Three years of observations by NASA’s Interstellar Boundary Explorer (IBEX) have shown that the Solar System has a tail shaped like a four-leaf clover. The stream of particles that blows away from the Sun, known as the solar wind, creates a bubble of charged (ionized) gas around the Solar System. Because of the Sun’s motion through the Galaxy, this bubble—the heliosphere—instead of being round, is drawn out so that it has a tail, called the heliotail. But before IBEX we had no way of mapping the structure and shape of this tail.
Launched October 19, 2008, by an Orbital Sciences Pegasus XL rocket, the 110-kilogram IBEX (a spacecraft in the Small Explorer program) moves in a very large, elongated orbit around the Earth that takes it out almost as far as the Moon. It builds up pictures of the boundary of the Solar System using a technique called energetic neutral atom imaging. Collisions between charged particles from the Sun and charged particles from interstellar space can produce neutral atoms, some of which are shot in our direction. It’s these atoms that IBEX looks for. Carrying no charge, they aren’t deflected by magnetic fields, and therefore travel in straight lines from their points of origin. So by observing them over time, IBEX can accurately trace out the edge of the Sun’s magnetic domain in every direction.
The new results, published in the July 10 edition of the Astrophysical Journal, reveal the heliotail to have a four-lobed structure. In the horizontal plane are two lobes containing slower-moving particles, while above and below, completing the four-leaf clover shape, are two other lobes with faster particles. Broadly speaking, this fits in with the pattern that over the past few years the Sun has been sending out slower particles from near its equator and faster ones from around its poles. Now that the Sun is reaching the peak of its activity, the bands of slow and fast particles in the solar wind are breaking down so that over time, as these newly-emitted particles penetrate into the heliotail, the lobed structure is likely to morph into a different shape.
Interestingly, the clover-leaf structure isn’t aligned exactly with the Solar System. The entire shape is skewed a bit, reflecting the fact that at greater and greater distances the Sun’s magnetic influence wanes and the tail starts to be affected more by the fields in interstellar space.
Finally, the newly-refined map of the Solar System’s magnetic envelope may shed light on how some powerful cosmic rays, coming from elsewhere in the Galaxy and beyond, manage to penetrate the heliosphere. The heliotail has long been suspected of being a weak spot in the Sun’s defenses against galactic high-energy particles. The data from IBEX may help show how cosmic rays are able to funnel in from the tail and reach the inner Solar System.
“Since first light in 2008, the IBEX mission team has amazed us with its discoveries at the interstellar boundary, including a previously unknown ribbon of energetic neutral particles stretching across it,” said Arik Posner, NASA’s IBEX program scientist. “The new IBEX image of the heliotail fills in a previously blank area on the map. We are first-hand witnesses of rapid progress in heliophysics science.”
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