Abstract:
Structuring on the west (east) wall of eastward (westward) drifting airglow depletion under quiet
(storm) time is common. Here, we report for the first time the rare occurrence of the east wall structuring
of an eastward drifting depletion near the crest of equatorial ionization anomaly (EIA) in India on 16 April
2012 under quiet geomagnetic conditions. We observed a linear depletion having a westward tilt of ∼25° and
eastward drift speed of ∼129 m/s in the beginning. First prominent structuring occurred on its west wall that
later swiftly surged poleward within next 6–12 min and bifurcated, that is, behaved like a secondary depletion
growing on the primary one. Second west wall structuring then occurred but had considerably subdued growth.
Meanwhile, main depletion bent slightly eastward above its branching node, and two structuring appeared on
the east wall in succession. Secondary depletion then slightly contracted equatorward, and structuring emerged
from its east wall, too, that later stretched eastward, got linked with main depletion and formed grid mesh
shaped feature. Second structuring on the west wall started fading; while, those on the east wall swiftly evolved,
surged poleward and gained prominence. Also, their westward tilt decreased. During the occurrence of these
events, gravity waves (GWs) and the retreat of EIA were active overhead. Current study serves as first imaging
evidence of Zalesak et al. (1982, https://doi.org/10.1029/JA087iA01p00151) simulations that the east wall of a
westward tilted depletion can become unstable and host secondary instabilities. Possibly these structuring were
excited by GWs via Rayleigh-Taylor instability.
