Sun’s Heliopause: A Moving Target

Isn't it counter-intuitive to name a spacecraft the Interstellar Boundary Explorer, knowing full well that it'll never venture more than about 200,000 miles (300,000 km) from its home planet?

Artist concept of the IBEX satellite.
NASA / Goddard Space Flight Center
That thought crossed my mind a couple of years ago, when IBEX was launched into a looping orbit that stretches halfway to the Moon but no farther.

However, within a year the spacecraft had amazed its science team. Its first all-sky map revealed, for the first time, the nature of the region 8 to 10 billion miles away where the Sun's magnetic bubble, the heliosphere, meets interstellar space.

How, you might ask, is that possible?

When the outward-racing solar wind reaches the heliosphere's edge, the heliopause, it mingles with atoms in the interstellar medium. That's where energetic solar-wind protons can steal electrons from the slower-moving atoms of interstellar hydrogen. This charge exchange turns the protons into electrically neutral hydrogen atoms. No longer controlled by the solar wind's magnetic field, and still moving fast, they zip away from the interstellar boundary in all directions.

Some of these "energetic neutral atoms," or ENAs, make it all the way back to Earth, where they're recorded by two particle detectors on IBEX. These instruments record the number and energy of atoms arriving from small spots of sky about 7° across (about the size of a tennis ball held at arm's length). IBEX is slowly rotating, with its spin axis always pointing at the Sun, so the detectors scan overlapping strips that gradually create an all-sky "snapshot" every six months.

The Interstellar Boundary Explorer (IBEX) completed its first all-sky map of the complex interactions occurring at the edge of the solar system in mid-2009. Labels show the current location of Voyagers 1 and 2 and of the leading "nose" of the Sun's heliosphere.
IBEX Science Team
The first IBEX map showed that the interactions around the heliopause are more varied and dramatic than imagined, punctuated by a long "ribbon" of strong ENA emission that had — and still has — space physicists scratching their collective heads. The team's first report of results ticked off six different ideas for what might cause it; other researchers later chipped in a seventh.

Yesterday mission scientists unveiled the spacecraft's second all-sky scan, and again the results were surprising. The interaction isn't static but instead has shape-shifted here and there. Most notably, a "hotspot" of ENA emission along the ribbon has smeared out. The full results were published in the September 27th issue of Journal of Geophysical Research.

A 135°-wide portion of IBEX's first two all-sky maps shows how the intensity and distribution of a "knot" of emission from energetic neutral atoms changed over a six-month period.
IBEX Science Team
David McComas (Southwest Research Institute), IBEX's principal investigator, mission and the paper’s lead author, did not see this coming. “We thought we might detect small changes occurring gradually throughout the Sun’s 11-year-long activity cycle, but not over just 6 months,” he admits. “These observations show that the interaction of the Sun with the interstellar medium is far more dynamic and variable than anyone envisioned.”

It'll be interesting to see how much the interstellar boundary changes in the years ahead. Right now solar activity is only slowly emerging from a deep sleep. Once solar maximum arrives (in mid-2013, the experts say), the solar wind will gust and fling protons at the heliopause with more energy and intensity. IBEX should see an uptick in ENA emission a year or so after that (the ENAs take a while to make their way back home).

Only two spacecraft, Voyagers 1 and 2, have ventured far enough to probe this region directly. While they have recorded hails from the heliopause as bursts of radio energy, they lack IBEX's ability to map the ENA emission. (Status check: Voyager 1 is now 115 astronomical units from Earth, while Voyager 2 is 93 AU away.)

Meanwhile, IBEX hasn't limited its discoveries to the threshold of interstellar space. It turns out that ENAs also arise much closer to home, along the edge of Earth's magnetosphere, the magnetopause. The solar-wind protons, racing outward at hundreds of miles per second, run into the exosphere, a very tenuous cloud of hydrogen atoms escaping from our atmosphere. It's never been clear how far the exosphere extends into space, but this crash scene is about 35,000 miles (55,000 km) up, over Earth's dayside hemisphere. "Where the interaction is strongest, there are only about 8 hydrogen atoms per cubic centimeter," explains Stephen A. Fuselier (Lockheed Martin Space Systems).

IBEX has even recorded an ENA signature coming from the Moon. Apparently some solar-wind protons must be bouncing off the lunar surface, becoming ENAs through charge exchange as they do.

Maybe McComas and his team ought to consider changing this versatile spacecraft's name. I just consulted the General Office of Far-Fetched Identifiers (GOOFI), whose experts responded with "SAEIL." That stands for "Something Amazing Everywhere It Looks."