Interannual variations of the intensity of narrowband VLF radio noise and radio station signals registered in Yakutsk in 2009-2017

Major natural sources of VLF radio noise registered in Northern Asia in summer are local thunderstorm activity and in winter thunderstorms of Northern India and China. A VLF propagation function is obtained by simultaneous registration in 2009-2017 in Yakutsk (62 N, 129.7 E) of the intensity of radio noise and signals of an Alpha navaid system: Novosibirsk (55.75 N, 82.45 E) and Khabarovsk (50.07 N, 136.6 E) transmitters. At a solar activity maximum (2014-2015) there is a decrease in the VLF signal attenuation. Taking into account the VLF radio signal propagation, changes in the VLF noise source intensity are obtained. There is a trend towards increased thunderstorm sources intensity, especially in summer (local thunderstorm activity). The intensity increase from 2009 to 2017 is about 4 dB (daytime) and 1.6 dB (nighttime). According to the World Wide Lightning Localization Network, the total number of lightning discharges for Northern Asia (40-80 N, 60-180 E) increased in summer by 5 dB, and for Northern India (17-36 N, 68-94 E) and Southern China (10-33 N, 94-126 E) the total number of discharges increased in winter by 2.3 dB (2009-2017). The thunderstorm activity increase may be caused by global warming.