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- Published: 04 Aug 2009
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- Author: NASAexplorer
The heliosphere is a bubble in space "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself. It was thought for decades that it extends in a long comet-like heliotail, but in 2009 data from the Cassini and IBEX show a different shape. As it begins to drop out with the interstellar medium, it slows down before finally ceasing altogether. The point where the solar wind slows down is the termination shock; then there is the heliosheath area; then the point where the interstellar medium and solar wind pressures balance is called the heliopause; the point where the interstellar medium, traveling in the opposite direction, slows down as it collides with the heliosphere is the bow shock.
As of June 2011, the heliosheath area is thought to be filled magnetic bubbles (each about 1 AU wide), creating a "foamy zone". The theory helps explain in situ heliosphere measurements by the two Voyager probes.
The solar wind consists of particles (ionized atoms from the solar corona) and fields (in particular, magnetic fields). As the Sun rotates once in approximately 27 days, the magnetic field transported by the solar wind gets wrapped into a spiral. Variations in the Sun's magnetic field are carried outward by the solar wind and can produce magnetic storms in the Earth's own magnetosphere.
In March 2005, it was reported that measurements by the Solar Wind Anisotropies (SWAN) instrument onboard the Solar and Heliospheric Observatory (SOHO) have shown that the heliosphere, the solar wind-filled volume which prevents the solar system from becoming embedded in the local (ambient) interstellar medium, is not axisymmetrical, but is distorted, very likely under the effect of the local galactic magnetic field.
The heliospheric current sheet is a ripple in the heliosphere created by the Sun's rotating magnetic field. Extending throughout the heliosphere, it is considered the largest structure in the Solar System and is said to resemble a "ballerina's skirt".
The shock arises because solar wind particles are emitted from stars 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 constant pressure associated with it; the pressure from the solar wind decreases with the square of the distance from the star. As one moves far enough away from the star, the pressure from the interstellar medium becomes sufficient to slow the solar wind down to below its speed of sound; this causes a shock wave.
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).
Going outward from the Sun, the termination shock is followed by the heliopause where solar wind particles are stopped by the interstellar medium, then the bow shock past which particles from the interstellar medium are no longer excited.
Evidence presented at a meeting of the American Geophysical Union in May 2005 by Dr. Ed Stone suggests that the Voyager 1 spacecraft passed 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.
The Interstellar Boundary Explorer (IBEX) mission gathered more data on the solar system's termination shock.
The current mission of the Voyager 1 and Voyager 2 space probes includes studying the heliosheath. In late 2010, Voyager 1 reached a region of the heliosheath where the solar wind's radial velocity is zero (i.e., it is flowing sideways relative to the Sun). In 2011, astronomers announced that the Voyagers had determined that the heliosheath is not smooth, but is filled with 100 million-mile-wide bubbles created by the impact of the solar wind and the interstellar medium. Voyager 1 and 2 began detecting evidence for the bubbles in 2007 and 2008, respectively. there exists a region of hot hydrogen known as the hydrogen wall between the bow shock and the heliopause. The wall is composed of interstellar material interacting with the edge of the heliosphere.
Another hypothesis suggests that the heliopause could be smaller on the side of the solar system facing the Sun's orbital motion through the galaxy. It may also vary depending on the current velocity of the solar wind and the local density of the interstellar medium. It is known to lie far outside the orbit of Neptune. The current mission of the Voyager 1 and 2 spacecraft is to find and study the termination shock, heliosheath, and heliopause.Voyager 1 reached the termination shock on May 23–24, 2005, and Voyager 2 reached it on August 30, 2007 according to NASA. Meanwhile, the Interstellar Boundary Explorer (IBEX) mission is attempting to image the heliopause from Earth orbit within two years of its 2008 launch. Initial results (October 2009) from IBEX suggest that previous assumptions are insufficiently cognisant of the true complexities of the heliopause.
When particles emitted by the sun bump into the interstellar ones, they slow down while releasing energy. Many particles accumulate in and around the heliopause, highly energised by their negative acceleration, creating a shock wave.
An alternative definition is that the heliopause is the magnetopause between the solar system's magnetosphere and the galaxy's plasma currents.
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For a video of the revised no-tail model see here. The new shape from the data is thought more like a spherical bubble, than a cometary shape. Initial interpretations suggest that "the interstellar environment has far more influence on structuring the heliosphere than anyone previously believed" "No one knows what is creating the ENA (energetic neutral atoms) ribbon, but everyone agrees that it means the textbook picture of the heliosphere—in which the solar system's enveloping pocket filled with the solar wind's charged particles is plowing through the onrushing "galactic wind" of the interstellar medium in the shape of a comet—is wrong."
"The IBEX results are truly remarkable! What we are seeing in these maps does not match with any of the previous theoretical models of this region. It will be exciting for scientists to review these (ENA) maps and revise the way we understand our heliosphere and how it interacts with the galaxy."
In October 2010, significant changes were detected in the ribbon after 6 months, based on the second set of IBEX observations.
Category:Space plasmas Category:Plasma physics Category:Sun Category:Voyager program Category:Trans-Neptunian region
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