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

<p>We report F-region airglow imaging of fossil plasma depletions around midnight that revived afresh under persisting thermospheric gravity wave (GW) activity. An all-sky imager recorded these events in OI 630 nm imaging over Ranchi (23.3° N, 85.3° E; mlat. <span class="inline-formula">∼19</span>° N), India, on 16 April 2012. Northward-propagating and east–west-aligned GWs (<span class="inline-formula"><i>λ</i>∼210</span> km, <span class="inline-formula"><i>v</i>∼64</span> m s<span class="inline-formula">⁻¹</span>, and <span class="inline-formula"><i>τ</i>∼0.91</span> h) were seen around midnight. Persisting for <span class="inline-formula">∼2</span> h, this GW activity revived two co-existing and eastward-drifting fossil depletions, DP1 and DP2. GW-driven revival was prominently seen in depletion DP1, wherein its apex height grew from <span class="inline-formula">∼600</span> to <span class="inline-formula">&gt;800</span> km, and the level of intensity depletion increased from <span class="inline-formula">∼17 <i>%</i></span> to 50 %. The present study is novel in the sense that simultaneous observations of thermospheric GW activity and the associated evolution of depletion 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.</p>