RELEASE NO: 98-210

For the first time, scientists are able to accurately determine the temperature of individual solar "sneezes," small explosions on the Sun called impulsive solar flares. The researchers used NASA's Advanced Composition Explorer spacecraft to observe a series of flares of this type in August, 1998.

"These measurements are a first step to understanding how solar flares accelerate particles from the Sun to extremely high velocities," said Dr. Eberhard Moebius of the University of New Hampshire, who will present the research December 6 at the Fall meeting of the American Geophysical Union in San Francisco, Calif.

"These flares are relatively modest, compared to a typical solar flare. Before ACE, we had to average over a group of them to get a temperature estimate," added Dr. Joseph Mazur of the Aerospace Corporation, El Segundo, Calif., a contributor to the research.

The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) instrument on board ACE derived the flare temperature by measuring the electric charge on high speed atoms shot from the flares. At high temperatures, electrons can be removed from atoms, giving the atoms a positive electric charge and allowing magnetic fields present in flares to accelerate them to high speeds. As temperatures rise, atoms lose more electrons until they have none left, a condition known as completely "stripped."

"The atoms of various elements detected, from hydrogen to silicon, had been completely stripped, and iron was almost fully stripped. This corresponds to a flare temperature of about 18 million degrees Fahrenheit, much hotter than the surface of the Sun, which is only 10,000 degrees," said Mazur.

"In the past, very often we were not sure whether these energetic particles came directly from solar flares or were accelerated between the Sun and the Earth. With the ACE payload, we have the means to exactly time their arrival and hence, infer the acceleration site, even for these very interesting small flares," said Moebius.

"Exactly how magnetic fields within flares accelerate particles and release energy is unknown. Strange things happen in them. For example, for some reason, impulsive solar flares prefer to accelerate helium 3 atoms. The concentration of helium 3, a rare isotope of helium, in matter ejected from these flares is as much as 1,000 times greater than its average concentration throughout the rest of the Universe," said Moebius.

As atoms lose electrons, they become electrically charged particles known as ions. Ions respond to magnetic fields; after their acceleration in a flare, they rush along invisible magnetic field lines extending from the Sun into interplanetary space, like race cars confined to a track. The Ultra Low Energy Isotope Spectrometer (ULEIS) instrument on ACE has identified many of these small flares when particles arrive directly along the interplanetary magnetic field lines (see the figure referenced at the end of this release).

"The sensitive particle detectors on ACE tell us details about the interplanetary magnetic field. If the field were uniform, showers of particles from different flares would all last about the same time, approximately a day or so as the slower particles trail the fast ones.

Occasionally, however, we see a particle shower from one flare suddenly 'turn off', while the shower from a different flare continues unaffected (see the figure referenced at the end of this press release, events # 5 & 6). Apparently some unknown feature of the interplanetary magnetic field must have severed the magnetic pathway between one of the events and ACE without severing the other. ACE provides another tool for diagnosing the structure of this unseen field," said Mazur. "The sensitive instruments on board ACE allow us to observe these events with a clarity and precision that has never been achieved before. As observations accumulate, hopefully we can unravel the mysteries of solar flares," said Mazur.

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