Tidal forcing effects on the zonal variation of solstice equatorial plasma bubbles

Equatorial plasma bubbles are elongated plasma depletions that can occur in the nighttime ionospheric F region, causing scintillation in satellite navigation and communications signals, and manifesting in ionograms as spread F. Equatorial plasma bubbles are believed to be Rayleigh-Taylor instabilities seeded by vertically propagating gravity waves. A necessary pre-condition for plasma bubble formation is believed to be a threshold vertical ion drift from the E region, which is required to produce the vertical plasma gradients conducive to such an instability. Factors affecting the zonal and seasonal variation of equatorial plasma bubbles therefore include magnetic declination, as well as the strength of the equatorial electojet, and neutral winds in the lower thermosphere controlling vertical plasma drifts via the wind dynamo. In most longitude zones, the above factors result in elevated occurrence rates of equatorial plasma bubbles during the equinoxes. The notable exception is over the central Pacific and African sectors, where equatorial plasma bubble activity maximizes during solstice. As the zonal separation of the two sectors is roughly half the Earth’s circumference, Tsunoda et al. (2015) [1] hypothesized that the solstice maxima in these two sectors could be driven by a zonal wavenumber 2 atmospheric tidal component in the mesosphere and lower thermosphere (MLT). In this study, we find that the post-sunset electron density observed by FORMOSAT-3/COSMIC during the boreal summer does indeed exhibit a wave-2 zonal distribution in both the equatorial and northern mid latitude regions. The equatorial wave-2 is consistent with results expected from elevated vertical ion drift over the Central Pacific and African sectors, while the mid- latitude wave-2 is consistent with the Mid-Summer Nighttime Anomaly. Using COSMIC, the seasonal, longitudinal, and local time variation of ionospheric tidal and stationary planetary wave (SPW) components that produce zonal wavenumber 2 disturbances when viewed in a constant local time frame is examined. Numerical experiments are also carried out using the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIE-GCM) to determine the effect of the aforementioned tidal and SPW components on vertical ion drift, showing a clear wave-2 modulation of vertical ion drift when subject to forcing from wave-2 atmospheric tidal components in the mesosphere and lower thermosphere. The aforementioned results are consistent with the solstice maxima hypothesis.