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RHESSI
CAPTURES NEW LIGHT FROM SUN, REVEALS SURPRISES IN SOLAR FLARES
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on image for animation. | The
Sun emits strong, localized bursts of high energy X-rays before the UV brightening
of large solar flares and high energy X-rays are constantly emitted from active
regions and elsewhere on the Sun. These
and other initial results from the Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) will be reported today by scientists from the University of California
at Berkeley, NASA's Goddard Space Flight Center, and other institutions at the
annual meeting of the American Astronomical Society in Albuquerque.
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2 | | Click
on image for animation. | On
April 21, 2002, a spectacular X-class (extremely large) solar flare exploded on
the western limb of the Sun and was captured by RHESSI and the Transition Region
and Coronal Explorer (TRACE) spacecraft, as well as many other spacecraft and
ground-based observatories. The combined RHESSI and TRACE data yield beautiful
time-lapse movies, and reveal important new physics. "By
combining RHESSI hard X-ray images with TRACE EUV data, we are able to follow
the cascade of energy in the largest explosions in the solar system," says
UC Berkeley professor Robert Lin, principal investigator of the RHESSI mission.
"We can determine exactly where and when energy is released in the solar
atmosphere, and identify its form: plasma heated to tens of millions of degrees,
and fast-moving electrons that stream from high in the corona to impact and heat
the gases below." RHESSI
observations show that just before a flaring region fills with hot gas (seen by
TRACE at about 2 million degrees C, or 3.6 million degrees F), it emits hard X-rays
associated with fast-moving electrons. The initial bursts are of ~20-keV photons,
similar to those used in medical X-rays. Even higher energy ~100-keV bursts reveal
where energy is deposited by the electrons before spreading throughout the flare
region. The
X-rays from the base of the active region are "bremsstrahlung", or "braking
radiation," caused by electrons slamming into the dense gases at the bottom
of the corona. The electrons are thought to be accelerated by the collapse of
stretched magnetic field lines high above the solar surface ("magnetic reconnection").
The impacts also heat the gas, which fills structures in the changing local magnetic
field to yield the spectacular patterns seen with TRACE, and emits its own thermal
X-rays as well. "We
were surprised to see the X-rays coming from the base of the flaring region well
before the initial brightening in the EUV," says Brian Dennis, RHESSI mission
scientist at NASA's Goddard Space Flight Center. "We expected to see X-rays
coming nearly simultaneously with the EUV brightening." RHESSI
also reveals that solar active regions, the strongly magnetized sources of solar
flares and coronal mass ejections, constantly produce a multitude of tiny X-ray
flashes, or "microflares," that last only a few minutes each. "RHESSI
is the first solar X-ray instrument with such high sensitivity in this energy
range," says Lin. "While large solar flares are the brightest sources
of X-rays, we can now see that active regions crackle and sputter with miniature
flares all the time." The
team has not yet performed a full analysis of the small X-ray flashes -- but there
is strong evidence that microflares with particle acceleration occur all the time
in the active corona. That's important because it could help explain the 60-year-old
mystery of how the active corona is heated to its observed temperature of more
than 200 times hotter than the surface of the Sun. RHESSI
uses a unique shadow-mask technique to generate images with X-rays so energetic
that no known broadband optics will focus them. RHESSI uses nine pairs of special
tungsten grids, with slits as fine as a human hair, at opposite ends of a 1.5-meter-long
(5 foot) tube. Only X-rays from specific directions can pass through each grid
pair to enter an energy-resolving detector. The spacecraft rotates at 15 RPM to
change the directions of admitted X-rays. Computers on the ground analyze the
cyclical changes in X-ray throughput to each detector, and reconstruct images
from them. Making
sharp images requires very precise knowledge of the grids' alignment, and ongoing
in-flight calibration will continue to improve even these early images as the
mission progresses. Ultimately, images with 2 arc second resolution will be possible.
(That's equivalent to the width of a human thumb at a distance of a mile). Dennis
explains, "We start with simple shadow patterns and reconstruct images that
have resolution equivalent to any telescope on the ground, but in gamma rays and
hard X-rays rather than visible light. No-one has been able to do that before."
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