Damping of magnetospheric cavity modes: A discussion

Impulsive changes in the solar wind can deposit energy into magnetospheric cavity modes. The authors discuss the coupling of cavity modes to the field line continuum, and show that the time scales for dissipative phase mixing and mode conversion to kinetic Alfven waves in the magnetosphere are long compared with lifetimes of MHD wave events with periods that vary continuously with radial distance. Therefore the ultimate dissipative sink for cavity mode energy should be the ionosphere. For model magnetospheres in which all wave components have a complex field aligned wave number kz = kz0 (1 + iκ) to simulate a Poynting flux into the ionosphere, they use simple arguments to show that the coupled cavity mode damping decrement is given approximately by γ/ω0 = (γ/ω0)c + (kz0/k)2κ. Here (γ/ω0)c is the damping via coupling to the field line continuum alone, and k is the total wave number magnitude. Detailed numerical calculations in a cylindrical magnetospheric model support this, although significant departures from the approximate expression can occur for small κ. Following earlier work by other authors, they emphasize that complex kz may not simulate the appropriate boundary condition for the coupled cavity mode outside resonance regions