The heliosphere is the magnetosphere, astrosphere, and outermost atmospheric layer of the Sun. It takes the shape of a vast, tailed bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstellar medium. The "bubble" of the heliosphere is continuously "inflated" by plasma originating from the Sun, known as the solar wind. Outside the heliosphere, this solar plasma gives way to the interstellar plasma permeating the Milky Way. As part of the interplanetary magnetic field, the heliosphere shields the Solar System from significant amounts of cosmic ionizing radiation; uncharged gamma rays are, however, not affected. Its name was likely coined by Alexander J. Dessler, who is credited with the first use of the word in the scientific literature in 1967.

Structure

thumb|The Sun photographed at a wavelength of 19.3 nanometers ([[ultraviolet)]]

Despite its name, the heliosphere's shape is not a perfect sphere. that can be described as a deflated croissant.

Solar wind

The solar wind consists of particles (ionized atoms from the solar corona) and fields like the magnetic field that are produced from the Sun and stream out into space. Because the Sun rotates once approximately every 25 days, the heliospheric magnetic field transported by the solar wind gets wrapped into a spiral. The solar wind affects many other systems in the Solar System; for example, variations in the Sun's own magnetic field are carried outward by the solar wind, producing geomagnetic storms in the Earth's magnetosphere.

thumb|The heliospheric current sheet out to the orbit of Jupiter

Heliospheric current sheet

The heliospheric current sheet is a ripple in the heliosphere created by the rotating magnetic field of the Sun. It marks the boundary between heliospheric magnetic field regions of opposite polarity. Extending throughout the heliosphere, the heliospheric current sheet could be considered the largest structure in the Solar System and is said to resemble a "ballerina's skirt".

Edge structure

The outer structure of the heliosphere 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 in the Earth's vicinity. At some distance from the Sun, well beyond the orbit of Neptune, this supersonic wind slows down as it encounters the gases in the interstellar medium. This takes place in several stages:

  • 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.

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.

thumb|Illustration of the heliosphere as released on 28 June 2013 which incorporates results from the Voyager spacecraft. The heliosheath is between the termination shock and the heliopause.

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. 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.

Heliopause

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. This is the boundary where the interstellar medium and solar wind pressures balance. The crossing of the heliopause should be signaled by a sharp drop in the temperature of solar wind-charged particles, Contrary to predictions, data from Voyager 1 indicates the magnetic field of the galaxy is aligned with the solar magnetic field.

On November 5, 2018, the Voyager 2 mission detected a sudden decrease in the flux of low-energy ions. At the same time, the level of cosmic rays increased. This demonstrated that the spacecraft crossed the heliopause at a distance of from the Sun. Unlike Voyager 1, the Voyager 2 spacecraft did not detect interstellar flux tubes while crossing the heliosheath.

NASA also collected data from the heliopause remotely during the suborbital SHIELDS mission in 2021.

Heliotail

The heliotail is the several thousand astronomical units long tail of the heliosphere,

The shape of the heliotail (as found by NASA's Interstellar Boundary Explorer – IBEX) is that of a four-leaf clover. The particles in the tail do not shine, therefore it cannot be seen with conventional optical instruments. IBEX made the first observations of the heliotail by measuring the energy of "energetic neutral atoms", neutral particles created by collisions in the Solar System's boundary zone.

thumb|The bubble-like heliosphere moving through the interstellar medium.

thumb|[[Energetic neutral atom (ENA) detection is more concentrated in one direction.]]

Outside structures

The heliopause is the final known boundary between the heliosphere and the interstellar space that is filled with material, especially plasma, not from the Earth's own star, the Sun, but from other stars. Even so, just outside the heliosphere (i.e. the "solar bubble") there is a transitional region, as detected by Voyager 1. Just as some interstellar pressure was detected as early as 2004, some of the Sun's material seeps into the interstellar medium.

The flow of the interstellar medium (ISM) into the heliosphere has been measured by at least 11 different spacecraft as of 2013. By 2013, it was suspected that the direction of the flow had changed over time. The wall is composed of interstellar material interacting with the edge of the heliosphere. One paper released in 2013 studied the concept of a bow wave and hydrogen wall.

