Low‐Latitude VLF Radio Signal Disturbances Due to the Extremely Severe Cyclone Fani of May 2019 and Associated Mesospheric Response

We present new results of lower ionospheric disturbances due to the extremely severe cyclonic storm Fani over northeastern part of the Indian Ocean. Very low frequency radio signal received at Kolkata, India, and mesospheric temperature and ozone concentration data from the NASA's TIMED satellite are used for this purpose. Significant wave‐like oscillations and strong amplitude anomalies in daytime and nighttime VLF signal were observed when the cyclone center was within ∼700 km from the receiver. Both the mesospheric ozone concentration and temperature showed maximum anomalies beyond 3σ during the cyclone period. Maximum ozone anomaly and maximum VLF anomaly occurred on the same day when cyclone intensity was maximum, while the maximum temperature anomaly occurred on the next day during landfall. Mesospheric temperature enhancement around VLF reflection heights indicates changes in the chemical composition and electron‐neutral balance in the D region ionosphere. Simulation of VLF signal showed that the D region reflection height decreased by 7.9 km and the D region electron density increased by ∼20 times compared to precyclone midnight values at the maximum perturbed area along the propagation path. Wavelet analysis confirmed significant enhancement of atmospheric gravity waves spectrum with periods 10 min to 2 hr around landfall. Further, a strong anticorrelation between total wavelet power in the wave band of periods 10–30 min and cyclone pressure suggests a possibility of monitoring cyclone intensity from mesospheric gravity waves using VLF radio measurements. Key Points VLF signal disturbance revealed maximum reduction of 7.9 km in the nighttime D region VLF reflection height due to the severe cyclone Fani Mesospheric temperature and ozone around VLF reflection heights showed significant enhancement during the cyclone Strong anticorrelation of gravity wave power and cyclone pressure indicates a possibility of monitoring cyclone intensity from VLF study