Magnetospheric magic frequencies as magnetopause surface eigenmodes

The persistent so-called magic magnetospheric frequencies are thought to be either directly driven by monochromatic solar wind pressure fluctuations or resonantly excited global (cavity/waveguide) or magnetopause surface eigenmodes. We distinguish between these cases by statistically investigating, using simultaneous observations, the magnetospheric response to jets in the subsolar magnetosheath. The broadband jets do not exhibit discrete frequencies but do drive waves at the discrete magic frequencies, with both direct and resonant driving. We show that the expected fundamental frequencies of magnetopause surface eigenmodes have two preferential values over a wide range of upstream conditions, corresponding to fast and slow solar wind, and that their harmonics are in good agreement with the magic frequencies. We also show that the waves are largely inconsistent with global modes outside the plasmasphere. Thus, we conclude that these magic frequencies are most likely due to magnetopause surface eigenmodes.