The heliosphere (from the Greek: solar sphere ) is the astrosphere of the sun . In space it describes a large area around the sun in which the solar wind with its magnetic fields is effective. In this area the particle flow from the sun displaces the interstellar medium . The orbits of the planets are well within the heliosphere.
The solar system is embedded in the interstellar medium , which consists primarily of extremely dilute gas as well as dust and magnetic fields.
The sun in turn emits a constant stream of particles, the solar wind . This mainly consists of ionized hydrogen and helium ( protons , helium nuclei and electrons ). At a distance of 1 AU from the sun (orbit of the earth), the particle density of the solar wind is one to ten million particles per cubic meter. In the case of coronal mass ejections , the particle density can increase more than a hundred times at this distance. The solar wind with its electrically charged particles and the accompanying interplanetary magnetic field displaces the interstellar medium and forms a "bubble" around the sun. That bubble is the heliosphere.
The solar system moves through the interstellar medium at a speed of about 23 km / s, coming from the direction of the constellation Taurus in the direction of the constellation Scorpio . This creates a "head wind" (interstellar wind). Whether and to what extent the Heliosphäre is deformed thereby - is pressed in the front and back a "Helio tail" (eng. Heliotail ) forms - is still largely unclear.
The heliosphere is structured by two borders:
- Edge shock wave or Terminationsschock (engl. Termination shock ) - the solar wind falls below sonic velocity , it enters a shock front on.
- Heliopause - the solar wind and the interstellar medium meet and are stopped. The ionized particles of the sun and the interstellar medium are in pressure equilibrium.
There is no special designation for the area within the edge shock wave. The area between the edge of the shock wave and Heliopause is Helio shell (engl. Heliosheath ) mentioned. Beyond the heliopause, by definition, the heliosphere ends and interstellar space begins.
The other side of the heliopause is another shock front, - It was long believed that - as seen with other stars bow shock (bow shock), where the interstellar wind is decelerated from over- to subsonic speed. According to more recent findings this does not exist (only a bow wave ) because the solar system moves relative to the interstellar medium at less than the speed of sound.
In the inner area of the heliosphere, the solar wind moves unaffected through space, as it flows at supersonic speed, i.e. H. its flow speed is greater than the speed with which disturbances of the density or pressure in the plasma move (speed of sound). Only electrically neutral atoms from the interstellar medium and a small part of the galactic cosmic radiation can penetrate this far into the heliosphere. Apart from the few particles that can do this, most of the particles there come from the sun.
Edge shock wave (shock termination)
At the edge shock wave ( termination shock ), the flow velocity drops below the speed of sound, so that the interstellar medium is influenced for the first time. The particles of the solar wind are decelerated abruptly - at low latitudes (ie near the ecliptic ) from approx. 350 km / s to approx. 130 km / s. This slowing down and the further flow of matter condenses and heats up the medium of the solar wind. As a result, there is still a significant increase in the magnetic field.
The Voyager 2 space probe measured a sudden rise in temperature from approx. 11,000 K to 180,000 K when it passed through the edge shock wave , which, however, was well below the predictions of some models that had predicted temperatures of a few million Kelvin. Together with the results of the STEREO probes, it was found that 70% of the kinetic energy of the solar wind is not converted into heat, but into the ionization of the matter encountered there. These could be electrically neutral hydrogen atoms that penetrated the helio envelope at a speed of around 25 km / s and advanced to the edge shock wave.
Voyager 1 was deflected 34 ° north of the ecliptic plane while flying past Saturn's moon Titan and reached the edge shock wave at a distance of 94 AU from the Sun; Voyager 2, however, which was deflected at Neptune 26 ° south, reached it at a distance of 84 AU. One possible explanation for this difference is that the interstellar magnetic field pushes the southern half of the heliosphere inward and arches the northern half outward. Another possible cause is the variable solar activity, since the measurements of the two Voyager probes were taken three years apart.
