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Ann. Geophys., 22, 3089-3098, 2004
www.ann-geophys.net/22/3089/2004/
© European Geosciences Union 2004
Seasonal-longitudinal variability of equatorial plasma bubbles
W. J. Burke1, C. Y. Huang2, L. C. Gentile2, and L. Bauer3
1Space Vehicles Directorate, Air Force Research Laboratory, Hanscom AFB, MA, USA
2Institute for Scientific Research, Boston College, Chestnut Hill, MA, USA
3Department of Physics, Air Force Academy, Colorado Springs, CO, USA
Abstract. We compare seasonal and longitudinal distributions of more than 8300
equatorial plasma bubbles (EPBs) observed during a full solar cycle from
1989-2000 with predictions of two simple models. Both models are based on
considerations of parameters that influence the linear growth rate,
γRT, of the generalized Rayleigh-Taylor instability in the context of
finite windows of opportunity available during the prereversal enhancement
near sunset. These parameters are the strength of the equatorial magnetic
field, Beq, and the angle, α, it makes with
the dusk terminator line. The independence of α and Beq from the solar cycle phase justifies our comparisons.
We have sorted data acquired during more than 75000 equatorial
evening-sector passes of polar-orbiting Defense Meteorological Satellite
Program (DMSP) satellites into 24 longitude and 12 one-month bins, each
containing ~250 samples. We show that: (1) in 44 out of 48
month-longitude bins EPB rates are largest within 30 days of when α=0°;
(2) unpredicted phase shifts and asymmetries appear in occurrence rates
at the two times per year when α≈0°; (3) While EPB
occurrence rates vary inversely with Beq, the relationships are very
different in regions where Beq is increasing and decreasing with
longitude. Results (2) and (3) indicate that systematic forces not
considered by the two models can become important. Damping by
interhemispheric winds appears to be responsible for phase shifts in maximum
rates of EPB occurrence from days when α=0°. Low EPB occurrence
rates found at eastern Pacific longitudes suggest that radiation belt
electrons in the drift loss cone reduce γRT by enhancing
E-layer Pedersen conductances. Finally, we analyze an EPB event observed
during a magnetic storm at a time and place where α≈-27°, to illustrate how electric-field penetration from high latitudes
can overwhelm the damping effects of weak gradients in Pedersen conductance
near dusk.
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