A puzzling knot of atoms at the edge of the solar system is fading, leaving space physicists with an even gnarlier mystery to unravel about the outer boundary of the heliosphere, where the vast bubble in which the solar system resides meets up with interstellar space.
Researchers reported last year that NASA’s Interstellar Boundary Explorer, or IBEX, spacecraft had discovered the knot — the densest part of an equally unexpected narrow ribbon of neutral atoms that makes nearly a complete circle around the fringes of the solar system (SN: 11/21/09, p. 15), about 100 to 125 times Earth’s distance from the sun.
Now a map compiled by IBEX just six months after the first sighting shows that the ribbon remains, but the bright, dense knot has dimmed dramatically, diffusing into the ribbon, says IBEX researcher David McComas of the Southwest Research Institute in San Antonio and the University of Texas at San Antonio. He and his colleagues report the finding online September 29 in the Journal of Geophysical Research–Space Physics. They also discussed the study during a telephone press briefing September 30.
“However shocking it was to see a narrow ribbon structure initially, it’s even more shocking that you could have something change on a time scale as short as six months,” McComas says. In contrast, large clouds and other scenery that the solar system encounters as it moves through space take thousands of years to change. McComas also notes that the sun’s magnetic field, which fills the heliosphere and might be expected to have something to do with the knot’s appearance, has an 11-year cycle. He suspects that the fading knot may be related to the unusually quiescent solar cycle that just ended and the shrinking of the heliosphere due to the reduction of the solar wind during that time.
Nonetheless, the “untying of the knot,” as McComas calls the dimming of the bright feature, “reinforces the whole basic idea that what we thought was going on in interactions at the edge of the solar system we just didn’t get right at all.” Two additional IBEX maps, taken six months and a year after the one analyzed in the new report, show a similar dimming.
Because the wind of ions blowing out from the sun and the solar magnetic field that threads through the heliosphere shield the solar system from galactic cosmic rays — energetic charged particles from the Milky Way — researchers are struggling to understand the processes that play out at the solar system’s edge.
“Models that try to explain the interaction between the solar system and the rest of the galaxy are missing key effects,” says Merav Opher of George Mason University in Fairfax, Va., who was not a participant in the new study.
During the most recent solar magnetic field cycle, in which the sun has been unusually quiescent, more galactic cosmic rays appear to have infiltrated the solar system, although a direct connection between changes in the cosmic ray population and global changes in the heliosphere are not definitive, says IBEX researcher Nathan Schwadron of the University of New Hampshire in Durham.
Schwadron says that last year he was “pretty well sold on the idea” that the ribbon and knot were generated by atoms from the solar wind that had streamed out beyond the heliospheric boundary and had been reflected back by the interstellar magnetic field. “However, the time variations we are seeing now throw yet another monkey wrench into that explanation,” he adds.
A combination of reflection, compression of the outer part of the heliosphere and events in which magnetic fields break apart and reconnect with neighboring fields may all be playing a role, he suggests.
Understanding the extent to which magnetic fields at the boundary of the solar system filter out harmful cosmic rays will not only guide future astronauts as they make longer and deeper forays into space but may also help perfect models for assessing the potential for habitable planets around other stars, Opher adds. Even a planet that had the right temperature to maintain liquid water could not sustain life if it were bombarded by a huge dose of galactic cosmic rays, she notes.
Time variations in the ribbon of neutral atoms at the edge of the solar system are shown in these maps created by NASA’s IBEX spacecraft.
Credit: IBEX Science Team, NASA’s Goddard Scientific Visualization Studio
This animation depicts a knot of neutral atoms at the solar system’s edge dissipating over a six-month interval.
Credit: IBEX Science Team, NASA’s Goddard Scientific Visualization Studio, ESA