Medium-scale thermosphere perturbations in the bottomside F region in tropical regions

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Friday, July 7, 2017

Thermospheric gravity waves in the bottomside F region have been proposed to play a key role in the generation of equatorial plasma bubbles (EPB). However, direct observations of such waves have been lacking.

Global distribution of gravity wave activity image
Global distribution of gravity wave activity at ~250 km altitude averaged over different seasons during 2010-2013. Left: GW activity derived from the density, in unit of 10^(−5); Right: GW activity derived from the zonal wind, in unit of m2/s2. March equinox covers March–May, June solstice covers June-August, Sept. Equinox covers September–November, and Dec. Solstice covers December– February. The average F10.7 index is about 110 sfu.

This study provides a systematic survey of medium-scale neutral atmosphere gravity wave perturbations at this critical altitude in the tropics, using in-situ satellite measurements of thermospheric density and zonal wind. The analysis reveals pronounced features on their global distribution and climatology: 1. A prominent 3-peak longitudinal structure exists in all seasons, with stronger perturbations over continents than over oceans. 2. Their seasonal variation consists of both semiannual oscillations (SAO) and annual oscillation (AO). The SAO component maximizes around solstices and minimizes around equinoxes, while the AO component maximizes around June solstice. 3. The perturbation is more intense at low solar flux level than at higher solar flux levels. Though the spatial distribution of these GWs overlaps with that of EPBs, their climatological variation shows opposite trend.