Implication of tidal forcing effects on the zonal variation of solstice
equatorial plasma bubbles
Abstract
Equatorial plasma bubbles (EPBs) are elongated plasma depletions that
can occur in the nighttime ionospheric F region, causing scintillation
in satellite navigation and communications signals. EPBs are believed to
be Rayleigh-Taylor instabilities seeded by vertically propagating
gravity waves. A necessary pre-condition for EPB formation is a
threshold vertical ion drift from the E region, which is required to
produce the vertical plasma gradients conducive to this instability.
Factors affecting the variation of EPBs therefore include magnetic
declination, the strength of the equatorial electojet, and the wind
dynamo in the lower thermosphere controlling vertical plasma drifts. In
most longitude zones, this results in elevated EPB occurrence rates
during the equinoxes. The notable exception is over the central Pacific
and African sectors, where EPB activity maximizes during solstice.
\citet{tsunoda_jgr2015} hypothesized that the solstice
maxima in these two sectors could be driven by a zonal wavenumber 2
atmospheric tide in the mesosphere and lower thermosphere. In this
study, we find that the post-sunset electron density observed by
FORMOSAT-3/COSMIC during the boreal summer from 2007 - 2012 does indeed
exhibit a wave-2 zonal distribution, consistent with results expected
from elevated vertical ion drift over the Central Pacific and African
sectors. Numerical experiments are also carried out which found that
forcing from the aforementioned tidal and stationary planetary wave
(SPW) components produced wave-2 modulations on vertical ion drift, ion
flux convergence, and midnight TEC. The relation between the vertical
ion drift enhancements and the midnight TEC enhancements are consistent
with the solstice maxima hypothesis.