Abstract
We present new observations of lower ionospheric disturbances due to the extremely severe cyclonic storm ‘Fani’ over northeastern part of the Indian Ocean. Very Low Frequency radio signals received from two places Kolkata and Cooch Behar, India revealed disturbances in the lower ionosphere namely in the D-region ionosphere. Mesospheric temperature and Ozone concentration data from the NASA’s TIMED satellite were also used to diagnose the disturbances in the lower ionosphere. Significant wavelike oscillations and strong amplitude anomalies in daytime and nighttime VLF signal were observed during the intense phase of the cyclone. Both the mesospheric Ozone concentration and temperature showed maximum anomalies beyond $3 \sigma$ during the cyclone period. Mesospheric temperature enhancement around VLF reflection heights indicates changes in the chemical composition and electron-neutral balance in the D-region ionosphere. Wavelet analysis of the VLF amplitudes indicates a strong anti-correlation of the total wavelet power in the wave-band of periods 10-30 min with the cyclone pressure which suggests a possibility of monitoring cyclone intensity from mesospheric gravity waves using VLF radio measurements.
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