Mass Loading

Mass-loading occurs when magnetized plasma and neutrals coexist in space and have different bulk velocities. When the neutrals are ionized, they accelerate to the plasma speed and gain a thermal speed equal to the component of the speed difference perpendicular to the magnetic field. The energy for this acceleration may come from the bulk flow in the case of stellar winds or from the rotation of a magnetized body. We discuss several situations where mass-loading is important. In planetary magnetospheres, the transfer of energy from the planetary rotation to the plasma via the pickup process powers magnetospheric processes such as aurora and radiative emission. The solar wind is slowed significantly by pickup ions and shocks and other solar wind structures are mediated by these hot ions. In the astrophysical context, mass-loading affects shocked subsonic pulsar winds, may lead to shells of high-temperature material, and may through interaction with clumps of neutrals produce non-spherical structure. We compare and contrast mass-loading processes in these different settings.