Energetic electrons and the ozone hole.

Energetic electrons and the ozone hole

High-energy electrons from the Earth's magnetosphere that dive into the upper atmosphere over Antarctica could be playing some role in creating the ozone hole each spring over that continent, proposes an atmospheric scientist from the University of Houston. William R. Sheldon reports that these electrons may stimulate the growth of cloud particles in the stratosphere. Polar stratospheric clouds, as they are known, are important because scientists think they foster chemical reactions that allow chlorine from chlorofluorocarbons to assume an active form that destroys ozone (SN: 10/15/88, P.249). Without the cloud particles, most chlorine atoms become locked up in inactive compounds.

Electrons enter the story as the first link in a chain of collisions starting high in the atmosphere. When these electrons, traveling near the speed of light in the outer Van Allen belt, rain down into the ionosphere, they hit atoms and produce X-rays. These in turn penetrate the underlying atmospheric layer, the stratosphere, where they ionize atoms. Ionization is known to stimulate droplets to form in supersaturated air.

Scientists have observed that X-rays reach into the Antarctic stratosphere during the summer, says Sheldon, but no one has studied their activity in the polar winter, when temperatures are cold enough for cloud particles to form. Measurements from the Defense Meteorological Satellite Project show far more outer-belt electrons raining down over the Antarctic than the Arctic, which, along with the colder temperatures in the south, could explain why more clouds grow in the Antarctic stratosphere, Sheldon suggests.