NASA’s Voyager 1 probe has been sailing towards the edge of the solar system since 1977, and is now in a turbulent region known as the heliosheath, where solar wind emanating from the sun is slowed by the pressure of interstellar gas. Recently, Voyager 1 entered a series of strange and unexpected transitional regions 11 billion miles from the sun. Now, thanks to a complimentary mission, scientists will gain a bigger picture of what Voyager will encounter as it speeds out of our solar system.
The new data come from IBEX, NASA’s Interstellar Boundary Explorer, which has achieved the first map of our solar system’s tail, the heliotail. According to IBEX, the heliotail’s shape resembles a four-leaf clover.
“Many models have suggested the heliotail might look like this or that, but we have had no observations,” said David McComas, IBEX principal investigator and lead author on the paper describing the new findings in the July 10 issue of the Astrophysical Journal. Quoted in NASA’s press release, McComas went on to say that scientists “always drew pictures where the tail of the solar system just trailed off the page, since we couldn’t even speculate about what it really looked like.”
McComas and colleagues combined three years of imagery taken by the IBEX satellite to ascertain the structure of the heliotail. Although telescopes have spied tails associated with other stars, our sun’s heliotail has proven elusive because its particles do not shine, like all particles in the heliosphere, the region influence by the sun.
IBEX uses a technique called energetic neutral atom imaging, which measures the neutral atoms created by particle collisions at the edge of the heliosphere. The neutral atoms travel in straight lines from the points of collision to IBEX, their paths unaffected by the sun’s magnetic field.
These IBEX data revealed the four-lobed shape of the heliotail, with two lobes of slower particles on the sides and two lobes of faster particles above and below. This shape is connected to the sun’s 11-year activity cycle, with faster solar winds emanating from the sun’s poles and slower winds from around its equator. The heliotail is not perfectly aligned with our solar system, but rather is rotated slightly, suggesting that the charged particles of the tail are forced into a new orientation as they move farther from the sun’s magnetic influence and towards the local magnetic field of the Milky Way. The heliotail apparently diminishes and becomes indistinguishable from the interstellar medium.
Unlike the local data collected by the Voyager 1 and 2 probes, the new IBEX data allow scientists to understand the three-dimensional interaction of the sun with surrounding interstellar space.