Heliosphere

The heliosphere is a bubble of charged particles in the space surrounding the Solar System, "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.

For the first ten billion kilometers of its radius, the solar wind travels at over 1,000,000 km/h. As it begins to interact with the interstellar medium, it slows down before finally ceasing altogether. The point where the solar wind begins to slow is called the termination shock; then the solar wind continues to slow as it passes through the heliosheath leading to a boundary where the interstellar medium and solar wind pressures balance called the heliopause.

Beyond the heliopause, where the interstellar medium collides with the heliosphere, it was once thought there was a bow shock. However, data from the Interstellar Boundary Explorer suggests that the velocity of the Sun through the interstellar medium is too low for a bow shock to form. Also, Cassini and IBEX data challenged the "heliotail" theory in 2009. Voyager data led to a new theory that the heliosheath has "magnetic bubbles" and a stagnation zone.

The 'stagnation region' within the heliosheath, starting around 113 AU, was detected by Voyager 1 in 2010. There the solar wind velocity drops to zero, the magnetic field intensity doubles, and high-energy electrons from the galaxy increase 100-fold. Starting in May 2012 at 120 AU, Voyager 1 detected a sudden increase in cosmic rays, an apparent signature of approach to the heliopause.