Solar system's tail is shaped like a four-leaf clover

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. The scientists detailed their discovery in a study in theAstrophysical Journal and during an online news media event  Wednesday. "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. While telescopes have spotted such tails around other stars, it has been difficult to see whether our star produced one. The particles found in the tail - and throughout the entire heliosphere, the region of space influenced by our Sun - do not shine, so they cannot be seen with conventional instruments. "By examining the neutral atoms, IBEX has made the first observations of the heliotail," said David McComas, IBEX principal investigator at Southwest Research Institute in San Antonio, Texas, and the paper's lead author. "Many models have suggested the heliotail might look like this or like that, but we have had no observations. We 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 said. IBEX measures the neutral particles created by collisions at the solar system's boundaries. This technique, called energetic neutral atom imaging, relies on the fact that the paths of neutral particles are not affected by the solar magnetic field. Instead, the particles travel in a straight line from collision to IBEX. Consequently, observing where the neutral particles came from describes what is going on in these distant regions. "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," said Arik Posner, Nasa's IBEX programme scientist in Washington. By combining observations from the first three years of IBEX imagery, the team showed a tail with a combination of fast and slow moving particles. There are two lobes of slower particles on the sides and faster particles above and below. This four-leaf clover shape can be attributed to the fact that the Sun has been sending out fast solar wind near its poles and slower wind near its equator for the last few years.This is a common pattern in the most recent phase of the Sun's 11-year activity cycle. The clover shape does not align perfectly with the solar system, however.The entire shape is rotated slightly, indicating that as it moves further away from the Sun and its magnetic influence, the charged particles begin to be nudged into a new orientation, aligning with the magnetic fields from the local galaxy. Scientists do not know how long the tail is, but think that it eventually fades away and becomes indistinguishable from the rest of interstellar space.