Scientists working on NASA's Interstellar Boundary Explorer mission have nabbed their first direct glimpse of the so-called heliotail, the long trailing edge of the solar wind. Much to their surprise, three years of data from IBEX, as the Earth-orbiting craft is known, showed that the tail has a sort of clover shape, with separate "lobes" of faster- and slower-moving solar wind.
"Scientists always presumed the heliosphere had a tail," IBEX mission scientist Eric Christian said during the conference. "But this is the first real data we have that gives us the shape of the tail. We've never taken a picture of it."
The heliosphere is the vast magnetic bubble around the solar system, created by solar wind emanating in all directions from the sun. As the heliosphere moves through the interstellar medium, its tail trails behind -- much like the tail of a comet, according to study lead author and IBEX principal investigator Dave McComas of the Southwest Research Institute in San Antonio.
IBEX's instruments measure neutral atoms created near the edge of the heliosphere that zoom back toward Earth. Because such particles move in a straight line, Christian explained, you can map them out to generate a picture of the structure they form.
In this case, IBEX's data indicated that a person viewing a cross-section of the tail would see two "lobes" of faster-moving particles, aligned more or less at the top and bottom of a circle, and two more lobes, composed of slower-moving particles, to the left and right. The overall effect is a clover shape, and was "really not expected," McComas said. He said the shape matched a pattern of high and low energy radiation associated with an earlier period in the solar cycle during which the measured particles probably escaped the sun.
The scientists also noticed that the clover shape seemed to tilt slightly, probably caused by magnetic fields flattening and twisting the heliotail.
During the media event, astrophysicist Brenda Dingus of the Los Alamos National Laboratory said that IBEX's findings would help researchers studying cosmic rays. McComas said that IBEX's data also aid the work being done by the Voyager 1 and Voyager 2 probes, which are hurtling toward the leading edge of the heliosphere (in the opposite direction from the tail).
"These two missions are incredible complementary," he said. "IBEX is like an MRI -- you take an image of the whole body to see what's going on -- and the Voyagers are like biopsies," focusing in more tightly on a particular spot.