Decrease of Schumann resonance frequencies and changes in the effective lightning areas toward the solar cycle minimum of 2008–2009

Abstract

For the period 1988–2002, it has been shown that the SR mode frequencies and Q-factors decrease with the decrease of X-ray radiation toward the solar cycle minimum (Sátori et al., 2005). In the present study, a decrease of frequencies and Q-factors of all first four SR modes from the latest solar cycle maximum to the anomalous minimum of 2008–2009 is found using the data from Modra Observatory. Decreases obtained here for the first two SR modes are evidently greater than those obtained in the study cited above. Moreover, effective thunderstorm areas are calculated from the monthly mean diurnal frequency range of the electric field component. Our study reveals that the difference between the northern and the southern hemisphere summer areas not only declines with the decrease in solar activity but almost vanishes during the deep solar minimum of 2008–2009. Semi-annual variation in the areas dominates in years of the deep solar minimum.