Plasmas in space

Plasma

PART III. ADDITIONAL READING

Practice writing abstract and summaries using the text given in thepart ADDITIONAL READING using words and word combinations given in Appendix 3.

The plasma state is often referred to as the fourth state of matter, an identification that resonates with the element of fire, which along with earth, water and air made up the elements of Greek cosmology according to Empedocles. Fire may indeed result in a transition from the gaseous to the plasma state, in which a gas may be fully or, more likely, partially ionized. For the present we identify as plasma any state of matter that contains enough free charged particles for its dynamics to be dominated by electromagnetic forces. In practice quite modest degrees of ionization are sufficient for a gas to exhibit electromagnetic properties. Even at 0.1 per cent ionization a gas already has an electrical conductivity almost half the maximum possible, which is reached at about 1 per cent ionization. The outer layers of the Sun and stars in general are made up of matter in an ionized state and from these regions winds blow through interstellar space contributing, along with stellar radiation, to the ionized state of the interstellar gas. Thus, much of the matter in the Universe exists in the plasma state. The Earth and its lower atmosphere is an exception, forming a plasma-free oasis in a plasma universe. The upper atmosphere on the other hand, stretching into the ionosphere and beyond to the magnetosphere, is rich in plasma effects. Solar physics and in a wider sense cosmic electrodynamics make up one of the roots from which the physics of plasmas has grown; in particular, that part of the subject known as magnetohydrodynamics – MHD for short – was established largely through the work of Alfven. A quite separate root developed from the physics of gas discharges, with glow discharges used as light sources and arcs as a means of cutting and welding metals. The word plasma was first used by Langmuir in 1928 to describe the ionized regions in gas discharges. These origins are discernible even today though the emphasis has shifted. Much of the impetus for the development of plasma physics over the second half of the twentieth century came from research into controlled thermonuclear fusion on the one hand and astrophysical and space plasma phenomena on the other.

To a degree these links with ‘big science’ mask more bread-and-butter applications of plasma physics over a range of technologies. The use of plasmas as sources for energy-efficient lighting and for metal and waste recycling and their role in surface engineering through high-speed deposition and etching may seem prosaic by comparison with fusion and space science but these and other commercial applications have laid firm foundations for a new plasma technology.

Thermonuclear burn in stars is the source of plasmas in space. From stellar cores where thermonuclear fusion takes place, keV photons propagate outwards towards the surface, undergoing energy degradation through radiation–matter interactions on the way. In the case of the Sun the surface is a black body radiator with a temperature of 5800 K. Photons propagate outwards through the radiation zone across which the temperature drops from about 107 K in the core to around 5 × 105 K at the boundary with the convection zone. This boundary is marked by a drop in temperature so steep that radiative transfer becomes unstable and is supplanted as the dominant mode of energy transport by the onset of convection.

Just above the convection zone lies the photosphere, the visible ‘surface’ of the Sun, in the sense that photons in the visible spectrum escape from the photosphere. UV and X-ray surfaces appear at greater heights. Within the photosphere the Sun’s temperature falls to about 4300K and then unexpectedly begins to rise, a transition that marks the boundary between photosphere and chromosphere. At the top of the chromosphere temperatures reach around 20 000K and heating then surges dramatically to give temperatures of more than a million degrees in the corona.

The surface of the Sun is characterized by magnetic structures anchored in the photosphere. Not all magnetic field lines form closed loops; some do not close in the photosphere with the result that plasma flowing along such field lines is not bound to the Sun. This outward flow of coronal plasma in regions of open magnetic field constitutes the solar wind. The interaction between this wind and the Earth’s magnetic field is of great interest in the physics of the Sun–Earth plasma system. The Earth is surrounded by an enormous magnetic cavity known as the magnetosphere at which the solar wind is deflected by the geomagnetic field, with dramatic consequences for each. The outer boundary of the magnetosphere occurs at about 10 R _E, where R _E denotes the Earth’s radius. The geomagnetic field is swept into space in the form of a huge cylinder many millions of kilometres in length, known as the magnetotail. Perhaps the most dramatic effect on the solar wind is the formation of a shock some 5 R _E upstream of the magnetopause, known as the bow shock. We shall discuss a number of these effects later in the book by way of illustrating basic aspects of the physics of plasmas.

Mechanisms of Electron Losses: Electron-Ion Recombination

The ionization processes were considered as a source of electrons and positive ions, e.g., as a source of plasma generation. Conversely, the principal loss mechanisms of charged particles, the elementary processes of plasma degradation, will now be examined. Obviously, the losses together with the ionization processes determine a balance of charge particles and plasma density. The variety of channels of charged particle losses can be subdivided into three qualitatively different groups.

The first group includes different types of electron-ion recombination processes, in which collisions of the charged particles in a discharge volume lead to their mutual neutralization. These exothermic processes require consuming the large release of recombination energy in some manner. Dissociation of molecules, radiation of excited particles, or three-body collisions can provide the consumption of the recombination energy.

Electron losses, because of their sticking to neutrals and formation of negative ions, form the second group of volumetric losses, electron attachment processes. These processes are often responsible for the balance of charged particles in such electronegative gases as oxygen (and, for this reason, air); CO₂ (because of formation of O−); and different halogens and their compounds. Reverse processes of an electron release from a negative ion are called the electron detachment.

Note that although electron losses in this second group are due to the electron attachment processes, the actual losses of charged particles take place as a consequence following the fast processes of ion-ion recombination. The ion-ion recombination process means neutralization during collision of negative and positive ions.

Finally, the third group of charged particle losses is not a volumetric one like all those mentioned previously, but is due to surface recombination. These processes of electron losses are the most important in low pressure plasma systems such as glow discharges. The surface recombination processes are usually kinetically limited not by the elementary act of the electron-ion recombination on the surface, but by transfer (diffusion) of the charged particles to the walls of the discharge chamber.

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