Physics of mass loaded plasmas

In space plasmas the phenomenon of mass loading is common. Comets are one o f the most evident objects where mass loading controls to a large extent th e structure and dynamics of its plasma environment. New charged material is implanted to the fast streaming solar wind by planets, moons, other solar system objects, and even by the interstellar neutral gas flowing through ou r solar system. In this review we summarize both the current observations a nd the relevant theoretical approaches. First we survey the MHD methods, st arting with a discussion how mass loading affects subsonic and supersonic g asdynamics flows, continuing this with single and multi-fluid MHD approache s to describe the flow when mass, momentum and energy is added, and we fini sh this section by the description of mass loaded shocks. Next we consider the kinetic approach to the same problem, discussing wave excitations, pitc h angle and energy scattering in linear and quasi-linear approximations. Th e different descriptions differ in assumptions and conclusions; we point ou t the differences, but it is beyond the scope of the paper to resolve all t he conflicts. Applications of these techniques to comets, planets, artifici al ion releases, and to the interplanetary neutrals are reviewed in the las t section, where observations are also compared with models, including hybr id simulations as well. We conclude the paper with a summary of the most im portant open, yet unsolved questions.