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

Chin S. Lin, Thomas J. Immel, Huey-Ching Yeh, Stephen B. Mende, and J. L. Burch


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 ~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 300Ð1000 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.

_______________
Journal of Geophysical Research, 110, A06304, doi:10.1029/2004JA010774, 2005.