RELEASE NO: 98-082

HIGH RESOLUTION IMAGES FROM TRACE SPACECRAFT CAPTURE AN EXTREMELY RAPID HIGH TEMPERATURE SOLAR EXPLOSION

NASA's Transition Region and Coronal Explorer (TRACE) spacecraft recorded a bright but extremely short-lived explosion in the atmosphere of the Sun. The explosion, called a flare, was observed on May 31, 1998 in extreme ultraviolet light using the telescope on board TRACE.

"TRACE is demonstrating that large scale events can happen very rapidly on the Sun," said Dr. Alan Title, the TRACE Principal Investigator from the Stanford Lockheed Institute for Scientific Research (SLISR) in Palo Alto, Calif. "Although less than 200 miles wide, the flare was about 55,000 miles long. It appeared and vanished in just a few minutes. TRACE was able to detect this explosion because it can maintain high cadence, high resolution imagings for long durations. At the time of the explosion, TRACE was taking images at a cadence of a frame every 86 seconds and an exposure time of 28 seconds."

High resolution movies of this explosion will be presented at a press conference Monday, June 8, 1998 at 2:30 A.M. PDT during the spring meeting of the American Astronomical Society in San Diego, Calif/

"TRACE is operated in conjunction with the ESA/NASA Solar and Heliospheric Observatory (SOHO) spacecraft. The Michelson Doppler Imager (MDI) on SOHO was producing magnetic and velocity maps at the time. These tell us where the flare occurred with respect to the Sun’s magnetic field, and further analysis may reveal evidence for Sun quakes.  The SOHO Extreme- Ultraviolet Imaging Telescope (EIT) was taking full disk images during the same interval at a 17 minute cadence. About an hour before the flare, the EIT movie shows that a coronal mass ejection (eruption of hot gas from the Sun, abbreviated CME) occurred on the edge of the Sun.  It is well known that such events trigger waves that can travel across the entire Sun. It may be that the trigger for the flare came from the blast wave associated with the CME. We are now studying the EIT data and will be on the look out for similar events in future data," said Title.

"The TRACE spacecraft had been following an active region's (NOAA 8227) passage across the solar disk since May 26, 1998. Suddenly, at 3:56:18 UTC on May 31, the region brightened in an area that was approximately ‘T’ shaped. The top and length of the ‘T’ were each about 55,000 miles (90,000 km) long and less than 200 miles (350 km) wide.  At the same time, the X-ray detectors on the National Oceanic and Atmospheric Administration’s (NOAA) GOES satellite recorded a doubling of their signals. The explosion is not visible in the frame taken a couple of minutes earlier at 3:54:52 UTC. This means that the event trigger, if it started from a point, had to travel at least 55,000 miles in a time period between 58 and 86 seconds long. This corresponds to a velocity of between 640 and 2,000 miles/second (2.3 to 7.1 million miles/hour)." said Title.

"The intensity in the exploding region increases by a factor of at least 220 between frames. We don't know exactly how much the increase is because we don't know when in the exposure the increase occurred. Almost as fast as the increase is the cooling. A frame 149 seconds later shows almost no evidence of the event," added Title.

"The TRACE spacecraft is unique in that it has both high spatial and temporal resolution in the extreme ultraviolet, a wavelength of light that reveals the multimillion degree Sun. We can image solar activity in finer detail than existing spacecraft, and we can take a new image once every few seconds. For example, there are 25 TRACE pixels for each one of SOHO EIT. Both high temporal and spatial resolution are necessary for our mission, which is to understand in great detail how energy is transported from the solar surface into the outer atmosphere. In the past, spacecraft of lower resolution were forced to average over much larger areas and periods of time. This made it difficult to get at the fundamental physics," said Title.

The TRACE spacecraft, launched from Vandenberg Air Force Base in Calif., Apr. 1, 1998, joins a multinational fleet of International Solar Terrestrial Physics project spacecraft studying the Sun during a critical period when solar activity is beginning its rise to a peak early in the new millennium. The Sun goes through an 11-year cycle from a period of numerous intense storms and sunspots to a period of relative calm and then back again. The coming months in the Sun's cycle will provide solar scientists with periods of intense solar activity interspersed with periods when the Sun is relatively passive and quiet. This will give TRACE the chance to study the full range of solar conditions, even in its relatively short planned lifetime.

TRACE is training it's powerful telescope on the so-called "transition region" of the Sun's atmosphere, a dynamic region between the relatively cool surface and lower atmosphere regions of the Sun (about 6,000 degrees Fahrenheit) and the extremely hot upper atmosphere called the corona (up to 3 million degrees Fahrenheit). Using portions of the telescope sensitive to extreme-ultraviolet and ultraviolet wavelengths of light, TRACE is studying the detailed connections between the fine scale surface features and the overlying,

changing atmospheric structures of hot, electrically charged gas called plasma. The surface features and atmospheric structures are linked by fine-scale solar magnetic fields. The solar atmosphere is constantly evolving because the magnetic fields that dominate the corona are continuously displaced by the convective motions in the outer layers of the Sun just below the photosphere.

The TRACE science team will also study the evolution of events, such as massive flarings and huge eruptions, in the Sun's atmosphere. These events originate at the Sun's visible surface, the photosphere, and travel upward through it's atmosphere (chromosphere and transition region), and then into its super-hot corona before speeding out into space, sometimes towards Earth.

The power of the TRACE telescope to do detailed studies of the solar atmosphere makes this observatory unique among the current group of spacecraft studying the Sun. The spacecraft has roughly 10 times the temporal resolution and 5 times the spatial resolution of previously launched solar spacecraft. These advances are possible because of the near Earth "Sun-synchronous" orbit and significant advances in optical coatings developed at the Lawrence Livermore National Laboratory, Livermore, Calif., and an optical system developed at the Smithsonian Astrophysical Observatory, Cambridge, Mass. A Sun-synchronous orbit is uninterrupted by Earth's shadow for eight months at a time, allowing the mission the greatest chance to observe the random processes which lead to flares and massive eruptions in the Sun's atmosphere.