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Practically anyone can recognize the distinct shape of a comet in the night sky: a small “match-head halo” with a long, feathery tail. Both the halo and tail are formed of material that sublimates off an icy nucleus and is then dragged out by the solar wind that fills interplanetary space. But the physics of the interaction between the solar wind and the comet material is surprisingly complex and hard to capture using conventional analysis or magnetohydrodynamic (fluid) simulations. Jan Deca, from the University of Colorado Boulder and NASA/SSERVI, and colleagues  have now produced detailed 3D particle-kinetic simulations of the intricate interaction between the solar wind and a comet. The results provide a clear “overview” of the rich physics of the most common cometary halos, including that of comet 67P/Churyumov-Gerasimenko observed by the Rosetta spacecraft.

The solar wind is a hypersonic flow of tenuous plasma that streams out from the Sun. Its existence was inferred in the 1950s from observations of comet tails  and finally detected directly with the Mariner 2 spacecraft during the early space program. More recently, long-exposure images of comet tails, observed with NASA’s STEREO spacecraft, have been used to probe large-scale turbulence and gustiness in the solar wind itself. The observed large-scale structure of the tails has also been used to probe the composition and volatile content of different comets.

Both the solar wind and cometary halo material are collisionless, that is, their constituent particles never interact directly with one another. Nevertheless, all of the particles making up the solar wind and any comet tails within it affect one another via the large-scale electromagnetic field that the particle ensemble generates. They form a multicomponent plasma in which different fluids of particles interact weakly while occupying the same space. Detailed interactions between individual populations of particles and the field are also important and percolate across spatial scales, yielding a hybrid system that is complex to treat numerically.

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