Variations of global thunderstorm activity derived from the long-term Schumann resonance monitoring in the Antarctic and in the Arctic

Abstract The paper analyzes the records of the magnetic components of the Schumann resonance signals obtained at the Ukrainian Antarctic station “Akademik Vernadsky” (coordinates: 65.25° S, 64.25° W) from March 2002 to March 2019 and at Svalbard in the Arctic (coordinates: 78.15° N and 16.05° E) from October 2013 to June 2019. The time-course is studied of the monthly average intensities and peak frequencies of the first Schumann resonance maximum, as well as the background noise intensity at the first minimum. Both observed parameters demonstrate periodic seasonal variations. The intensities are in-phase in different hemispheres while the peak frequencies are in anti-phase. Another feature of the resonance signals is interannual trend of the resonance intensity and peak frequency observed in both hemispheres being synchronous with 11-year solar cycle. The background noise intensities demonstrate the absence of appreciable interannual variations. We propose the following interpretation of these results. Seasonal changes in peak frequencies are determined by the regular meridian drift of the world thunderstorm centers up to ±(18°–26°) in latitude from the equator. Seasonal variations of the intensity are controlled by changes of thunderstorm activity that increase of about 1.5 times during the boreal summer compared with the winter. World thunderstorm activity is practically independent on the phase of the solar cycle. The observed interannual trends in the parameters of the first Schumann mode are determined by the global changes in the lower ionosphere. They can be explained by increasing of the upper (magnetic) characteristic height by 2–3 km due to additional ionization during the years of maximum solar activity. Another explanation might be an increase of the lower (electric) characteristic height by 1–1.5 km during the years of the quiet Sun due to variation of the flux of galactic cosmic rays. Moreover, we showed that involvement of ionosphere conductivity profiles in the point source model accounting their seasonal drift provides reliable estimating the global thunderstorms’ centers activities. To explain a behavior of the intensity changes recorded in the Antarctic observatory a mechanism of additional modulation of the local resonant cavity parameters within the solar cycle is required.