American Scientist

July-August 2015

Volume: 103 Number: 4

For more than two decades, the Voyager 1 and Voyager 2 probes have been headed toward the edge of the heliosphere, the enormous bubblecreated by the Sun’s extended atmosphere. That edge, called the heliopause, is defined by the pushback from the interstellar medium against thesolar wind, the charged particles that stream from the Sun. Until Voyager 1 reached the heliopause in 2012, scientists knew little about thiscritical boundary, not even how far away it is; it turns out to be around 18.2 billion kilometers from Earth. Researchers expected the regionbefore the heliopause, called the heliosheath, to be smooth and calm, but Voyager 1’s measurements show that it is anything but. In this artist’sinterpretation of the data, magnetic field lines (red and blue) connect back to the Sun, but the transition region is filled with magneticbubbles. The heliopause is not a continuous shield between the solar atmosphere and the interstellar medium after all, but a porous membranewith fingers and indentations. (Image courtesy of NASA/Goddard Space Flight Center/CI Lab.)