Astronauts aboard the space shuttle have long told tales of shimmering auroras flickering high above Earth’s atmosphere. Standard textbooks say that auroras cannot form at such rarefied heights, leading many scientists to doubt the reports. But a sun-spying satellite has now confirmed that the astronauts are right and the textbooks wrong.
The Solar Mass Ejection Imager satellite (SMEI), launched by the U.S. Air Force in January 2003, tracks giant bubbles of energized particles as they belch off the sun. Auroras usually occur when the most energetic of those particles flow down Earth’s magnetic field and collide with atmospheric atoms such as oxygen and nitrogen, which then emit light.
This process was thought to occur only at altitudes below about 60 miles, where there are enough atoms to produce a visible glow. But while poring over SMEI data collected during a solar outburst last May, research physicist Andrew Buffington of the University of California at San Diego and his collaborators noticed intense lights more than 500 miles above Earth—450 miles higher than most of the auroras visible from the ground. Subsequent solar storms produced a similar high-altitude show.
Buffington is perplexed by the cause of the glow, which seems to have a lacy structure and may be reddish in color (SMEI doesn’t have the filters that would tell for sure). One idea is that energized ion plumes may be blasting up from the lower atmosphere and infusing the upper atmosphere with aurora-producing charged particles. Solar physicist Bernard Jackson of UCSD, head scientist on the SMEI mission, says that the new aurora could also merely be the wispy outer fringes of more conventional aurora, too faint to be seen either from the ground or with other instruments. “We just have a very sensitive instrument,” says Jackson. “My colleagues may not agree with this, though, since they think a new process may be involved. We are just beginning the study.”
Mystery aside, solar researchers actually find this new type of aurora a bit of a nuisance: “When there is a really bright aurora, about one-quarter of our image of the sun is wiped out, and we can’t see the coronal mass ejections,” Jackson says.