Because of its free electrons, a plasma is a good conductor of electricity, much better than than copper, silver, or gold. Lightning offers one of the most dramatic manifestations of this property. As a thunderstorm develops, negative charges accumulate along the cloud base, causing positive charges to build up on the ground below. The resulting electrical field between the concentrations becomes so strong that it ionizes the air. This creates a conducting path of free electrons and ions—a plasma—through which the lightning discharges.
A young engineer and chemist working for General Electric Company gave plasma its name. In 1923 Irving Langmuir, who went on to win the Nobel prize in chemistry, was fascinated with the effect of electrical discharges on gases. He borrowed the term plasma from medicine because it fitted the unstable, almost lifelike behavior of the ionized material with which he experimented.
While all matter is subject to gravitational forces, the negatively charged electrons and positively charged ions in a plasma also react to electric and magnetic forces that are 10^36 times as strong. Because of these additional interactions, plasma display structures and motions that are fare more complex than those found in neutral solids, liquids, or gases. Langmuir was among the first to note the separation of highly conducting plasma into charged-particle sheaths or cellular-like walls. This structure appears wherever samples with different densities, temperatures, or magnetic-field strengths come into contact.
Like flashes of lightning, terrestrial plasmas are by and large transient. Even in a neon or fluorescent bulb, the mixture of free electrons and ions remains only as long as the power is turned on. Extraterrestrial plasmas are much more long-lived, but until recently only a handful of scientists had speculated about the universal extent and character of such matter. Yet almost all of the observable universe is plasma. Stars, for example, are gravitationally bound plasmas, while all of interstellar and intergalactic space is plasma.
Wherever plasma exist, they produce prodigious amounts of electromagnetic radiation. In particular, X- and gamma rays from beyond the solar system are likely produced by free electrons with energies corresponding to temperatures of more than 1 million degrees—the realm of hot, magnetized plasmas. We call the overall picture obtained from these energetic emissions the plasma universe.
Hot plasma also emit radiation of lower energy, such as visible and radio waves (we can both see lightning and hear it on a receiver). However, the emission does not always have a thermal origin. For example, unknowing human have viewed synchrotron radiation (from electrons spiraling at nearly the speed of light in a magnetic field) from the Crab nebula for centuries.
Synchrotron radiation is named after the particle accelerators developed in the 1930's and 1940's to produce high-energy electrons. In 1950 Hannes AlfvÈn, Nicolai Herlofson, and Karl Kiepenheuer brought this form of plasma radiation to astronomer's attention. Alfvén, who later won a Nobel prize in physics for his solar studies, proposed hat streams of electrons move at nearly the speed of light along magnetic-field lines not only in Earth's magnetosphere and above the Sun, but also through the cosmos. If so, sheets and ropes of electric current should crisscross the universe in ever-increasing sizes.
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Left: In the laboratory filaments are produced when a pulse-power generator delivers 10 trillion watts to a plasma only a few centimeters long. Middle: Similar structure is seen in solar prominences, but in this case the lengths are measured in hundreds of thousands kilometers. Right: Filaments at the center of Galaxy stretch out over 120 light years. |
These currents, Alfvén thought, should give the universe a cellular and filamentary structure. At the time of this suggestion, supporting evidence in the form of huge filaments, sheets, and walls of galaxies were unknown.
Modern plasma space science has been heavily influenced by the earlier research of Norwegian scientist Kristian Birkeland. At the turn of the century he suggested that electrical currents due to "corpuscular rays" (plasma) from the Sun caused the aurora borealis. Such currents were considered impossible until they were discovered by an artificial satellite in 1974. Enormous Birkeland currents connecting Jupiter and its moon Io were recorded by the Voyager spacecraft in 1979.
In 1984 Farhad Yusef-Azdeh, Don Chance, and Mark Morris found an example of Birkeland currents on a galactic scale. Working with the Very Large Array radio telescope, they discovered an arc of radio emission some 120 light-years long near the center of the Milky Way. The structure is made up of narrow filaments typically 3 light-years wide and running the full length of the arc (see the image above). The strength of the associated magnetic field is 100 times greater than previously thought possible on such a large scale, but the field is nearly identical in geometry and strength with computer simulations of Birkeland currents in studies of galaxy formation.