NASA's RHESSI spacecraft aims to unravel an explosive
mystery: the origin of solar flares.
June 12, 2001Astronauts
love space walks. Floating weightless hundreds of kilometers above
Earth, the terrain below racing by at 17,000 mphno space traveler
wants it to end. But it only takes two words to send one of those
brave explorers racing back to their craft: "Solar flare!"
"Solar flares are the biggest explosions in the
solar system," says Robert Lin of UC Berkeley's Space Science
Lab. "They erupt near sunspots with the force of a hundred
million hydrogen bombs." Astronauts caught spacewalking during
a solar flare or one of their cousins, a coronal mass ejection,
can absorb a radiation dose equivalent to 100 chest x-raysreason
enough to dash for shelter.
Flares pose little direct danger to Earth dwellers
because our planet's atmosphere protects us from their deadly radiation.
But unpredictable solar explosions do affect our lives. They can
disable satellites, scramble aircraft navigation, and interrupt
high-frequency radio communications for hours.
"One of the most amazing things about solar flares,"
says Brian Dennis of NASA's Goddard Space Flight Center, "is
the efficient way they accelerate subatomic particles to energies
exceeding 109 eV." As much as 50% of the total explosion energy
emerges as electrons and atomic nuclei traveling at nearly the speed
of light. "Flares operate much more efficiently than any particle
accelerator we've been able to build here on Earth."
"How do flares do that?" he asks. We don't
know, but terrestrial particle physicists would love to find out.
What ignites solar flares? How do they unleash so
much energy so quickly? And is it possible to predict when they
Such questions have vexed astronomers since 1859 when
Lord Carrington spotted a solar flare for the first time. "I
was [counting sunspots on a projected image of the Sun]," he
recalled, when suddenly "two patches of intensely bright and
white light broke out" near a remarkably large sunspot group.
"Flurried by the surprise," Carrington rushed from his
telescope to call a second witness, but by the time he returned
minutes later the outburst had vanished.
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