Impact of Two Tropical Cyclones on the Radio Atmospherics Observed Using VLF Receivers

Abstract

The response of electric field intensity of VLF radio atmospherics during two tropical cyclones Fani (May 2019) and Amphan (May 2020) has been presented in this paper. VLF radio atmospherics (or VLF sferics) received at Coochbehar (CHB) and Kolkata (CUB) at three discrete frequencies (4 kHz, 7 kHz, and 9 kHz) showed clear amplitude anomalies with respect to the reference level during the two cyclonic storms. This is explained using the electrical structure and distribution of cloudto-ground lightning associated with the cyclones. Effects of 'local lightning' and 'distant lightning' have been identified for both the CUB and CHB receivers. Field intensity of VLF sferics at CUB station was found to get enhanced for both types of lightning events. But the intensity of VLF sferics at CHB station was found to be reduced for 'distant lightning' and enhanced for 'local lightning', possible reasons of which are also explained.