plasma Natural plasmasstate of matter

It has been suggested that the universe originated as a violent explosion about 10 billion years ago and initially consisted of a fireball of completely ionized hydrogen plasma. Irrespective of the truth of this, there is little matter in the universe now that does not exist in the plasma state. The observed stars are composed of plasmas, as are interstellar and interplanetary media and the outer atmospheres of planets. Scientific knowledge of the universe has come primarily from studies of electromagnetic radiation emitted by plasmas and transmitted through them and, since the 1960s, from space probes within the solar system.

In a star the plasma is bound together by gravitational forces, and the enormous energy it emits originates in thermonuclear fusion reactions within the interior. Heat is transferred from the interior to the exterior by radiation in the outer layers, where convection is of greater importance. In the vicinity of a hot star, the interstellar medium consists almost entirely of completely ionized hydrogen, ionized by the star’s ultraviolet radiation. Such regions are referred to as H II regions. The greater proportion by far of interstellar medium, however, exists in the form of neutral hydrogen clouds referred to as H I regions. Because the heavy atoms in such clouds are ionized by ultraviolet radiation (or photoionized), they also are considered to be plasmas, although the degree of ionization is probably only one part in 10,000. Other components of the interstellar medium are grains of dust and cosmic rays, the latter consisting of very high-energy atomic nuclei completely stripped of electrons. The almost isotropic velocity distribution of the cosmic rays may stem from interactions with waves of the background plasma.

Throughout this universe of plasma there are magnetic fields. In interstellar space magnetic fields are about 5 × 10^-6 gauss (a unit of magnetic field strength) and in interplanetary space 5 × 10^-5 gauss, whereas in intergalactic space they could be as low as 10^-9 gauss. These values are exceedingly small compared with the Earth’s surface field of about 5 × 10⁻¹ gauss. Although small in an absolute sense, these fields are nevertheless gigantic, considering the scales involved. For example, to simulate interstellar phenomena in the laboratory, fields of about 10¹⁵ gauss would be necessary. Thus, these fields play a major role in nearly all astrophysical phenomena. On the Sun the average surface field is in the vicinity of 1 to 2 gauss, but magnetic disturbances arise, such as sunspots, in which fields of between 10 and 1,000 gauss occur. Many other stars are also known to have magnetic fields. Field strengths of 10^-3 gauss are associated with various extragalactic nebulae from which synchrotron radiation has been observed.