How Do Schumann Resonance Frequency Changes in the Vertical Electric Field Component Reflect Global Lightning Dynamics at Different Time Scales?

The electromagnetic waves in the Schumann resonance (SR) frequency range (<100 Hz) radiated by natural "lightning antennas" excite the Earth-ionosphere cavity confined between the Earth's surface and the lower ionosphere. The peak frequencies of SR are known to vary with source-observer distance (SOD), while the daily frequency range (DFR: f(max) - f(min)) is also indicative of the average size of thunderstorm regions. This paper provides observational evidence for these relationships based on SR frequency observations of the vertical electric (E-Z) field component at Nagycenk (NCK), Hungary in Central Europe from the period 1994-2015. Variations of the peak frequencies are considered on the annual, seasonal and diurnal time scales as well as during a specific event when squall-line formation of lightning activity in South America moves toward NCK. DFR is studied in relation to the El Ni & ntilde;o Southern Oscillation (ENSO). Increasing area of lightning activity in mid-high Northern hemisphere latitudes has been identified by DFR variations during the transition from warm to cold episodes of the ENSO in 1998 and 2010. The extension of the lightning area is considered as a consequence of energy released in the tropics and exported to higher latitudes with some months of delay from the end of the El Ni & ntilde;o episodes. The frequency variations are interpreted via model calculations and supported with satellite-based optical lightning observations (Optical Transient Detector, Geostationary Lightning Mapper). The described variations of SR peak frequencies and DFR yield information on the global/regional lightning dynamics and on this basis they have important application to climate issues as well. Plain Language Summary Schumann resonances (SR) are global electromagnetic resonances maintained by global lightning activity in the cavity resonator formed by the Earth's surface and the lower ionosphere. This paper provides observational evidence for the relationships between the variations of SR peak frequencies of the first three resonance modes and the global/regional lightning dynamics on different time scales based on SR observations of the vertical electric (E-Z) field component. As lightning is one of the essential climate variables that provides valuable information on the state of the atmosphere, the lightning source-observer distance and lightning area-dependent frequency variations demonstrated in this paper have important applications to climate issues as well.