Magnetospheric field line resonances: A comparative planetology approach

Planetary magnetospheres are natural laboratories for many interesting plasma physical processes which are difficult to study under normal laboratory conditions. Among the major processes occurring in space plasmas are the reconnection phenomenon and field line resonances. This paper deals with the second of these processes. A field line resonance is the resonant coupling between an isotropic mode and an anisotropic mode in a magnetized plasma. Field line resonances allow us to understand many features of ultra-low frequency oscillations in the terrestrial magnetosphere, that is resonant mode coupling is the current paradigm to explain geomagnetic pulsations. A brief historical introduction as well as a physical description of the field line resonance is given. Resonant mode coupling is discussed for the terrestrial, Hermean (Mercury), and Kronian (Saturn) magnetospheres, which represent natural laboratories with different conditions such as size of the laboratory, the background plasma density and composition, and the strength of the magnetic field. This comparative approach allows a deeper insight into the critical coupling problem than an isolated study of the terrestrial field line resonance phenomenon. Finally, resonant mode coupling between elastic wave modes in the solid Earth is briefly tackled and compared with the magnetospheric situation.