Simultaneous observations of equatorial plasma depletion by IMAGE and ROCSAT-1 satellites

[1] Simultaneous observations of the equatorial ionosphere by the ROCSAT-1 and IMAGE satellites were used to study zonal propagation characteristics of equatorial plasma bubbles. IMAGE far ultraviolet (FUV) nighttime images have indicated signatures of depression in the brightness of equatorial airglow arcs. Using the list of airglow brightness depression events observed by IMAGE, we surveyed ROCSAT-1 Ionospheric Plasma and Electrodynamics Instrument data and found three ROCSAT- 1 passes with simultaneous plasma observations in the same local time sector. Brightness depressions seen in FUV images are found to correlate with equatorial plasma bubbles detected by ROCSAT- 1 at 600 km altitude with a separation distance between bubbles varying from 300 to 1000 km. Successive FUV images are averaged to produce a keogram with UT versus longitude, which is then used to deduce zonal drift velocity for plasma density depletions. The drift speed determined from FUV images is compared with ion drift velocities measured by ROCSAT- 1. The analysis indicates that the bubbles drift faster than the ambient plasma. The relative zonal velocity of plasma bubbles with respect to the ambient plasma is large, over 200 m/s when plasma bubbles are first created in the early evening hours. The relative zonal velocity decreases rapidly with magnetic local time (MLT) from over 200 m/s at 20 MLT to similar to 30 m/s at 22 MLT. ROCSAT-1 has detected quasiperiodic structures for equatorial plasma bubbles, corresponding to the quasiperiodic appearance of FUV brightness depression. The quasiperiodic structure of plasma bubbles has been interpreted as evidence of zonal propagation of atmospheric gravity waves with a wavelength in the range of about 300 - 1000 km. The observed zonal drift velocity of plasma bubbles is shown to be consistent with the horizontal propagation velocity of acoustic gravity waves. The dependence of the horizontal group velocity of acoustic gravity waves on various parameters is further investigated. The vertical wavelength of acoustic gravity waves is found to be the most sensitive parameter for determining the horizontal group velocity of acoustic gravity waves and thus presumably the zonal propagation of equatorial plasma bubbles. The relative zonal propagation of equatorial plasma bubbles might imply strong coupling between atmospheric gravity waves and the Rayleigh-Taylor instability. The obtained results suggest that atmospheric gravity waves play an important role not only for initiating plasma bubbles but also for maintaining the horizontal motion of equatorial plasma bubbles.