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 space can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself.
For the first ten billion kilometres of its radius, the solar wind travels at over a million kilometres per hour. As it begins to collide with the interstellar medium, it slows down before finally ceasing altogether. The point where the solar wind slows down is the termination shock; the point where the interstellar medium and solar wind pressures balance is called the heliopause; the point where the interstellar medium, travelling in the opposite direction, slows down as it collides with the heliosphere is the bow shock.
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 heliosphere's outer structure is determined by the interactions between the solar wind and the winds of interstellar space. The solar wind streams away from the Sun in all directions at speeds of several hundred km/s (about 1,000,000 mph) in the Earth's vicinity. At some distance from the Sun, well beyond the orbit of Neptune, this supersonic wind must slow down to meet the gases in the interstellar medium. This takes place in several stages:
The termination shock is the point in the heliosphere where the solar wind slows down to subsonic speed (with respect to the star) due to interactions with the local interstellar medium. This causes compression, heating, and a change in the magnetic field. In our solar system the termination shock is believed to be 75 to 90 astronomical units from the Sun. In 2007, Voyager II passed through the sun's termination shock. Voyager II actually passed through the termination shock five times because the shock boundary fluctuates in its distance from the sun as a result of fluctuations in solar flare activity; i.e., changes in the ejections of gas and dust from the sun.
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 will attempt to gather more data on the solar system's termination shock.
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. Its distance from the Sun is approximately 80 to 100 astronomical units (AU) at its closest point; however, 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. The current mission of the Voyager 1 and Voyager 2 space probes includes studying the heliosheath.
The heliopause is the theoretical boundary where the Sun's solar wind is stopped by the interstellar medium; where the solar wind's strength is no longer great enough to push back the stellar winds of the surrounding stars.
According to one hypothesis, 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. It is anticipated that both missions may ultimately reach the heliopause itself. In the mean time, the Interstellar Boundary Explorer (IBEX) mission will attempt 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 cogniscent 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.
It is hypothesized that the Sun also has a bow shock produced in its travels within the interstellar medium, as shown in the figure. The shock is named from its resemblance to the wake left by a ship's bow and is formed for similar reasons, though of plasma instead of water. Bow shocks will occur if the interstellar medium is moving supersonically "toward" the sun, since its solar wind moves "away" from the sun supersonically. When the interstellar wind hits the heliosphere it slows and creates a region of turbulence. NASA's Robert Nemiroff and Jerry Bonnell believe the solar bow shock may lie at around 230 AU from the Sun.
This phenomenon has been observed by NASA's orbital GALEX telescope. The red giant star Mira in the constellation Cetus has been shown to have both a cometlike debris tail of ejecta from the star, and a distinct bowshock preceding it in the direction of its movement through space (at over 130 kilometers per second).
The precise distance to, and shape of the heliopause is still uncertain. Interplanetary/interstellar spacecraft such as Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 are traveling outward through the solar system and will eventually pass through the heliopause.
Initial data from Interstellar Boundary Explorer (IBEX), launched in October 2008, revealed a previously unpredicted "very narrow ribbon that is two to three times brighter than anything else in the sky." 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."
Rather than a comet-like shape, the heliosphere appears to be bubble-shaped according to data from Cassini's Ion and Neutral Camera (MIMI / INCA). Rather than being dominated by the collisions between the solar wind and the interstellar medium, the INCA (ENA) maps suggest that the interaction is controlled more by particle pressure and magnetic field energy density.