Line splitting of modes: day/night and asymmetry

In an ideal, spherically symmetric cavity each mode n is degenerate — all (2n+1) azimuthal configurations m have the same frequency. The real cavity, however, has no symmetry: day/night asymmetry (the illuminated hemisphere has a different ionospheric conductivity and height than the night one), inhomogeneities and the effective rotation of the cavity lift the degeneracy. The consequence is line splitting of modes — one resonance is split into several close lines.

The phenomenon is pronounced especially for higher modes, where the degeneracy is greater. The observed spectra therefore exhibit a fine structure that the idealized symmetric model cannot capture. It can only be reproduced by three-dimensional models, typically using the finite element method (FEM), which include the spatial inhomogeneity of the cavity in the calculation Goncharov 2019.

Line splitting of modes is therefore valuable diagnostically: its magnitude and shape carry information about the global asymmetry of the ionosphere and about the distribution of sources (storm regions). It is directly related to the lifting of degeneracy described for the eigenmodes (see #16).