The continuous expansive motion of the coronal gas gives rise to the SOLAR WIND. At the distance of the Earth its velocity is normally about 400 km s”1, the particle density about 107nr3, and the temperature approximately 105 K. Even though the total outflow of material from the Sun is about one million tonnes per second the mass-loss has a negligible influence on the Sun’s evolution. The high coronal temperatures ensure that the constituents of the solar wind (mainly electrons, protons and alpha particles) are ionized; the low density of particles prevents the recombinations of ions and electrons even at large distances, where the temperature may have dropped considerably. This ensures that the solar wind is electrically high conducting. Consequently it can be influenced by magnetic fields, and indeed it drags some of the magnetic field of the Sun with it into interplanetary space. The wind itself blows approximately radially outwards from the corona, but the inter¬planetary magnetic field, firmly rooted to the Sun, is drawn out into spirals on account of the rotation of the Sun; this is frequently referred to as the garden-hose effect. The large-scale regions of different magnetic polarity on the Sun, mentioned above, are reflected in the sectorial structure of the solar wind . Magnetic fields in the wind scatter low-energy cosmic rays, thus preventing us from measuring directly the interstellar cosmic-ray flux.

Back in the last century, the aurorae were related to streams of charged particles emitted from the Sun. Later, a similar hypothesis was used to explain magnetic storms and changes in cosmic-ray counting rates after solar flares. In the 1950s, a model invoking more uniform emission of particles was proposed by L. Biermann to explain why comets’ tails point away from the Sun; ordinary electromagnetic radiation pressure is insufficient for this. The first direct evidence for the solar wind (a phrase coined by E.N. Parker, a pioneer in this field) came in 1959 and 1962 from charged-particle detectors carried in early space probes to the Moon and Venus. These results have since been amply confirmed, and now the solar wind had been detected out beyond Jupiter. Somewhere beyond the orbit of Pluto the solar wind probably merges with the interstellar medium.

At times of solar activity, such as a solar flare, much plasma is ejected from the Sun both in the form of relativistic (energies exceeding 1 Ge V) and non-relativistic particles. The less energetic plasma is preceded by a shock wave, which greatly distorts the interplanetary magnetic fields as it passes them. The interaction of first the shock front and then the compressed, hot plasma with a planetary magnetosphere leads to a magnetic storm being recorded down on the surface of the planet.

Inhomogeneities and fluctuations in the coronal and solar wind density near the Sun can cause the SCINTILLATION OP RADIO SOURCES. This phenomenon occurs in much the same manner as the optical twinkling of stars observed on Earth, which is attributable to turbulence in our atmosphere. A point source of light, such as Sirius, twinkles violently to the eye, but the twinkling of individual parts of the apparent disc of Jupiter tends to even out and render it relatively steady. Similarly only the smallest radio sources show scintillation at radio wavelengths, caused by natural variations in the density of the corona and solar wind. Studies of scintillation have improved our knowledge of the outer corona especially in directions as yet inaccessible to spacecraft, and have also permitted estimates of the angular sizes of small radio sources.

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