Nearly two months after the Feb. 5 launch of NASA's solar flare satellite, HESSI, the University of California, Berkeley, scientists who designed and build it couldn't be happier. The satellite is working flawlessly and has already captured numerous explosive flares as well as X-ray and gamma ray flashes from elsewhere in the cosmos.

"It's absolutely beautiful," said Robert Lin, principal investigator for the HESSI mission and professor of physics. "We have seen some really neat stuff."

An artist's drawing of RHESSI. Photo: NASA

Lin also is pleased that the satellite has been rechristened RHESSI - the Reuven Ramaty High-Energy Solar Spectroscopic Imager - in honor of the late NASA scientist who pioneered the fields of solar-flare physics, gamma-ray astronomy and cosmic ray research. Ramaty died last year of Lou Gehrig's disease after a long and distinguished career in the Laboratory for High Energy Astrophysics at the NASA Goddard Space Flight Center in Greenbelt, Md.

"He was one of the people who pushed real hard to get HESSI funded," said Lin, who proposed to NASA that it change the satellite's name to honor Ramaty. "He was really looking forward to HESSI's launch, but he passed away before the spacecraft went up. It's appropriate that we name it after him."

RHESSI to date has imaged more than 50 small and medium X-ray flares, though so far no large solar flares emitting gamma rays. Flares are among the most powerful events on the Sun, and can occasionally disrupt satellites, communication systems and power grids on Earth. Scientists believe solar flares are powered by the violent release of magnetic energy, but how this happens is a mystery.

"Their behavior is really amazing, much more complicated than people guessed beforehand," Lin said. "They will take a while to analyze."

NASA created a movie of one of the flares superimposed on solar images from the SOHO satellite (Solar & Heliospheric Observatory) and the ground-based Big Bear Solar Observatory in California. This was an explosion Feb. 20 in the atmosphere on the southern hemisphere of the Sun, in an active region designated AR 9830. The blast was equal to one million megatons of TNT and gave off powerful bursts of X-rays.

"We are thrilled to be making the first high-resolution movies of flares using their high-energy radiation," said Brian Dennis, the RHESSI mission scientist at Goddard. "We want to understand how solar flares can explosively release so much energy. RHESSI shows us the high-energy radiation emitted by flares: their X-rays and gamma rays. This radiation reveals the core of the flare - the exact time and place where the energy is released."

RHESSI also has seen several non-solar events, including a soft gamma ray burster and X-rays that may be from the black hole binary Cygnus X-1. These X-rays and gamma rays penetrated the sides of the spacecraft and hit the detectors, so they couldn't be turned into an image. But because RHESSI has the best energy resolution for such high-energy radiation of any satellite, Lin hopes the instruments can provide unique information on these and other X-ray and gamma ray sources by searching for emission and absorption lines and obtaining Doppler shifts indicative of chemical composition and motions, respectively. RHESSI also can study X-ray pulsars, which are stars that flash at regular intervals.

The immediate goal of the UC Berkeley RHESSI team, however, is to switch the spacecraft over to automatic operation, so that the operations team doesn't have to scramble every time the satellite passes over Berkeley and downloads data, which can be in the middle of the night. Lin is hopeful this will be achieved in early April.

During its planned two-year mission, RHESSI will study the secrets of how solar flares are produced in the Sun's atmosphere. RHESSI is the first NASA Small Explorer mission managed in the "principal investigator" mode, where Lin and his team are responsible for most aspects of the mission, including the science instrument, spacecraft integration and environmental testing, and spacecraft operations and data analysis.

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.