Mirror mode expansion in planetary magnetosheaths: Bohm-like diffusion.

Observed scale sizes of mirror modes in planetary magnetosheaths tend to be equal or larger than those that correspond to the maximum growth rate of the mirror instability: 9 {rho}{sub p} (proton gyroradius). These phenomena can be accounted for by introducing a diffusion process (Bohm) that shifts the spectra to lower wave numbers as the mode convects away from the source to the observation point. The theory is applied to data obtained in the magnetosheaths of Earth, Jupiter, Saturn, and the heliosheath, and shown to provide reasonable agreement to past spacecraft observations. Further observational tests of the theory are suggested.