Limitations of mass-loading models

The mass-loading concept is discussed in relation to the dynamics of magnetoplasma streaming through rarefied background gas. Changes in energy and momentum flux (generally losses) can outweigh the increases in mass flux. Suprathermal ion components cannot be simply described in fluid terms: as shown by the probes to comet Halley, the main cometary ions are depleted by interaction with the background gas faster than they are scattered and thermalised by plasma turbulence. MHD instabilities tend to isotropize pitch angles but do not thermalise the ions, while wave steepening into a bow shock occurs outside positions expected from mass-loading. In the strongly-loaded subsonic region, charge exchange of suprathermal ions causes energy losses that can be more significant than further increases of mass. Non-parallel pick-up of new implanted ions, large gyroradii and finite spatial scales also limit the validity of fluid models.