Simultaneous OI 630 nm imaging observations of thermospheric gravity waves and associated revival of fossil depletions around midnight near the equatorial ionization anomaly (EIA) crest

Abstract

We report F-region airglow imaging of fossil plasma depletions around midnight that revived afresh un der persisting thermospheric gravity wave (GW) activity. An all-sky imager recorded these events in OI 630 nm imag ing over Ranchi (23.3° N, 85.3° E; mlat. ∼ 19° N), India, on 16 April 2012. Northward-propagating and east–west aligned GWs (λ ∼ 210 km, v ∼ 64 m s−1 , and τ ∼ 0.91 h) were seen around midnight. Persisting for ∼ 2 h, this GW activity revived two co-existing and eastward-drifting fos sil depletions, DP1 and DP2. GW-driven revival was promi nently seen in depletion DP1, wherein its apex height grew from ∼ 600 to > 800 km, and the level of intensity deple tion increased from ∼ 17% to 50 %. The present study is novel in the sense that simultaneous observations of thermo spheric GW activity and the associated evolution of deple tion in OI 630 nm airglow imaging, as well as that around local midnight, have not been reported earlier. The current understanding is that GW phase fronts aligned parallel to the geomagnetic field lines and eastward-propagating are more effective in seeding Rayleigh–Taylor (RT) instability. Here, GW fronts were east–west-aligned (i.e., perpendicular to the geomagnetic field lines) and propagated northward, yet they revived fossil depletions.