Solar wind

The solar wind is a stream of charged particles that flows constantly from the sun in all directions. Compared to the stellar wind of other fixed stars, it is relatively weak, but must have been stronger in the initial sun.

Occasionally, the wrong term sun dust is used (similar to Stardust ), which was especially the case in the press reporting on Genesis probe.

Origin and composition

The solar wind consists mainly of protons and electrons and helium nuclei ( alpha particles ); other atomic nuclei and non-ionized ( electrically neutral ) atoms are hardly contain, so the solar wind is a so-called plasma.

Although he comes from the outer layers of the Sun, it reflects the element frequency of these layers does not object exactly. For by fractionation (FIP effect) are enriched some elements in the solar wind or diluted. Inside the solar element abundances were amended by there running nuclear fusion; but since the outer solar layers are not mixed with the inner whose composition corresponds to that of the still primeval mist from which the solar system was formed. The exploration of the solar wind is therefore interesting to include both the chemical composition and the isotopic abundances of the nebula can.

Near the Earth, the solar wind has a density of ≈ 5 × 106 particles per cubic meter. The sun loses by the solar wind per second, about one million tons of its mass.

Speed

One distinguishes the slow and the fast solar wind: The slow solar wind doubles its speed of 150 km / s at a distance 5 × R0 to 300 km / s at a distance 25 × R0. For most of the 30 solar radii large observational field corresponding to a constant acceleration of 4 m / s ².

The fast solar wind, which escapes from the coronal holes is between 1.5 and 2.5 · R0 · R0 remarkably accelerated and has in the middle of the field, ie, at 2 · R0, a speed of 300 km / s The oxygen ions are much faster than the lighter proton. The measurements by the Ultraviolet Coronal Spectrometer ( UVCS ) of the research satellite Solar and Heliospheric Observatory showed that the fast solar wind is accelerated considerably faster over the sun's poles than can be explained by thermodynamics. This theory predicts that the speed of sound about four solar radii should be exceeded above the photosphere. In fact, we find this limit already in about 25% of this distance. The cause of this acceleration Alfvén waves are considered.

Effects of the solar wind

Since the solar wind is a plasma, it deforms both the magnetic field of the sun and the earth. The earth's magnetic field keeps the particle showers from for the most part of the earth. Only with a strong solar wind particles can penetrate into the high layers of the atmosphere where they can cause auroras, as well as on other planets with a magnetic field. Strong solar winds also affect the propagation of electromagnetic waves and can interfere with short wave radio and communication with satellites, among others. Solar wind and its impact on the technology have been known for, for example, 1847, 1859, 1921 and 1940, because there have been incidents in telegraphy, at signals of the web, to radio communications and sometimes even the explosive by braising transformers ( to a transformer failure has occurred, for example, on March 13, 1989 in Quebec). It is considered possible that particularly strong solar winds could lead to a global total failure of power supply and computer functions.

A clearly visible indication of the existence of the solar wind provide the comet: comet tails always point away from the sun, because the gas and dust particles, which form the coma and the tail will be carried away by the solar wind.

The solar wind extends far away beyond the outer planets orbits. It drives, the interplanetary matter from the solar system, forming a kind of bubble in space, which is called the heliosphere. The boundary of the heliosphere where the particles of the solar wind is decelerated is called the heliopause. It is often regarded as the boundary of the solar system. The exact distance is not known, observations of the Voyager 2 spacecraft give reason to believe that the heliopause is roughly four times the distance of Pluto.

Within the heliosphere, there is a layer in which the solar magnetic field changes polarity. This gives rise to electric currents in the solar wind, which could be measured by spacecraft. This layer is irregularly shaped and is called Heliospheric current sheet.

Discovery and Exploration

Already during the solar storm of 1859 the researcher Richard Carrington observed a correlation between solar flares and time-shifted the Earth's magnetic storms, which - although inexplicable at that time - was an early indication of the existence of the solar wind. Beginning of the 20th century represented the Norwegian physicist Kristian Birkeland held that the auroras were triggered by particle beams of the sun. His idea was, however, taken no more seriously as those of the German physicist Ludwig Biermann, who accepted a "Solar particle radiation " in order to explain the direction of the comet tails can. Astronomers had noticed that the comet tails were not directed exactly away from the Sun, but had a small angle to it. Biermann said this property in 1951 through the motion of the comet in a moving stream of particles is also, to some extent lateral drifting through the flow. E. N. Parker in 1959 the English name solarwind introduced and proposed a magneto- hydrodynamic theory for the description of the solar wind.

The existence of the solar wind could be confirmed in 1959 by the Soviet Lunik 1 and 1962 by the American space probe Mariner 2 on its way to Venus experimentally. Another milestone in the study of the solar wind were the solar wind sails that were set up with the exception of Apollo 13 and 17 moon landings at all and provided data on the isotopic abundances of the noble gases helium, neon and argon in the solar wind. Many other issues have contributed to the understanding of the solar wind. The space probes Pioneer 10/11, Voyager 1 /2 and the Ulysses mission provided data of the solar wind outside the Earth's orbit, while Helios 1/2 and the Mariner and Pioneer missions to Venus and Russian Vega probes provided data from within the Earth's orbit. IMP 1-8, AIMP 1/2, ACE, ISEE 1-3 probes and the solar observatory SOHO and Wind spacecraft delivered solar wind data near the Earth. The Ulysses mission also provided data on the solar wind outside the ecliptic. In 2001, the Genesis mission was launched, in which high-purity crystals in one of the Lagrange points ( L1) of the Sun-Earth system have been exposed to the solar wind and should be then returned to the study of the earth. The mission failed in its conclusion in 2004, because the capsule with the solar wind has not slowed down, but crashed on the ground. The Voyager 1 spacecraft has reached the termination shock in December 2004, and in August 2007, Voyager 2 reached this limit, and provided measurement data.

There are efforts, the solar wind using solar sails to propel spacecraft to use.