Studies of the variations of the first Schumann resonance frequency during the solar flare on 7 March 2012

Abstract

Abstract 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.