The example of Voyager 2 also showed that the edge shock wave is not a consistent fixed boundary, but a dynamic event that behaves similar to the surf on a beach. There are density fluctuations in the solar wind, caused by coronal mass eruptions or superimposition of the fast and slow solar winds, which are comparable to the waves in the sea and thus extend further into the Helio envelope. Due to the differential rotation of the sun and the great distance from the sun, large jumps in the absolute distance from the sun can be possible at relatively short intervals. Voyager 2 passed the edge shock wave five times within a few days before it finally passed on August 30, 2007.
Helio shell (heliosheath)
Beyond the edge shock wave is the heliosheath , in the area of which solar wind particles continue to occur, but now with a reduced flow velocity at higher density and temperature. This area is still dominated by the solar wind, but particles of the local interstellar medium mix into it. Measurements by the Voyager space probes showed that there is a kind of "foam" of magnetic bubbles with a diameter of typically 1 AU in which the charged particles of the solar wind are trapped.
Based on model calculations and observations on other stars, it has long been assumed that the helio hull is only 10 AU thick in the direction of the sun's own motion, while it is deformed in the opposite direction by the interstellar wind into a long "helio tail" of up to 100 AU. Measurements by the IBEX probe in 2013 indicated a helio's tail with a clover-leaf structure, believed to be caused by the uneven activity of the sun during its 11-year cycle. Combined long-term measurements with the space probes Voyager 1 and 2, Cassini and IBEX on the other hand suggest that the Helio hull is more spherical. The reason is the unexpectedly strong interstellar magnetic field of around 0.5 nanotesla , which keeps the heliosphere in shape. In addition, electrically neutral particles can penetrate the heliosphere unhindered and are charged through interaction with solar wind particles. Such high-energy “pick-up ions” were detected by the New Horizons space probe and could cause the heliosphere to have more of the shape of a croissant in a certain energy range .
The heliopause is the outermost limit of the heliosphere. Behind it, by definition, begins interstellar space. The solar wind no longer exerts any material influence on the interstellar gas. The particles of the solar wind mix with the interstellar gas and have no recognizable prominent flow direction in comparison with the gas surrounding the heliosphere.
Voyager 1 passed the heliopause around August 25, 2012 at a distance of 121.7 AU from the Sun. The measuring devices recorded a dramatic decrease in the counting rate of solar particles by more than a factor of 100 and a significant increase in high-energy cosmic radiation. Voyager 2 reached the heliopause on November 5, 2018 at a distance of 119.0 AU. The plasma spectrometer recorded a sharp drop in the speed of solar particles. In the radial direction (out of the solar system) the solar wind was completely stopped. This measurement could not be carried out with Voyager 1 because the probe's plasma spectrometer failed completely in the 1980s.
Voyager 2 crossed the heliopause in just half a day, showing a thin, stable boundary layer. Voyager 1, on the other hand, had already registered a slowdown in the plasma flows almost two years before the heliopause and then turbulence in the surrounding interstellar plasma - an indication of an unstable but thick border region. The reason for the difference could be the time-varying solar activity. According to measurements by Voyager 1, the magnetic field lines of the heliosphere were connected to those of interstellar space. Along a thus resulting "magnetic highway" (magnetic highway) may pass charged particles from the solar system in interstellar space and vice versa. Voyager 2 found no such connection but a magnetic barrier on this side of the heliopause, which presumably builds up again with the cycle of solar activity and acts as an additional shield against galactic cosmic radiation.
The heliopause is sometimes referred to as the "limit of the solar system". In fact, the orbits of the known planets are well within the heliosphere ( Neptune as the outermost planet with 30 AU), and the same is true for Pluto and the Kuiper Belt in general (30–50 AU). In the meantime, however, trans-Neptunian objects have been found whose orbits extend beyond the heliopause, and the Oort cloud, suspected by astrophysicists, is even further away . Gravity from the sun is still present in these objects, so it is not generally accepted to define the heliopause as a limit.
