This picture shows what happens when the molecules coming from the comet encounter interplanetary space.
Click on image for full size
JPL
The comet's interaction with interplanetary space, part 1
Why the magnetic field slows down to slip around the comet
When evaporation begins, the gas is propelled from the nucleus at supersonic speed (depicted by arrows in the figure). Because of the low gravity in space, this means that the molecules from the nucleus extend very far into space, and form a huge cloud around the nucleus. (Due to the scale of this picture, the nucleus is too small to be seen in this picture).
Interplanetary space is not empty, however, but contains dust, particles with an electric charge, and the magnetic field of the sun (also called the IMF, or Interplanetary Magnetic Field). The particles of interplanetary space also travel at supersonic speeds.
When the cloud of gas from the comet encounters the cloud of particles which fill interplanetary space, particles of the two clouds collide with each other and are forced to slow down to subsonic speeds. This process causes two shocks to form, a bow shock, the same sort of bow shock which exists as part of every planetary environment, and an internal shock which is unique to comets. Between these two shocks, the two clouds interact.
Because the particles slow down, the magnetic field must also slow down. The comet tail forms as the slowed magnetic field tries to slip around the coma of the comet.
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