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Journal of Geophysical Research






American Geophysical Union

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We report image measurements of plasma depletions in the equatorial F region ionosphere over Christmas Island (2.1°N, 157.4°W; dip latitude 2.8°N) in the central Pacific Ocean. The observations were made during the equinox period, September‐October 1995, using a Utah State University CCD imaging system filtered to observe thermospheric O I (630.0 nm) airglow emissions centered at ∼280 km altitude. Well‐defined magnetic field‐aligned depletions were observed on 18 nights during the campaign, including strong postmidnight fossilized structures, enabling detailed measurements of their morphology and dynamics. The number of depletions was influenced by their initial onset times and their persistence. The separations between adjacent depletions ranged from ∼150 to ∼250 km in good agreement with prior observations from other sites. However, measurements of their eastward zonal drift speeds indicated normal behavior peaking around 90–100 m/s prior to local midnight, with exceptionally high velocities, ∼80 m/s during the postmidnight period that persisted until dawn. These results differ markedly from optical measurements at similar equatorial latitudes but at different longitude sectors, suggesting that the zonal drift velocities can have a significant longitudinal dependence. Model drift velocities calculated using a simple electric field model with winds defined by the horizontal wind model (HWM‐07) produced an eastward drift throughout the night, but their postmidnight magnitudes were much smaller than observed. Using a modified HWM‐07 wind field, a basic nighttime trend similar to the Christmas Island trend was successfully obtained.


Published by the American Geophysical Union in Journal of Geophysical Research: Space Physics.

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