Relationship of the parameters to the source-observer distance (SOD)

The source-observer distance (SOD) is the distance between the storm region and the measuring station along the Earth's surface. This distance fundamentally affects the observed frequency of the resonance peaks: the position of the peak in the spectrum depends not only on the properties of the Earth-ionosphere cavity, but also on how far from the observer the sources are located Satori 2024.

Physically this follows from the modal structure of the field in the cavity: for different SODs the relative contributions and interference of the individual modes change, which shifts the apparent frequency of the peak. The relationship between SOD and the observed frequency is therefore the key to interpreting frequency variations — without it, changes in frequency could not be correctly attributed to either a change in the cavity's conductivity or a relocation of the sources Satori 2024.

It is precisely this dependence that makes it possible to infer the geography of lightning activity from frequency data, and thus links the diurnal (#43) and seasonal (#44) variations with the specific distribution of sources.