Plasmas in space

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 10R_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 5R_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.