solar_wind_991214 SAN FRANCISCO -- An unusual diminishing of the solar wind last spring sent a stream of charged particles flowing directly from the sun to the Earth, allowing an unprecedented glimpse at our stars corona, a group of physicists reported Monday.
The group said that between May 10 and 12, the velocity of the solar wind -- a constant stream of plasma sprayed out in all directions from supersonic expansions of the suns corona -- dropped to about 625,000 m.p.h. from 1 million-plus m.p.h. (1 million from 1.6 million kilometers per hour). The density of the wind also dropped, to just 0.2 protons per cubic centimeter -- from 5 to 10 protons. Put another way, that is about 2 percent of normal.
That strong tailing off of the solar wind allowed highly hot and energetic particles to flow directly from the sun to the Earth in narrow beams called strahl, measurements from a flotilla of international satellites show.
"This event provides a window to see the suns corona directly," said Keith Ogilvie, a space physicist at NASAs Goddard Space Flight Center. The results were presented Monday at the fall meeting of the American Geophysical Union.
(X-ray images of the North Pole reveal the "polar rain" of electrons that lit up Earth's upper atmosphere over the North Pole. Credit: Lockheed Martin/NASA)
Those energized particles are normally scattered on their nearly 100-million-mile (160-million- kilometer) journey from the sun to the Earth through space, making their appearance here an unheralded rarity.
"By all arguments, this is primary material, propagated across the interstellar medium," said Jack Scudder, a University of Iowa space physicist, of the energetic electrons. Once at Earth, the sun-sent electrons caused a so-called "polar rain" directly over the North Pole, producing a glow sufficiently energetic to make it visible to X-ray-sensing satellites.
The polar glow of highly charged electrons occurred inside the normal auroral doughnut that circles the pole. Electrons excited "locally" -- that is, within the Earths magnetosphere -- normally cause the ovoid auroral glow.
The disappearance of the solar wind also had a large effect on the Earths magnetosphere, or the extent of its magnetic field. Typically, the solar winds velocity and density exert pressure on the magnetosphere, "blowing" it away from the Earth on the side facing away from the sun. And since the wind carries an electric charge, it flows around the Earths magnetic field, not dispersing it, but rather molding it into a comet-like shape.
(This pair of images in visible light compares the northern auroral regions on May 11, 1999, and a more typical day on November 13, 1999. Credit: University of Iowa/NASA)
During the nearly three-day disappearance of the solar wind, that pressure was about 1 percent of normal, prompting the now-unconstrained magnetosphere to balloon outward to a volume never before measured.
Typically, the magnetosphere extends 40,000 miles (64,375 kilometers) into space from the Earth on the side facing the sun; on May 11, it reached nearly 235,000 miles (378,195 kilometers) -- about as far as the moon. Although physicists have dubbed the events "The Day the Solar Wind Disappeared," they still do not have an explanation for what caused it.
Observations made of the sun itself during that period show no evidence of unusual activity.
"We basically havent found any smoking guns back at the sun," said David Webb, a Boston College research physicist.
One possible reason is that the heat that drives the expansion was deposited too low in the solar atmosphere, producing not a bang but a whimper of a solar wind, Scudder said.
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