Thunderstorm–Induced Ionospheric Perturbations Observed Over the Indian Equatorial Sector Using VHF Radar and GNSS Data

Electromagnetic waves and atmospheric gravity waves (AGWs) attributed to thunderstorm/lightning events can alter the ionospheric conductivity and produce total electron content (TEC) fluctuations at acoustic and gravity wave (GW) frequencies. This study investigates the thunderstorm-induced
$E$
- and
$F$
-region irregularities over the near-equatorial Indian sector on October 16, 2019 and April 9, 2022, based on the observations from a 205 MHz VHF radar at Cochin (10.04°N, 76.33°E), India and global navigation satellite system (GNSS) receiver at a nearby station, Changanacherry (9.44°N, 76.55°E). Concurrent with the thunderstorm activity,
$E$
-region irregularities were observed on both the days following the lightning strikes. Lightning-induced sporadic
$E$
(
$E_{s}$
) with a vertical width of 2 km was observed on April 9, 2022 and lasted for 2 h longer than the case observed on October 16, 2019. Subsequent to the disappearance of
$E_{s}$
, quasi-periodic irregularities spanning from 60 to 550 km altitude were observed. Lightning strikes induced wave-like fluctuations in TEC with subsequent enhancements in amplitude scintillations. Vertically propagating AGW-induced ionospheric acoustic waves (AWs) of
$\sim $
4 min periodicity on October 16, 2019 and
$\sim $
3 min periodicity on April 9, 2022 were observed. GW periodicity of 12–13 min became prominent at ionospheric heights after 1–2 h of the lightning strike. The mesoscale convective system with multiple cells developed over the Indian-near equatorial sector could have caused the early AWs and delayed GWs. This observation showcases the potential application of ground-based systems in exploring the lower atmosphere-ionosphere coupling process during thunderstorms.