In August 2018, long-term studies about the hydrogen wall by the New Horizons spacecraft confirmed results first detected in 1992 by the two Voyager spacecraft. Although the hydrogen is detected by extra ultraviolet light (which may come from another source), the detection by New Horizons corroborates the earlier detections by Voyager at a much higher level of sensitivity.

Bow shock

It was long hypothesized that the Sun produces a "shock wave" in its travels within the interstellar medium. It would 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. A bow shock was thought to possibly occur at about 230 AU,

This phenomenon has been observed outside the Solar System, around stars other than the Sun, by NASA's now retired orbital GALEX telescope. The red giant star Mira in the constellation Cetus has been shown to have both a debris tail of ejecta from the star and a distinct shock in the direction of its movement through space (at over 130 kilometers per second).

Observational methods

Detection by spacecraft

The precise distance to and shape of the heliopause are still uncertain. Interplanetary/interstellar spacecraft such as Pioneer 10, Pioneer 11 and New Horizons are traveling outward through the Solar System and will eventually pass through the heliopause. Contact to Pioneer 10 and 11 has been lost.

Cassini results

Rather than a comet-like shape, the heliosphere appears to be bubble-shaped according to data from Cassinis 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. revealed a previously unpredicted "very narrow ribbon that is two to three times brighter than anything else in the sky", now known as the IBEX ribbon.

"No one knows what is creating the ENA (energetic neutral atoms) ribbon, ..." In October 2010, significant changes were detected in the ribbon after 6 months, based on the second set of IBEX observations. IBEX data did not support the existence of a bow shock, As it begins to interact with the interstellar medium, its velocity slows to a stop. The point where the solar wind becomes slower than the speed of sound is called the termination shock; the solar wind continues to slow as it passes through the heliosheath leading to a boundary called the heliopause, where the interstellar medium and solar wind pressures balance. The termination shock was traversed by Voyager 1 in 2004,

Voyager data led to a new theory that the heliosheath has "magnetic bubbles" and a stagnation zone. Also, there were reports of a "stagnation region" within the heliosheath, starting around , detected by Voyager 1 in 2010. In the summer of 2013, NASA announced that Voyager 1 had reached interstellar space as of 25 August 2012.

In December 2012, NASA announced that in late August 2012, Voyager 1, at about from the Sun, entered a new region they called the "magnetic highway", an area still under the influence of the Sun but with some dramatic differences. Pioneer 10 was the first spacecraft to detect sodium and aluminum ions in the solar wind, as well as helium in the inner Solar System. It was also contacted in 2003 when it was a distance of 7.6 billion miles from Earth (82 AU), but no instrument data about the solar wind was returned then.

Voyager 1 surpassed the radial distance from the Sun of Pioneer 10 at 69.4 AU on 17 February 1998, because it was traveling faster, gaining about 1.02 AU per year. On July 18, 2023, Voyager 2 overtook Pioneer 10 as the second most distant human-made object from the Sun. Pioneer 11, launched a year after Pioneer 10, took similar data as Pioneer out to 44.7 AU in 1995 when that mission was concluded.

In 2012 Voyager 1 is thought to have passed through heliopause, and Voyager 2 did the same in 2018.

The twin Voyagers are the only man-made objects to have entered interstellar space. However, while they have left the heliosphere, they have not yet left the boundary of the Solar System which is considered to be the outer edge of the Oort Cloud. The observations complement data from NASA's IBEX mission. In 2025, NASA launched Interstellar Mapping and Acceleration Probe (IMAP) to capitalize on Voyager observations.

  • 1958: Eugene Parker published a paper that predicted solar wind; his theory was initially rejected by scientific community.
  • January 1959: Luna 1 becomes the first spacecraft to observe the solar wind.
  • 1962: Mariner 2 detects the solar wind.
  • 1972–1973: Pioneer 10 becomes the first spacecraft to explore the heliosphere past Mars, flying by Jupiter on 4 December 1973 and continuing to return solar wind data out to a distance of 67 AU.

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Sources

Further reading

  • Voyager Interstellar Mission Objectives
  • NASA GALEX (Galaxy evolution Explorer) homepage at Caltech
  • A Big Surprise from the Edge of the Solar System (NASA 06.09.11)