In 1958 the very first American satellite, Explorer I, discovered space weather when it passed through the lower Van Allen radiation belt that encircles the earth. Its primitive sensors registered the radiation and, after some study, researchers realized its significance: space around the earth is not empty. All the various phenomena that exist or develop in that near-earth environment whose behavior and interactions directly affect the planet and human technologies on and in orbit around it make up space weather. The chief actors in the origin and development of space weather are the sun, the solar wind, and the earth. However, the details of how all the separate manifestations of space weather relate to each other, how they interact, and what may cause specific phenomena, remain subjects of intense investigation.
Space weather originates with the sun. At least four solar phenomena, not one of which is well understood, release matter from the sun into the solar system and together are the starting point for understanding the processes by which space weather develops and affects near-earth space: coronal mass ejections, solar flares, coronal holes, and solar prominences.
Coronal mass ejections consist of matter thrown out from the corona, the sun’s outer atmosphere. The corona’s reach, its density, and most other characteristics are structured by strong magnetic fields that are generated by conditions beneath the sun’s visible surface. When the magnetic fields loop back on themselves, great masses of the electrically-charged solar atmosphere become confined within their lines of force. At some point the highly charged gas or plasma—which may amount to as much as a billion tons of matter—can suddenly and violently explode, blowing out huge plumes of solar atmosphere at speeds of several million miles an hour.
Solar flares are intense releases of energy that originate in the sun’s chromosphere, a thin layer of mostly hydrogen that lies between the sun’s visible surface and the corona. Solar flares can last for minutes or hours, and are the largest explosive events in the solar system. The energy released in a single flare can be the equivalent of 40 billion Hiroshima-size atomic bombs.
Coronal holes, viewed in the x-ray waveband, show as exactly what their name states: holes in the corona. The holes can last for months to years, and are rooted in large cells of unipolar magnetic fields that emanate from the sun’s surface. The field lines of the cells extend far away from the sun, allowing a continuous outflow of high-velocity solar material to travel along them into open space.
Solar prominences originate as clouds of solar material held above the sun’s surface by fields of magnetic force. The clouds remain suspended, relatively quiescent—until they erupt, releasing large amounts of solar matter into space.
The matter blown out from the sun moves away from it in a continuous but varying flow that is called the solar wind. The solar wind was first monitored between the earth and sun in 1962 by the Mariner 2 spacecraft and was found to have a velocity that varied between a little less than a million miles per hour to over one and a half million miles per hour.
The solar wind’s density has been measured near the earth, however, to be only about six ions per cubic centimeter. (In other words, the solar wind is actually closer to being a vacuum than the best vacuum obtainable in laboratories on earth.) As insubstantial as six ions might seem to be, their constant flow exerts enough pressure to push—for one very visible example—the tails of comets always away from the sun.
The make-up of the solar wind generally resembles what the sun is made of: mostly protons, or, hydrogen ions—the hydrogen nucleus stripped of its electron—and about five percent helium ions and smaller fractions of ions of oxygen and other elements.
The composition of any particular volume of solar wind will vary, however—as will its density and velocity—depending on what kind of event on the sun gave rise to it. Solar wind that originated in a coronal mass ejection, for example, generally will be more dense when it reaches the earth than solar wind that originated in a coronal hole.
Solar flares, however, also radiate enormous bursts of electromagnetic energy, including gamma and x-rays. Since gamma and X-rays travel at the speed of light, this radiation will reach earth’s orbit days ahead of the solar matter released at the same time in the same event.
And there is further complexity: the solar wind carries with it the imprint of the magnetic fields it encountered at the sun. Also, particular volumes of the wind will generate new electromagnetic fields depending on the density, composition, and speed of the particles they hold and the conditions those particles encounter on their journey toward earth.
The earth contributes to space weather because it has a magnetic field and an atmosphere. The magnetic field sends most of the solar wind sliding around the planet. It’s as if the earth were inside a loosely inflated balloon.
That "balloon," whose invisible boundaries are drawn by lines of magnetic force, is called the magnetosphere. The magnetosphere is shaped by the solar wind into something like a tear drop. Its front, toward the sun, where it first encounters the solar wind, begins roughly 40,000 miles from the earth—a distance of about 10 or 11 earth radii, the unit of measurement generally used in describing the magnetosphere. The "tail" of the magnetosphere extends behind the earth as much as 200 radii (800,000 miles), where it’s been routinely observed—well beyond the distance to the moon.
Where the solar wind and the magnetosphere actually come into contact is called the magnetopause. The magnetopause is in constant flux. It shrinks or expands as the electromagnetic and particle characteristics of the solar wind change. The fluctuations can be pronounced. When matter thrown out by a coronal mass ejection reaches the magnetopause, for example, in the balloon analogy given above, it’s effect is like a fist punching deep into the balloon, its skin—the magnetopause—"stretching" inward to absorb the shock.
Such intrusions can push into the magnetosphere by as much as five or six earth radii—well past the orbit where geosynchronous satellites are stationed.
Inside the magnetopause there is additional structure and significant activity. The structures include: the inner magnetosphere, where the Van Allen radiation belts reside; the geomagnetic tail, which stretches back from the earth well beyond the orbit of the moon and stores energy from the solar wind—which it periodically releases, sending hot plasma into the inner magnetosphere and producing active aural displays; and the "geocorona," a large cloud of neutral hydrogen that surrounds the earth. These regions interact closely with each other, and, in complex ways, with the solar wind and the earth’s upper atmosphere.
The earth’s upper atmosphere consists of two distinct layers, the mesosphere and the thermosphere, which together extend from about 30 miles above the planet to well over 500 miles. These two layers absorb energy—mostly ultraviolet and x-ray radiation from the sun and other, minor, sources—which ionizes a small fraction of the atoms that make up the layers. Those ionized atoms form the ionosphere.
The ionosphere is electrically conducting, so it interacts strongly with the earth’s magnetosphere that surrounds it, reacting quickly to changes there and in the solar wind. One visible manifestation of this interaction is the aurora. Additionally, electromagnetic "storms" can transfer great amounts of energy into the ionosphere, thereby heating and thus expanding the atmosphere—which in turn increases atmospheric drag on satellites
At the same time, intense electric currents continually flow from the magnetosphere through the ionosphere. These currents can also induce large currents and other effects on the earth below—which in turn can affect people and human technology on the ground.