Volume 1, Issue 7
A Flare Up In Solar Physics
The sun is the solar system's biggest powder keg. Solar flares, tremendous explosions on the sun's surface, can release the equivalent energy of 10 billion megatons of TNT in just a few minutes. The explosions sometimes interfere with terrestrial radio communications and might put astronauts' lives in jeopardy, but the complex phenomena that ignite the flares remain much of a mystery. Six hundred kilometers above the earth though, a small satellite operated by UC Berkeley scientists is shedding some light on the sun's unrest.
Scientists believe that solar flares occur when energy stored in the magnetic fields of the sun's corona is suddenly released. Electrons and ions are explosively flung toward the surface of the sun, heating the solar atmosphere to tens of millions degrees Celsius. Sometimes, up to a billion tons of gas is ejected into space. In recent years, scientists determined that perhaps as much as half of the energy released by the flares is contained in accelerated electrons and ions.
Robert Lin at the Space Sciences Laboratory. Visible in the background is the eleven meter-diameter dish in the Berkeley hills used to communicate with the RHESSI satellite. (David Pescovitz photo)
"Solar flares are the most energetic particle accelerators in the solar system," says UC Berkeley astrophysicist Robert Lin, director of the Space Sciences Laboratory. "But how the sun releases the energy and accelerates the particles with such high efficiency is a puzzle that couldn't be solved on the ground."
Three years ago, NASA and UC Berkeley launched the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), a satellite designed at the University to observe the energy released by the flares before it's absorbed in the Earth's atmosphere. RHESSI is outfitted with novel imagers that measure x-ray and gamma rays produced when electrons and ions collide with the solar atmosphere. Those rays, Lin says, provide the most direct signatures of the location and motion of the particles released by the flares. The trick is imaging them.
The rays are of such high energy, Lin explains, that they cannot be focused by the lenses or mirrors of traditional optical telescopes. To solve that problem, Lin's research group developed the RHESSI Imaging Spectrometer based on grids that block some photons while allowing others to fly through the gaps. Photon detectors behind the grids not only count the particles passing through, but also precisely measure the energy of those photons.
As the satellite flies over UC Berkeley, those measurements are transmitted to an eleven-meter diameter dish on the hillside outside the Space Sciences Laboratory. The data is then combined to construct high-resolution spectrographic movies of the flares.
A superposition of RHESSI images of gamma-ray and X-ray emissions with a TRACE satellite extreme ultraviolet image taken 90 minutes later of the July 23, 2002, solar flare. The superposition clearly shows the large separation between the high-energy emissions. Solar physicists expected to see X-rays and gamma rays emerging from the same spots at the base of the flare loops.
"When we image the acceleration of the particles, we can see where the energy is coming from relative to the sun's magnetic field," Lin says. "And the spectroscopy enables us to measure the conditions in that region such as density and temperature. Once we know where the activity occurs and the conditions surrounding it, we can then begin to understand the physics behind the flares."
Already, RHESSI has enabled Lin's team to produce dazzling movies of the unusual phenomenon with unprecedented clarity. Towards the end of October 2004, the satellite measured some of the biggest flares that the sun may have ever unleashed. "It's these big flares that really push the limits of our understanding," Lin says.
Solar flares are produced by the sun's massive magnetic fields. Those fields are commonly seen as sunspots, "active regions" where the solar magnetic field may be several thousand times that of the Earth. As those magnetic fields twist like tightly-coiled springs, the tension eventually becomes too much and they snap, spewing the charged particles that make up a solar flare.
The RHESSI Imaging Spectrometer contains nine germanium detectors that are positioned behind the nine grid pairs on the telescope. The detectors convert incoming x-rays and gamma-rays to pulses of electric current. The amount of current is proportional to the energy of the photon, and is measured by sensitive electronics designed at the Lawrence Berkeley National Laboratory and the Space Sciences Lab., Berkeley. (NASA photo)
Before last year's observations, solar physicists expected that the accelerated ions and electrons would be seen along the same magnetic pathways. Both gamma rays and x-rays, the scientists believed, should originate from the feet of massive loops that arch hundreds of thousands of miles across the surface of the star. However, once RHESSI's data made it home, the scientists were pleasantly surprised.
"What we found is that the electrons and ions were separated by tens of thousands of kilometers," Lin says. "We don't completely understand why that's the case, but it suggests that the particles are accelerated in different ways."
The massive amounts of data already delivered by the satellite will take years to fully analyze, he adds. As long as funding continues, Lin says, RHESSI's spectroscopic eye will remain open for the foreseeable future.
"If you truly understand what causes the flares, someday you might be able to look for the right conditions and predict when the flares will occur," he says.