VLF radio signal anomaly associated with geomagnetic storm followed by an earthquake at a subtropical low latitude station in northeastern part of India

This paper presents a case study of very low frequency (VLF) radio signal anomaly during a moderate geomagnetic storm (Kp = 6, Ap = 80 and Dst = − 60 nT) on September 10, 2018 followed by a moderate earthquake (M = 5.3) on September 12, 2018 which occurred near to (~ 80 km) a subtropical low latitude station Cooch Behar (CHB), India (26.345°N, 89.448°E). Signal amplitudes from the two transmitters, VTX (18.2 kHz) and NWC (19.8 kHz), are analyzed for this study. Seven VLF parameters, such as nighttime amplitude fluctuation (dAn), amplitude difference between pre-sunrise signal and sunrise minimum signal (dAm), amplitude difference between sunset minimum signal and post-sunset signal (dAe), time (Tm) and amplitude (Am) during sunrise minimum, time (Te) and amplitude (Ae) during sunset minimum have been examined. Anomalous behaviors of all the parameters have been observed during September 10–12, 2018. D-region electron density is estimated using the long wave propagation capability code which revealed a maximum increment of ~ 20 electron/cm3 at altitude 85 km over the VTX-CHB path ~ 2 days before the earthquake event. The anomalous behaviors of the VLF signals ~ 1–2 days before the earthquake are identified as possible precursor of the earthquake which could be linked with various pre-earthquake seismo-electromagnetic processes.