Our neighboring planet, Mars, which has little or no magnetic field, is thought to have lost much of its former oceans and atmosphere to space [cover story, Scientific American, Nov. '96]. This loss was caused, at least in part, by the direct incidence of the solar wind on Mars' upper atmosphere. Our other close planetary neighbor, Venus, has no appreciable magnetic field, either. Venus is also thought to have lost nearly all of its water to space, in large part owing to solar wind-powered ablation.
The outer parts of the geomagnetic field, with field line footpoints in the polar regions of Earth, are swept back by the wind to form a long tail (100s of Earth radii). In the geomagnetic tail, plasmas are heated to more than 10,000,000 degrees Celsius, and then accelerated back toward the Earth where they reach temperatures 100 times higher (1,000,000,000 degrees C!). These plasmas, 1000 times hotter than the Sun's corona, are responsible for geomagnetic or space plasma storms that damage spacecraft and ground electrical systems, inflate the atmosphere causing increased drag on low orbiting satellites, and interfere with radio communications.
The solar wind is the expanding atmosphere of the sun, a fully ionized and electrically-conducting plasma at about 1,000,000 degrees Celsius, and travelling past the Earth at well over 1,000,000 kilometers per hour. Thanks to the Earth's magnetic field, the solar wind is stopped about 10 Earth radii upstream, and deflected around the Earth so that it is not incident directly on our atmosphere.