic equator.