Suppose the Earth were the size of an apple, about 3 inches across. The biosphere, consisting of the ocean, land surface, and lower atmosphere in which all life exists, would be no thicker than the apple's skin. Astronauts lifted into "outer space" atop powerful rockets would be orbiting about 1/32 inch above the apple's surface. Already at that height the air is almost nonexistent, so diffuse that what is left constitutes a vacuum better than any we can make in a laboratory. From this perspective, it hardly seems possible that the Earth could affect much of the space around it, or that the few particles passing by the Earth from the Sun beyond the equivalent of, say, an inch from the apple's surface could have any affect on the Earth.

Well before the advent of spacecraft there were hints that a larger region is important. The mysterious Northern Lights-shimmering curtains and wisps of green and pink often seen in the sky at high latitudes, but on occasion as far South as Washington, DC-hint of an other-worldly origin. Balloon flights more than a half-century ago revealed that energetic particles were coming from the space beyond the atmosphere, sometimes penetrating to the Earth's surface. There was evidence, too, that the Sun was not just a source of light: the gas tails of comets seemed swept outward from the Sun as if blown by a cosmic wind. Hints such as these suggested that the space beyond the biosphere was not empty and passive, but rather the site of dynamic, if invisible, activity.

If air is virtually gone very close to the Earth's surface, what could extend the Earth's outer "boundaries"? Imagine a magnet in the core of the apple: if we lay the poles horizontally and sprinkle iron filings on a sheet of paper dividing the apple's hemispheres, we would see a much more distant influence. The discovery long ago of the compass needle showed us that the Earth does have some kind of magnet inside it. Of course, we would not expect the Earth to be surrounded by iron particles from the Sun, but any atom, when stripped of one or more electrons to become charged, will respond to the magnetic field. Thus if the wind from the Sun that blows the comets' tails consists of charged particles (a "plasma"), these particles will encounter the Earth's influence at great distances. The particles tend to be tied to and follow the field lines, and thus they will at times be able to rain down along the field to the Earth's atmosphere, providing possible origins for both what was observed in the balloon flights and for the auroras. Although weak at large distances from the Earth, the magnetic field can still affect the particles because they have so little mass, and thus we might expect that the Earth's influence could extend well beyond the space next to the surface.

A host of spacecraft launched since 1950 have brought the above rough picture into a clear focus. The ever-changing hot solar atmosphere continuously accelerates particles to high speeds (millions of miles per hour), and these stream out from the Sun taking the solar magnetic field with them. (The Sun, too, is a huge magnet.) This solar wind contains high- and low-speed regions, gusts, shock waves, and more exotic objects such as 6 4magnetic clouds." The Earth's magnetic field forms a partial barrier to the solar wind, and because of this the simple iron filings picture is distorted such that the Earth's influence extends (in the apple analogy) to about a 2 feet toward the Sun and 4 feet or so to the sides, but more than twenty yards downstream as the Earth's field is dragged into a long, invisible, comet-like tail. The region where the Earth's magnetism dominates-the magnetosphere-is continuously changing in response to the wind changes, and these changes can be measured on the Earth's surface. Sometimes the effects are large enough to disrupt communication systems and power grids, leading to blackouts over large regions, but more often the influence is that of the more subtle auroral displays and energetic particle effects.

The Earth-apple of the first paragraph thus extends its influence to many times its size into a realm far beyond the "outer space" of the orbiting astronauts, and even far beyond the orbit of the Moon. Astronauts on missions to other planets, although they would not be affected by the force of the wind because it is so diffuse, will find a harsh realm of charged particles and magnetic fields unshielded by the Earth's larger influence. This is a realm that is continually in turbulent motion, dynamically evolving and influencing the Earth in subtle ways. V;While these regions seem exotic by terrestrial standards, they are more typical of the Universe as a whole, and thus, besides their intrinsic interest, they also provide us with a laboratory for understanding the complex processes occurring in and around distant stars and galaxies.