SR tomography of the ionosphere

Besides reconstructing the sources, the inversion can also be turned "upward" — toward deriving the state of the ionosphere. The modal parameters of the Schumann resonance (the resonance frequencies of the individual modes and their quality factor Q) depend on the conductivity profile of the cavity, especially on the lower edge of the ionosphere formed by the D-region. From the measured frequencies and Q-factors one can therefore infer the conductivity profile and the overall state of the D-region.

This principle is sometimes called SR tomography of the ionosphere: the SR field carries integral information about how the cavity is "tuned", and changes in the modal parameters reflect changes in the height and conductivity of the reflecting layer. Because both the frequency and Q respond to the ionization of the upper layers of the atmosphere, this link is closely related to the topics of space weather (cluster H) — for example solar flares and proton events, which change the ionization of the D-region.

This is an actively developing area: the reconstruction of the conductivity profile from a few modal numbers is again ill-posed, and the interpretation depends on assumptions about the shape of the profile. For this reason this method is rather a complement to direct ionospheric measurements than a replacement for them.