The heliosphere is a bubble in space "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself. It was thought for decades that it extends in a long comet-like heliotail, but in 2009 data from the Cassini and IBEX show a different shape. However, depiction of the heliotail is still common. Another change is that the heliosheath area is not smooth but filled with magnetic bubbles.NASA 2011
For the first ten billion kilometres of its radius, the solar wind travels at over a million km per hour. As it begins to drop out with the interstellar medium, it slows down before finally ceasing altogether. The point where the solar wind slows down is the termination shock; then there is the heliosheath area; then the point where the interstellar medium and solar wind pressures balance is called the heliopause; the point where the interstellar medium, traveling in the opposite direction, slows down as it collides with the heliosphere is the bow shock.
As of June 2011, the heliosheath area is thought to be filled with magnetic bubbles (each about 1 AU wide), creating a "foamy zone". The theory helps explain in situ heliosphere measurements by the two Voyager probes.
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