July 15, 2013 | 1
Astronomers just got their first good look in the solar system’s rearview mirror. What they see is a magnetic tail that twists like a corkscrew.
Although we tend to think of the sun as stationary with the planets orbiting about it, in fact the solar system is plowing through the galaxy at a million miles per hour. All the time the sun is throwing off charged particles, called the solar wind, in all directions. As the sun moves through the galaxy’s magnetic fields, the solar wind gets flattened into a bullet shape, with a long tail that drags behind.
The observations were made with the Interstellar Boundary Explorer, or IBEX, a satellite, launched by NASA in 2008. Unlike a conventional telescope, IBEX doesn’t see light, it sees atoms. At the boundary of the solar wind and the interstellar medium (the cold, dark gas and dust that fills the space between the stars) ions from the solar wind steal electrons from the atoms of the interstellar medium. This changes the charge of the solar ions from positive to neutral, freeing them from the sun’s magnetic field. They fly off in different directions—some of them reaching the detectors on board IBEX, which translates the atoms’ direction and energy into a map of the bullet-shaped heliosphere.
By compiling data recorded between 2008 and 2011, mission scientists assembled a map of the structure of the heliotail, which drags behind the solar system like the wake of a boat. Because we’re looking down the length of the tail, it’s hard to measure how far out it stretches. From the energies of the detected particles, mission scientists estimate that the tail is evaporating along a length 1000 times greater than the distance between Earth and the sun—roughly 150 billion kilometers.
“What’s new about this [finding] is we were never able to look away from the direction we are moving through the interstellar medium,” said David McComas, IBEX principal investigator and assistant vice president for space science and engineering at Southwest Research Institute (SwRI), at a press conference on July 10.
As the tail stretches out, the researchers found, it corkscrews slightly. “This tilt is a really interesting aspect of the observations,” said McComas. The twisting “can’t be caused by the sun.” The most likely culprit is the galactic magnetic field tugging on the solar wind.
“Imagine putting bungee cords around a beach ball,” McComas explained. “By pulling on [the cords] they exert a force. That’s the same thing a magnetic field does: it exerts force on the outer boundary of the heliosphere. That force squeezes the heliotail so it’s no longer circular but is actually flattened like an egg. But it also twists the tail and turns it to align with the magnetic field.”
The IBEX team is using data from the Voyager probes to piece together the structure of the heliosphere—the bullet shaped bubble carved out by the forward motion of the solar system through the galactic medium. The twin Voyager probes were launched in 1977 and are currently passing through the front end of the heliopause, 18 billion kilometers from home. The Voyagers are sending back data on the interaction between the solar and galactic magnetic fields. “IBEX is like an MRI,” McComas said, “where we can take an image of the entire body and see the big picture, whereas Voyager is more like a biopsy. They’re extremely precise local measurements that are only at one or two specific locations.”
With data from IBEX, Voyager and many other satellites looking at the sun itself, researchers believe they are entering a new era of solar research. “Whenever the sun coughs or has an eruption, we can see it from all sides,” said Arik Posner, the IBEX program scientist.
The IBEX mission is scheduled to continue operating through 2016. This will give astronomers a chance to watch how the heliosphere evolves and reacts to changes from the sun. “IBEX basically sees solar wind that was produced years earlier. During solar maximum, [predicted to occur later this year], that structure breaks down. Over the next several years, those particles will travel out and get bent down the heliotail. Our expectation is that we should be able to see the changing sun on the heliotail structure. We hope to be able to make those measurements with IBEX if it stays healthy.”