Eigenfrequencies in the complex plane for m = 1 from the 2D solver (δ = 0, νt = 250 km2 s−1) and the 1D solver (νt = 250 km2 s−1). Modes with positive imaginary frequencies are self-excited (unstable). The red vertical line shows the observed frequency of the high-latitude symmetric mode at −86.3 nHz. The red symbols indicate the modes from the models, which have frequencies and surface eigenfunctions close to those observed.
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