The strength of the Voyagers is that they directly measure magnetic fields and how the fields change over distance as the spacecraft travel farther out from the sun. In a study recently published in the Astrophysical Journal Letters, Rankin and her team of researchers paint the first clear picture of the draped region by bringing together independent measurements from the twin Voyager probes and a model of the heliosphere-interstellar boundary sourced from NASA’s Interstellar Boundary Explorer (IBEX), an Earth-orbiting satellite launched in 2008. But exactly what this draping looks like and how it gives way to the pristine interstellar medium have remained open questions-that is, until now. And just like this rippling wake can reveal a ship’s outline, the bending of interstellar magnetic fields around the heliosphere as our star moves through the Milky Way can provide important clues about the size and shape of the bubblelike boundary between our solar system and the rest of the galaxy. This “draping” effect is similar to how flowing water piles up around a ship’s bow and along its flanks, tailward. “Retrospectively it made sense that there should be a transition region where the interstellar magnetic field bunches up and drapes against the heliosphere,” says Jamie Rankin, deputy project scientist for the Voyager mission and a space physicist at Princeton University. But one measurement didn’t fit expectations: the spiral magnetic field didn’t straighten out like it was supposed to if the spacecraft had indeed crossed over. Just more than a decade ago, NASA’s Voyager 1 beamed back data suggesting it had finally exited the heliosphere to enter interstellar space. Yet despite the heliosphere being known and studied since the late 1950s, its hazy boundaries have only relatively recently come to light-with a surprising discovery. To venture beyond where this wind wanes against the greater flows of plasma coursing through our galaxy is, in a very real sense, to leave our solar system behind. This ionized gas flows outward along magnetic field lines emerging from our star, spooling out in radial spirals tied to the sun’s rotation. It’s called the heliosphere, and it’s made of tenuous plasma billowing from the sun.
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