The Solar Wind Is Plasma – Space Plasma Cannot Be Duplicated in the Lab Yet
Ions and electrons in space are usually intimately mixed, in a “soup” containing equal amounts of positive and negative charges. Such a mixture is known as a plasma (the same term has a different meaning in medicine; (see the history of plasma). In many respects it behaves like a gas, but when electric and magnetic forces are present, additional properties come to light, quite unlike those of ordinary gases.
The ionosphere above our heads is a plasma. Unlike air, it conducts electricity, and in fact, the ionosphere in the polar regions carries large electric currents, as is discussed in a later section. The electric conductivity of the ionosphere, unlike that of metals or seawater, is very much influenced by the Earth’s magnetic field. This is a rather special plasma, because the ionosphere also contains a fairly high number of neutral atmospheric molecules, with which the ions and electrons constantly collide.
In contrast, collisions are extremely rare in the solar wind. If this were an ordinary gas, or if the Earth lacked a magnetic field, the solar wind would have penetrated all the way to the top of the atmosphere and would then have flowed around the Earth, the way water flows around a rock in a stream. Something like that in fact happens at the planet Venus, which seems to have no magnetic field of its own. At Earth, however, a strong magnetic field confronts the solar wind, forming a much bigger obstacle than the Earth itself. Because the solar wind is a plasma, it is forced to detour around the Earth’s field, creating a large shielded cavity around the Earth —the magnetosphere.
The explanation of space phenomena thus requires a good understanding of plasma physics. Unfortunately, no laboratory can duplicate the large dimensions and the very low particle collision rates found in space plasmas. The behavior of such plasmas can be sometimes simulated by computers, but ultimately, to figure what actually happens, one needs to send instruments into space and study their observations.