5.3 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Heliosphere | 2/6 | https://en.wikipedia.org/wiki/Heliosphere | reference | science, encyclopedia | 2026-05-05T13:27:47.547602+00:00 | kb-cron |
The solar wind is traveling at supersonic speeds within the Solar System. At the termination shock, a standing shock wave, the solar wind falls below the speed of sound and becomes subsonic. It was previously thought that once subsonic, the solar wind would be shaped by the ambient flow of the interstellar medium, forming a blunt nose on one side and comet-like heliotail behind, a region called the heliosheath. However, observations in 2009 showed that this model is incorrect. As of 2011, it is thought to be filled with a magnetic bubble "foam". The outer surface of the heliosheath, where the heliosphere meets the interstellar medium, is called heliopause. This is the edge of the entire heliosphere. Observations in 2009 led to changes to this model. In theory, heliopause causes turbulence in the interstellar medium as the Sun orbits the Galactic Center. This turbulence results from the pressure of the advancing heliopause against the interstellar medium. However, the velocity of the solar wind relative to the interstellar medium may be too low for a bow shock.
=== Termination shock ===
The termination shock is the point in the heliosphere where the solar wind slows down to subsonic speed (relative to the Sun) because of interactions with the local interstellar medium. This causes compression, heating, and a change in the magnetic field. In the Solar System, the termination shock is believed to be 75 to 90 astronomical units from the Sun. In 2004, Voyager 1 crossed the Sun's termination shock, followed by Voyager 2 in 2007. The shock arises because solar wind particles are emitted from the Sun at about 400 km/s, while the speed of sound (in the interstellar medium) is about 100 km/s. The exact speed depends on the density, which fluctuates considerably. The interstellar medium, although very low in density, nonetheless has a relatively constant pressure associated with it; the pressure from the solar wind decreases with the square of the distance from the Sun. As one moves far enough away from the Sun, the pressure of the solar wind drops to where it can no longer maintain supersonic flow against the pressure of the interstellar medium, at which point the solar wind slows to below its speed of sound, causing a shock wave. Further from the Sun, the termination shock is followed by heliopause, where the two pressures become equal and solar wind particles are stopped by the interstellar medium. Other termination shocks can be seen in terrestrial systems; perhaps the easiest may be seen by simply running a water tap into a sink creating a hydraulic jump. Upon hitting the floor of the sink, the flowing water spreads out at a speed that is higher than the local wave speed, forming a disk of shallow, rapidly diverging flow (analogous to the tenuous, supersonic solar wind). Around the periphery of the disk, a shock front or wall of water forms; outside the shock front, the water moves slower than the local wave speed (analogous to the subsonic interstellar medium). Evidence presented at a meeting of the American Geophysical Union in May 2005 by Ed Stone suggests that the Voyager 1 spacecraft passed the termination shock in December 2004, when it was about 94 AU from the Sun, by virtue of the change in magnetic readings taken from the craft. In contrast, Voyager 2 began detecting returning particles when it was only 76 AU from the Sun, in May 2006. This implies that the heliosphere may be irregularly shaped, bulging outwards in the Sun's northern hemisphere and pushed inward in the south.
=== Heliosheath === The heliosheath is the region of the heliosphere beyond the termination shock. Here the wind is slowed, compressed, and made turbulent by its interaction with the interstellar medium. At its closest point, the inner edge of the heliosheath lies approximately 80 to 100 AU from the Sun. A proposed model hypothesizes that the heliosheath is shaped like the coma of a comet, and trails several times that distance in the direction opposite to the Sun's path through space. At its windward side, its thickness is estimated to be between 10 and 100 AU. Voyager project scientists have determined that the heliosheath is not "smooth" – it is rather a "foamy zone" filled with magnetic bubbles, each about 1 AU wide. These magnetic bubbles are created by the impact of the solar wind and the interstellar medium. Voyager 1 and Voyager 2 began detecting evidence of the bubbles in 2007 and 2008, respectively. The probably sausage-shaped bubbles are formed by magnetic reconnection between oppositely oriented sectors of the solar magnetic field as the solar wind slows down. They probably represent self-contained structures that have detached from the interplanetary magnetic field. At a distance of about 113 AU, Voyager 1 detected a 'stagnation region' within the heliosheath. In this region, the solar wind slowed to zero, the magnetic field intensity doubled and high-energy electrons from the galaxy increased 100-fold. At about 122 AU, the spacecraft entered a new region that Voyager project scientists called the "magnetic highway", an area still under the influence of the Sun but with some dramatic differences.