Although the 293 kg (645 lb) satellite, known as the High Energy Solar Spectroscopic Imager (HESSI), carries only one scientific instrument, it will make a major contribution to studies of our nearest star.

HESSI will capture the first pictures and high resolution spectra of high-energy ("hard") X-ray and gamma-ray emissions from solar flares - the signatures of electrons and ions accelerated during the flare process. Sequences of such images will show in unprecedented detail what happens at high energies when flares erupt from the surface of the Sun.

By analysing HESSI`s images and spectra, scientists hope to discover more precisely where flares are born, what triggers them, and how they generate such huge power in the form of fast particles as well as heat and bulk motion.

Previous observations indicate that the explosions are created when energy stored in intense solar magnetic fields is suddenly released. As a result, large amounts of thermal radiation, including lower energy "soft" X-rays, are created, as the gases in the Sun`s corona (outer atmosphere) are heated to tens of millions of degrees.

At the same time, the electrons and ions in the searingly hot gas (known to physicists as "plasma") are accelerated, resulting in the productioÛæ™Ú other particles (neutrons and pions), hard X-rays and gamma-rays.

HESSI will be the first spacecraft to simultaneously trace the different kinds of particles being accelerated, locate the regions where the acceleration occurs and determine when the particles get accelerated. The most direct tracers of these accelerated particles will be the X-ray and gamma-ray emissions that they produce as they travel through the solar atmosphere.

Among those looking forward to studying and analysing the images and spectral data from the spacecraft is the Astronomer Royal for Scotland, Professor John Brown of Glasgow University, where the tradition of solar activity studies dates back to Alexander Wilson (1760).

Professor Brown and his team are the sole UK HESSI Co-Investigators. They will be analysing the data using image/spectral processing codes and models of particle acceleration and propagation which they have developed, in order to improve current theories about how our violent Sun works, particularly at high energies.

"HESSI will help us to understand the ultra-high temperature and non-thermal processes taking place in the Sun and the Universe as a whole," he said. "It is important to understand how such high temperatures (possibly up to one billion degrees), and accelerated particle beam energies and fluxes, are generated in the Sun`s outer atmosphere."

"It is also important to understand how high energy particles are created and accelerated to near light speed – about 300,000 km per second," he added. "As well as being a key problem in plasma astrophysics, such particles and their radiation pose a hazard to personnel in space and high altitude aircraft, and have been implicated in upper atmospheric chemistry which may affect climate."

"The Sun is a perfect natural laboratory for exploring plasmas and high energy processes," he said. "Because of their large scale and the relative proximity of the Sun, solar flares enable us to study energy release in magnetised plasmas that cannot be studied in such detail anywhere else. Indeed, the Universe is teeming with accelerated particles from explosive events such as supernovae, and the Sun gives us a nearby test bed to study these particles."

"The timing of this mission is particularly significant since the Sun has just passed the peak in its 11-year cycle of activity, when the largest explosive events are expected to be most frequent," said Brown.

Editor's notes:

The HESSI mission is scheduled for launch on 5 February, some 18 months later than originally planned. The satellite had to be rebuilt after it was severely damaged during vibration testing in March 2000. The launch was rescheduled for March 2001, but problems with the Pegasus launcher pushed it back to February 2002.

The rocket and satellite will be launched from beneath a Stargazer L-1011 aircraft flying over the Atlantic Ocean. After being dropped from the aircraft, the engine of the Pegasus rocket will fire to deliver the spacecraft to a circular orbit 600 km (373 miles) above the Earth, inclined at 38 degrees to the equator.

Solar flares occur when magnetic fields twist, snap and recombine, accelerating particles to near light speed (about one billion kilometres per hour), and heating gas in the corona – the Sun’s outer atmosphere which appears as a halo of light during a total solar eclipse - to tens of millions of degrees. As a result, the solar atmosphere teems with high energy X-rays (20-100 keV) and gamma-rays (0.51 MeV and above), while accelerated proton and electron particles zoom into the Solar System, sometimes impacting Earth`s upper atmosphere. Radiation and particles from solar flares sometimes affect orbiting spacecraft, interfering with communications and threatening astronauts.

HESSI has the finest angular and spectral resolution of any hard X-ray or gamma-ray instrument ever flown in space. Powerful X-rays and gamma-rays penetrate all materials, to some extent, and cannot be easily focused, so researchers are using another technique to form images. HESSI`s sole instrument - an imaging spectrometer - will construct a flare image from patterns of light and shadows produced by high-energy radiation that passes through the telescope`s grids while the spacecraft rotates. Using this new method, HESSI is expected to gather data on hundreds of flares during its two-to three-year mission.

By working together with ground-based flare observatories and with other solar spacecraft - the Solar and Heliospheric Observatory (SOHO), Geostationary Operational Environmental Satellites (GOES) and the Transitional Regional and Coronal Explorer (TRACE) for flare radiation, and Wind, the Advanced Composition Explorer (ACE), Ulysses, and Voyager for particle detection - HESSI will provide key insights into the processes of energy release and particle acceleration at the Sun.

HESSI is the sixth Small Explorer spacecraft built by NASA. The HESSI mission costs about 85 million dollars, which includes the spacecraft, launch vehicle, mission operations and data analysis.

Principal Investigator for the mission is Robert Lin of the Space Sciences Laboratory at the University of California, Berkeley. ( Tel: +1510-642-1149, e-mail: boblin@ssl.berkeley.edu)

CONTACT:
Professor John Brown,
Astronomer Royal for Scotland,
Dept. of Physics and Astronomy,
University of Glasgow,
Glasgow,
G12 8QW
Tel: +44 (0)141-330-5182
Fax: +44 (0)141-330-3183
E-mail: john@astro.gla.ac.uk

MORE INFORMATION AND IMAGES are available on the Web at:
http://hesperia.gsfc.nasa.gov/hessi
or
http://hessi.ssl.berkeley.edu/

University of Glasgow Dept. of Physics & Astronomy:
http://www.astro.gla.ac.uk/research/index.shtml

For further information, please contact:
Professor John Brown
University of Glasgow
john@astro.gla.ac.uk
+44 (0)141-330-5182