Contact: Daniel Baker


University of Colorado at Boulder

Earth’s magnetic field expanded immensely after the day the solar wind ran out of gas

Note to Editors: The American Geophysical Union Press Room phone number is (415) 905-1007.

A rare space weather event May 11 marked by a sharp decrease in solar wind helped cause Earth’s magnetosphere to balloon to more
than 100 times its normal volume, reaching nearly to the moon in the process, according to a new study.

Daniel Baker, director of the University of Colorado’s at Boulder1s Laboratory for Atmospheric and Space Physics, said the density
and speed of the particle-toting solar wind creates a dynamic pressure that confines Earth’s magnetic field to form its magnetosphere.
Under normal conditions, a cross-section of Earth1s magnetosphere resembles a halved apple, although a comet-like tail sweeps back
from the side of the planet facing away from the sun.

But on May 11, the solar wind density – the number of energetic electrons and protons per cubic centimeter — was only about 2 percent
of normal, and the wind1s speed dropped by more than half. This caused the pressure on the magnetic field to plummet by more than 99
percent, said Baker, lead author on the study. The event was dubbed by some scientists as “The Day the Solar Wind Ran Out of Gas.”

“This was a very rare occurrence that has been seen only a few times since satellites began taking solar wind measurements 35 years
ago,” he said. While the magnetosphere generally reaches out about 40,000 miles from Earth toward the sun, it stretched to nearly
235,000 miles on May 11, about the distance from the Earth to the moon.

A paper on the subject by Baker, CU students Sean Yarborough and Niescja Turner and CU Research Associate Xinlin Li was
presented at the fall meeting of the American Geophysical Union held Dec. 13 to Dec. 17 in San Francisco. Other co-authors included
Shrikanth Kanekal of NASA’s Goddard Space Flight Center, J.B. Blake of the Aerospace Corp. in Los Angeles and Howard Singer of
the National Oceanic and Atmospheric Administration1s Space Environment Center in Boulder.

Baker and his colleagues used data from the SAMPEX, POLAR, GOES, GPS and many other satellites to take data on Earth’s Van
Allen radiation belts, which are deeply embedded in the magnetosphere.

The radiation belts became much more symmetric during the event, with the comet-like tail of radiation apparently disappearing in the
process. Although the density of energetic electrons in the solar wind returned to normal on May 12, as did the solar wind speed, the
density of very high-energy electrons in the magnetosphere dropped mysteriously once again on May 13 and remained “severely
depleted” for an extended period, said Baker.

“It appears this episode fundamentally changed the magnetosphere by causing the energetic electrons to all but disappear for about two
months,” said Baker. “Everything the solar wind was throwing at the magnetosphere should have re-started its engine after May 13, but
for some reason it failed.”

Although scientists once thought Earth’s radiation belts slowly waxed and waned and were not particularly dynamic, they recently have
been shown to be powerful particle accelerators for the magnetosphere1s energetic electrons, he said. The solar wind spins off the
rotating sun much like a circular lawn sprinkler sprays water, accelerating up to nearly 1 million to 2 million miles per hour by the time it
hits Earth’s magnetosphere, said Baker.

“It1s possible that a large ‘vacuum bubble1 formed on the edge of the sun and was blown toward Earth, causing the solar wind to slow
and become less dense during the May 11 event,” he said. “This illustrated the solar wind, like the radiation belts, are highly dynamic
and variable.”

Although the shape of the magnetosphere and the density of its charged particles returned to normal levels after about two months, the
surprising 1999 phenomenon may prove significant to space-weather experts like Baker. “I think these rare, extreme events eventually
will help us to understand how the magnetosphere’s engine works under normal circumstances,” he said.