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The area around the Sun where the solar wind exerts its force is known as the heliosphere, but the name is misleading. We've known for a long time its shape isn't spherical, but it took longer to make it reveal its true form to us. Now, however, the heliosphere's boundary has been mapped for the first time, offering insight into the forces that sculpt our bubble from the outside.
As the solar wind pushes outwards from the Sun it gradually loses strength, eventually becoming unable to overcome even the weak winds of interstellar space. The location where this happens is called the heliopause, and the area inside is known as the heliosphere.
Both Voyager spacecraft have crossed the heliopause, beaming back its location in the directions they left the Solar System. However, sending probes on enough extra-system missions to create a detailed map would be slow and prohibitively expensive. A faster method has been described in the Astrophysical Journal.
"Just as bats send out sonar pulses in every direction and use the return signal to create a mental map of their surroundings, we used the Sun's solar wind, which goes out in all directions, to create a map of the heliosphere," Dr Dan Reisenfeld of Los Alamos National Laboratory said in a statement.
Sonar of course relies on a return signal. In this case, it's hydrogen and oxygen atoms known as energetic neutral atoms (ENAs) produced by collisions between particles in the solar and interstellar winds. As its name suggests, NASA's Interstellar Boundary Explorer (IBEX) satellite was designed with one capacity in mind: to detect ENAS and identify the direction from which they come.
IBEX checks each seven by seven piece of the sky every six months. Variations in Solar activity produce an uneven solar wind, which is in turn reflected in the number of ENAs that IBEX detects. It takes two to six years for a peak to reach the heliopause and the ENAs produced to travel back to IBEX's location in an orbit that dips in and out of Earth's magnetosphere. The longer the delay, the further the heliopause must be in the direction the ENAs are coming from.
The primary reason the heliosphere is not, in fact, spherical is that the Sun is moving through the galaxy, and therefore through the interstellar medium. It's 110–120 Astronomical Units (AU) to the heliopause in the direction the Sun is traveling, and at least 350 AU behind. The trailing distance could be greater, however, since IBEX isn't sensitive enough to detect a more distant heliopause. External influences, such as inconsistencies in the interstellar medium, play a subsidiary role, for example producing a ribbon of high ENAs.
The true shape of the heliosphere has been a matter of discussion in astronomical circles for some time, with many attempts to model it. One paper combined less complete IBEX data with information from the Cassini and New Horizons space probes to propose the heliosphere resembles a “deflated croissant”.
The heliosphere may seem distant, but that doesn't mean it can't affect us. It serves as a bubble in which the Earth, and other planets, are shielded from the high-energy radiation between the stars, like a vastly larger version of our own ionosphere.
