Rosetta observations of the birth and growth of a solar wind cavity around a comet
Different collisionless plasma regions do not mix easily. When two plasma components meet, structures form, such as collisionless shocks and cavities in stellar and interstellar winds. These boundaries extend over much smaller scales than the mean free path for binary collisions of the gas particles. Current knowledge of how different interplanetary plasma regimes interact stems primarily from spacecraft observations obtained during fast passages through boundaries around planets.
The initial formation and microphysics of such structures, however, remain elusive. The Rosetta mission provided detailed observations of the growth of a cavity in the solar wind around comet 67P. As the comet approached the Sun, the plasma of cometary origin grew enough in density and size to present an obstacle to the solar wind. Our results demonstrate how the initial slight perturbations of the solar wind prefigure the formation of a cavity, with a particular interest placed on the discontinuity passing over the spacecraft. The slowing down and heating of the solar wind can be followed and understood in terms of single particle motion. We propose a simple geometric model which can explain the observations, and shows how a cometary magnetosphere and bow shock are seeded from the gradual steepening of an initially slight solar wind perturbation.
Comets might be the only natural experiment within our reach for in-situ study, where shocks and cavities in the ambient plasma stream periodically form and vanish. These plasma structures are to be found in various environments and scales through the universe, and these observations shed a new light on the microphysics of such interactions.
Created 2016-10-28 11:37:53 by Uwe Raffalski Last changed 2016-10-28 11:37:53 by Uwe Raffalski