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UC Berkeley/NASA satellite RHESSI captures new light from sun, reveals surprises in solar flares
07 June 2002

By Robert Sanders, Media Relations
and Nancy Neal, NASA Goddard Space Flight Center

Berkeley - An April 21 solar flare powerful enough to interfere with radio communications on Earth was captured by the recently launched RHESSI satellite, revealing never-before-seen detail of the high-energy emissions from these huge explosions on the sun.

"We are in that exploratory phase where everything we are seeing is new, the whole scenario is new, and there are quite a few surprises," said Robert Lin, principal investigator for the Reuven Ramaty High-Energy Solar Spectroscopic Imager mission and professor of physics at the University of California, Berkeley.

Lin and colleague Brian Dennis, the RHESSI mission scientist at NASA Goddard Space Flight Center, discussed the RHESSI data at a press conference Wednesday (June 5) during the national meeting of the American Astronomical Society in Albuquerque, New Mexico.

They also released a movie of RHESSI's April 21 X-ray observations superimposed on video of extreme ultraviolet emissions recorded by the TRACE (Transition Region and Coronal Explorer) satellite. The movie shows clearly the relationship between the hot X-ray emissions from high energy particles and the cooler EUV emissions. They found that the sun emits strong, localized bursts of high energy X-rays before the ultraviolet brightening of large solar flares, while high energy X-rays are constantly emitted from active regions and elsewhere on the sun.

RHESSI, designed and built by scientists at UC Berkeley's Space Sciences Laboratory, was launched by NASA on Feb. 5 on a two-year mission to study high-energy emissions from solar flares. With data from on-board X-ray and gamma ray detectors, the scientists hope to discover what triggers flares and how energy stored in the solar magnetic fields is suddenly released to accelerate particles to very high speeds, often heating the gases in the solar atmosphere to tens of millions of degrees.

The April flare was a spectacular X-class (extremely large) solar flare that exploded on the western limb of the Sun and was captured by many spacecraft and ground-based observatories.

"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," Lin said. "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 Celsius or 3.6 million degrees Fahrenheit), it emits hard X-rays associated with fast-moving electrons. The initial bursts are photons at an energy of about 20 keV (kiloelectron volts), similar to those used in medical X-rays. Even higher energy photon bursts around 100 keV 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, a process known as 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," Dennis said. "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," Lin said. "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 is important, Lin said, because it could help explain the 60-year-old mystery of how the active corona is heated to its observed temperature, which is 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 (five foot) tube. Only X-rays from specific directions can pass through each grid pair to enter an energy-resolving detector. The spacecraft rotates 15 times every minute 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 grid alignments, 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 - the equivalent of seeing the width of a human thumb at a distance of a mile.

The RHESSI scientific payload is a collaborative effort among UC Berkeley, Goddard, the Paul Scherrer Institut in Switzerland and the Lawrence Berkeley National Laboratory. The mission also involves additional scientific participation from France, Japan, The Netherlands, Scotland and Switzerland.