The solar wind is like any gust on Earth: It carries along a raft of different particles that can interact with each other. Now scientists have shown that a few of the positively charged protons that float like dust in the solar wind transform into hydrogen atoms on the trip to Earth by scavenging an electron. This is the first detection of the predicted flux of hydrogen atoms, which may one day provide early warning of approaching solar storms.
The space between Earth and the sun may look empty, but it's not. The solar wind of charged particles and magnetic fields blows through a fog of primordial dust and neutrally charged atoms created in nearby stars or the uppermost reaches of Earth's atmosphere. Theorists had predicted that about one in 10,000 of the solar wind's protons would snatch an electron from a passing neutral atom to become a neutral hydrogen atom itself. But proof had to wait for a satellite sensitive enough to separate the faint neutral hydrogen breeze from the other particles in the solar wind. Last year the wait ended, when NASA launched the Imager for Magnetopause to Aurora Global Exploration spacecraft, which carries the Low Energy Neutral Atom (LENA) imager.
Now, new results verify the estimated the strength of the neutral hydrogen wind. "The exact number depends on how hard the solar wind is blowing," says LENA team member Thomas Moore of Goddard Space Flight Center in Greenbelt, Maryland, but the team figures that indeed about one in 10,000 protons becomes a hydrogen atom en route to Earth. They report the results in the 15 March issue of Geophysical Research Letters.
These neutral hydrogen atoms might also signal the approach of solar storms--gusts in the solar wind that can disrupt satellites, radio communications, and electrical power grids. Once a proton combines with an electron, it breaks free of the magnetic field that usually keeps it spinning in circles. The liberated atom then speeds ahead of the slower moving charged particles in the wind. If a satellite could spot fluctuations in the neutral wind caused by the approaching storm, "we could get a few hours of warning," says physicist John Hsieh of the University of Arizona in Tucson.
ScienceNOW story about predicting solar storms (24 June 1999)