Research with space probes on site is difficult because immense distances have to be overcome, on top of that against the braking gravitational force of the sun. The two probes in the Voyager program are the only man-made objects that have ever been shown to have penetrated the Helio hull. Although they were accelerated by several swing-by maneuvers, this took them more than a quarter of a century; they reached the heliopause after 35 and 41 years, respectively. Pioneers 10 and 11 returned data up to 63 AU and 35.6 AU, respectively, before breaking contact. It is uncertain whether New Horizons will still have enough energy when the probe reaches the surface shock wave around 2035.
The two Voyager probes were launched in 1977. They were only designed for exploring the outer planets and a lifespan of a few years, but remained functional for much longer. With their detectors for magnetic fields, cosmic rays, plasma particles (only Voyager 2) and plasma waves , the probes transmitted measurement data from the helio envelope and interstellar space.
Voyager 1 reached the edge shock wave on December 16, 2004, Voyager 2 on August 30, 2007. On approximately August 25, 2012, Voyager 1 passed the heliopause and entered interstellar space; Voyager 2 followed on November 5, 2018.
Solar Terrestrial Relations Observatory (STEREO)
Originally designed to study the earth's magnetosphere in connection with the sun's eruptions, the Solar Terrestrial Relations Observatory (STEREO) was able to indirectly detect neutral atoms from the interstellar gas. The probes detected high-energy, electrically neutral atoms that came primarily from the direction in which the sun was moving. Unaffected by the magnetic field of the heliosphere, these atoms were able to penetrate to the STEREO probes. Apparently these are originally charged particles ( ions ) from the solar wind, which were heated to high energies in the region of the edge shock wave, in the helium envelope their charge was lost to low-energy neutral atoms from the interstellar gas and scattered back. This agrees with the measurement results of Voyager 2, which showed a lower temperature than expected beyond the edge shock wave.
Interstellar Boundary Explorer (IBEX)
The NASA research satellite Interstellar Boundary Explorer (IBEX, to German about explorer of the interstellar limit ) mapped the interstellar medium out around the sun by means of measuring neutral atoms from an earth orbit. There were first indications of a Helios tail in 2013. In 2016, a band structure was discovered that is believed to result from the flow around the heliosphere in the interstellar magnetic field. In addition, IBEX in 2012 gave the surprising result that the solar system moves so slowly through the interstellar medium that there is no shock front (bow shock wave).
In 2016, IBEX observed the effects of increased solar activity: in the second half of 2014, the density and speed of the solar wind had increased, increasing its pressure by 50%. Two years later, IBEX detected solar wind particles that had reached the edge of the heliosphere and were scattered back from there as neutral atoms. Model calculations showed that the increased solar wind had shifted the edge shock wave by 7 AU and the heliopause by up to 4 AU.
Shielding the galactic cosmic rays
The heliosphere - especially the helio envelope - shields the earth from about 3 ⁄ 4 of the galactic cosmic radiation . Currently, the solar system moves in interstellar space through the local bubble , which has a relatively low density. If the sun were to cross an area with a much higher density, the heliosphere could be pushed back further at the front. For example, for crossing a molecular cloud with a density 30 times higher, model calculations show that the heliopause in the direction of movement would be closer by a factor of 4–5. The galactic cosmic radiation on earth would increase by a factor of 1.5-3, the anomalous cosmic radiation by a factor of 10. This fact, had it ever occurred in the 4.5 billion years since the existence of the solar system, could be investigated of sediments . However, there is no evidence that the Sun has passed through a molecular cloud in its previous lifespan. Likewise, it is not to be expected that the sun will immerse itself in a region with greater density in the next millions of years.
- IBEX homepage of the Southwest Research Institute (English)
- STEREO homepage of NASA (English)
- Current data from the Voyager mission (English)
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