The National Park Service lists numerous natural sources: moonlight, starlight from individual stars and planets, the Milky Way (also called galactic light, or integrated starlight), zodiacal light (sunlight reflected off dust particles in the solar system), airglow (a faint aurora caused by radiation striking air molecules in the upper atmosphere), wildfire, lightning strikes and meteors.
"There is a constant flux of ultrahigh-energy cosmic rays striking the atmosphere," Dr. Giddings said.
Cosmic rays striking the atmosphere are a strong source of beryllium-10, carbon-14, and chlorine-36, which are deposited in rain and snow, whence their migration may be followed.
A simple phenomenon, rays striking the atmosphere, no more mysterious than the beam from a flashlight — yet the lights convey a sense of being at the very edge of the world and looking out at the endless, empty universe through which we are all careening.
"The strike of 1970 had turned whispered resentment into shouts of defiance... Our own defiant attitudes, nurtured in other struggles, flourished in this new post-strike atmosphere where the bewildered foreman could do little but keep his resentment of us to himself, and where our gestures of disrespect for authority were popular on the crew.
When the Earth runs through these dirty trails, specks of dirt strike our atmosphere and burn up — giving us wonderful meteors.
The isotope has always occurred naturally on Earth, formed wherever incoming cosmic rays strike the atmosphere, but some of the current supply also comes from nuclear-weapons tests.
Most of those, Dr. Halzen said, are low-energy neutrinos produced when cosmic rays, mostly atomic nuclei flying through the space from the Sun and elsewhere, strike the atmosphere.
Converting the number of meteorites into mass, and adding in the contribution from space dust that strikes our atmosphere constantly, gives about 37,000-78,000 tonnes per year, or 101 214 tonnes per day.
When a high-energy cosmic array strikes the atmosphere, it disappears in an avalanche of lower energy particles.
