Within auroral ovals, some 100 to 1,000 km (62 to 620 mi) over our heads, high-energy electrons from solar plasma collide with
air molecules in Earth's upper atmosphere.
One possible explanation of the gamma rays' origin, Dixon says, is that they are created when electrons traveling near the speed of light collide with lower-energy, infrared photons.
According to one theory, some massive, dark-matter particles occasionally collide with each other and either generate gamma rays or produce particles that decay into gamma rays.
If the components of these "contact binary asteroids" pull apart but remain gravitationally bound, traveling together as they collide with
a planet, they might produce the double craters detected on Earth, the moon, and, most recently, Venus.
One near-Earth asteroid more than 1 kilometer across may collide with the sun every 100,000 years or so, report Paolo Farinella of the University of Pisa in Italy and his colleagues in the Sept.
At most, about 20 percent of the current near-Earth asteroids might fall into the sun -- not enough to forestall worries that one of them might someday collide with Earth, he says.
Like the energetic electrons before them, these exiting particies collide with atoms in the chromosphere, creating a faint glow of gamma rays that can last for hours after the main flare peters out.
The pictures indicate that the charged particles that collide with atoms to create these X-rays must have originated at the top of a magnetic loop high in the corona.
The third stage, due in 1995, will be a second 3-TeV beam to collide with