Space Plasmas

Many people believe the space in between the the Sun and its planets is empty, a vacuum devoid of energy or matter. But space is not empty. Our Sun constantly emits plasma, a superheated state of matter, which moves out in all directions at very high speeds to fill the entire solar system and beyond.

By studying processes that occur in the earth's magnetosphere (where earth's magnetic field has a greater influence than the Sun's interplanetary field), in interplanetary space, and around other planets, we are better able to appreciate the important role of plasmas throughout our plasma universe. This space plasma laboratory is truly our window to the stars.

The earth's magnetosphere is normally invisible because the dominant hydrogen and helium ions coming in the solar wind do not scatter light in visible wavelengths. However, comets emit heavier ions that are visible and which result in spectacular neutral and ion (plasma) tails. Images of earth's magnetosphere would show it to be a very large comet-like interaction region.

The Earth's magnetic field acts in many ways as a buffer between us and space. Over and around the field flows the solar wind, a dilute but persistent stream of protons, electrons, and other ions. This flow of charge, with its associated electromagnetic fields, distorts Earth's own field, compressing it on the dayside and stretching it out on the nightside into a long tail much like a comet's. The resulting field is called the magnetosphere.


The dynamic structure of Earth's magnetosphere, revealed by detailed satellite investigation,is responsible for the aurora on Earth. These spectacular phenomena occur in two "auroral arcs" lying at polar latitudes in both the northern and southern hemispheres, and are caused by plasma electrons flowing down in sheets along the Earth's magnetic field. These sheets of electrons, or electrical currents, filament up to form the rapid waving curtains of light in an auroral display, a result of the electrons interacting with and exciting molecules in the upper atmosphere.


The Sun, a gravitationally bound plasma at the center of the solar system, displays a rich variety of instabilities that have come to be identified with this energetic state of matter. The photosphere, the chromosphere, and the corona are layers of plasma superimposed on the Sun like onion skins. At more than a million degrees, electrons and ions tend to escape the outer corona. This "solar wind" of plasma stretches out and permeates the entire solar system, interacting with the magnetic fields of the planets to form magnetospheres, eventually diffusing into the interstellar plasma between stars. While evidence of the solar wind came from the existence of comet tails, the growing stream of plasma from the nucleus of a comet as it nears the Sun, confirmation was not obtained until 1959 when Soviet Luna 2 and 3 rockets measured the wind in situ.

Since that time, spacecraft have probed the atmospheres of Venus, the systems of Jupiter and Saturn, and the enigmatic planet Uranus. These craft, in addition to radio astronomy, have uncovered phenomena which were previously unsuspected. For example, "ropes" of magnetic-flux-entwined plasma were discovered on Venus while strong electrical discharges were observed on Jupiter, its satellite Io, and Saturn; a phenomena that apparently also occurs on the inner planet Mercury. In mimicry to the Jupiter-Io torus, Voyager 1 discovered a gigantic torus of plasma around Saturn, that begins far beyond its ring system and extends 25 times the radius of the planet.

Like Earth, Jupiter, Saturn, and Uranus were found to be intense sources of radio waves. This radiation is intermittent and intimately connected to polar aurora observed on all these planets. The source of this radiation derives from charged particles trapped and accelerated in the magnetospheric plasmas of these planets, producing powers several orders of magnitude greater than radio broadcast transmitters. These waves cannot penetrate the protective layer of plasma covering Earth (the ionosphere) and so were unsuspected until the advent of satellites.

Thus, in the course of the last thirty years, space research, including radio and high-energy astronomy, has given us a glimpse of a plasma dominated solar system, a system very much different than that based on observations in the narrow visual octave of the electromagnetic spectrum. As we shall see, the extension of these space plasma discoveries to regions beyond the reach of space craft gives a picture of the plasma universe, a picture drastically different from the traditional visual universe.