The ELF measurements at the YS station in China during the X5.4 solar flare on 7 March 2012 are examined. The first modal Schumann resonance (SR) frequencies of the horizontal magnetic field components were found to increase by 0.1-0.2 Hz during the X-ray burst. During the enhancement of the proton flux, the first modal frequency of the east-west magnetic field component decreases by approximately 0.6 Hz at most, while the variation in the north-south magnetic field component is less well defined. The mechanisms of the variations are simulated with a finite difference time domain technique by modeling the perturbed conductivity profile in the day-night asymmetric Earth-ionosphere cavity and modeling the global lightning source with the raw flash data measured by satellites. The simulated varying trends of the SR frequencies observed near the ground with the altitudes of the conductivity perturbations are nearly the same as those previously reported and are interpreted by the two characteristic height model first proposed by Greifinger and Greifinger. It is concluded that the SR frequencies increase for enhanced conductivity above the altitude of 60-70 km because of the lowered magnetic height and decrease for enhanced conductivity below this altitude due to the lowered electric height. This finding can explain the opposite behaviors of the SR frequencies during X-ray bursts and strong solar proton events (SPEs). The simulation model in this work proved to be effective, with the simulated shifts in the values of SR frequencies during X-ray bursts and SPEs being close to the practical measurements.
IntroductionThe Schumann resonance (SR) is excited by the global thunderstorms and propagates with low attenuation in the closed waveguide confined by the Earth's surface and the lower ionosphere. The SR parameters, including the modal frequencies, amplitudes, and Q values, have robust relationships with the global lightning activity, and the inverse electromagnetic problem aimed at reducing the distribution of global lightning activity from the SR records has been proven to be feasible and has become a popular topic of study in the past few decades [Nickolaenko et al., 1998;Shvets et al., 2009Shvets et al., , 2010Ondrášková et al., 2011]. Due to the close correlations between the global lightning activity and the climate, the SR can also be used to detect the slight fluctuations in the tropical atmosphere temperature, the content of the upper tropospheric water vapor and the position of the lightning activity in El Niño and La Niña years; these studies have received great attention in the past few decades [Williams, 1992;Kent et al., 1995;Sátori and Zieger, 1999;Price, 2000;Sekiguchi et al., 2006;Sátori et al., 2009].Meanwhile, the characteristics of the SR also depend on the electrical properties of the lower ionosphere, which are strongly associated with the solar activity at different time scales. There are multiple studies demonstrating clear links of the SR parameters with the solar X-ray radiation variati...