A
surprising solar flare sent a stream of pure hydrogen at Earth.
"We've
detected a stream of perfectly intact hydrogen atoms shooting out of an X-class
solar flare," said researcher Richard Mewaldt of Caltech. "If we can
understand how these atoms were produced, we'll be that much closer to
understanding solar flares."
Solar
flares occur when the sun's
twisted magnetic fields suddenly release their stored energy. The energy
jolts are classified into three categories, with X-class solar flares being
major events that can trigger radio blackouts around the world and long-lasting
radiation storms in the upper atmosphere. (M-class are medium-sized flares and C-class
are small flares with few consequences on Earth.)
The event
occurred on Dec. 5, 2006, when a large sunspot rounded the sun's eastern limb
and with little warning it exploded. On the "Richter scale" of
flares, which ranks X1 as a big event, the blast registered X9, making it one
of the strongest flares over the past 30 years.
Such a
ferocious blast usually produces a blizzard
of high-energy particles dangerous to both satellites and astronauts. But
an hour later, when NASA's twin Solar Terrestrial Relations Observatory (STEREO)
spacecraft detected the particles, they were of an unexpected type.
"It
was a burst of hydrogen atoms," Mewaldt said. "No other elements were
present, not even helium (the sun's second-most abundant atomic species). Pure
hydrogen streamed past the spacecraft for a full 90 minutes."
Next came
30 minutes of quiet. Then a second wave of particles enveloped the STEREO
spacecraft. The astronomers said these particles were the ions typically
produced by solar flares, including protons and heavier ions, such as ions of
helium, oxygen and iron.
So how did
hydrogen atoms survive the energy blast of a
solar flare, which packs a punch equal to a hundred million hydrogen bombs?
Turns out, the hydrogen atoms weren't so hardy. The researchers think the atoms
were obliterated, beginning their journey toward Earth in pieces, as protons
and electrons.
"Before
they escaped the sun's atmosphere, however, some of the protons captured an
electron, forming intact hydrogen atoms," Mewaldt said. "The atoms
left the sun in a fast, straight shot before they could be broken apart again."
Since
hydrogen atoms are electrically neutral, this hydrogen stream would have shot
straight out of the sun without any interference from the sun's magnetic
fields. That wasn't the case for the ions, which did show a delay, not being
detected for a half hour or so after the hydrogen burst.
"Ions
are electrically charged and they feel the sun's magnetic field. Solar
magnetism deflects ions and slows their progress to Earth," Mewaldt said,
adding, "The hydrogen atoms reached Earth almost two hours before the ions."
Mewaldt said
that all strong flares might emit hydrogen bursts, but scientists just haven't
noticed them before now.
The
discovery will be detailed in a forthcoming issue of the Astrophysical
Journal Letters.
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