Damping of magnetospheric cavity modes: A discussion

Abstract

Impulsive changes in the solar wind can deposit energy into magnetospheric cavity modes. We 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 Alfvén 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, we use simple arguments to show that the coupled cavity mode damping decrement is given approximately by γ/ω0 = (γ/ω0)c + (kz0/k)² κ. 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, we emphasize that complex kz may not simulate the appropriate boundary condition for the coupled cavity mode outside resonance regions.

Magnetospheric Cavity Resonance Oscillations with Energy Flow across the Magnetopause.

“…Previous works placed the inner boundary at the plasmapause (for example, the box model used by Southwood (1985, 1986) and the dipole model used by Lee and Lysak (1989)) or at the ionosphere (Fujita and Patel, 1992). These theoretical studies examined the effect of the ionospheric dissipation on coupled oscillations (Allan et al, 1986a;Krauss-Varban and Patel, 1988;Allan and Poulter, 1989) and the ionospheric transmission property (Kivelson and Southwood, 1988).…”

A Possible Signature of Magnetic Cavity Mode Oscillations in ISEE Spacecraft Observations.

“…These discrete cavity modes decay by coupling to the transverse Alfven waves on L shells where the frequency of the field line resonances matches that of the cavity mode. They can also decay by depositing Poynting flux from the wave into the low latitude ionosphere (Allan and Poulter, 1989) or by leaking energy into the magnetotail. In this latter case, the cavity may be better approximated as a wave guide (Samson and Harrold, 1992) with the azimuthal wavelength imposed by the source.…”

“…They interpreted this low damping rate as evidence that the wave was driven by a compressional magnetospheric cavity mode stimulated by an earlier sudden pressure pulse in the solar wind. Numerical analyses (Allan and Poulter, 1989;Allan and McDiarmid, 1989) have provided insight into the relation between damping produced by direct energy losses to the ionosphere and damping arising from coupling to the continuous spectrum of field line resonances. Their analysis supports the interpretation of Crowley et al (1987).…”

Field line resonances in discretized magnetospheric models: an artifact study

“…The time scales of the packeting clearly relate to the described mechanism. Allan et al (1985) and Allan and Poulter (1989) have shown amplitude modulations near a resonance as a result of beating due to the frequency mismatch between the driving cavity mode and the natural Alfve n frequency on one particular ®eld line. In contrary the interaction presented in this work is caused by the frequency mismatch between two azimuthal perturbations on two neighboring ®eld lines.…”