Spectral analysis, Lorentzian fit and the I-LOR method

From the digitized signal, the modal parameters of the SR (frequency, amplitude and Q-factor of the individual modes) are obtained by spectral analysis. As standard, the power spectrum is computed using the FFT, and the individual resonance peaks are fitted with a Lorentzian function (Lorentzian fit), from which the parameters of each mode are read off.

A more advanced approach is the I-LOR method (Isolated Lorentzian, Mushtak and Williams), which suppresses intermodal interference — the mutual overlap of neighboring modes — as well as the influence of cultural noise. It thereby provides more stable and accurate estimates of the modal parameters than a simple fit of the whole spectrum Guha 2017.

An important step is the removal of large transients. Strong discharges (Q-bursts) distort the background spectrum, and so they are filtered out by median filtering with a threshold of approximately 16 CSD (core standard deviations) Guha 2017. After removing these events, the background spectrum stabilizes in roughly 10-12 minutes, which is the typical time needed to obtain a reliable estimate of the parameters of the individual modes.

*Cluster E file — part of the Schumann resonance knowledge base.*