The heliosphere is a region around the Sun that includes its magnetic field, outer atmosphere, and other layers. It forms a large, bubble-shaped area with a tail. In terms of plasma physics, it is a space created by the Sun in the area between stars. The "bubble" of the heliosphere is kept inflated by plasma from the Sun, called the solar wind. Beyond the heliosphere, this solar plasma is replaced by plasma from other parts of the galaxy. As part of the interplanetary magnetic field, the heliosphere protects the Solar System from most harmful cosmic radiation. However, gamma rays are not affected. The name "heliosphere" was probably named by Alexander J. Dessler, who first used it in scientific writing in 1967. The study of the heliosphere is called heliophysics, and it includes studying space weather and space climate.

The solar wind travels through the Solar System for billions of kilometers until it reaches the "termination shock," where its movement slows due to pressure from space between stars. The "heliosheath" is a wide area between the termination shock and the outer edge of the heliosphere, called the "heliopause." The shape of the heliosphere looks like a comet, being roughly spherical on one side, about 100 astronomical units (AU) in size, and tail-shaped on the other side, called the "heliotail," which extends for thousands of AU.

Two spacecraft from the Voyager program studied the outer parts of the heliosphere, passing through the termination shock and the heliosheath. Voyager 1 reached the heliopause on August 25, 2012, when it measured a sudden increase in plasma density by 40 times. Voyager 2 reached the heliopause on November 5, 2018. Since the heliopause is where material from the Sun meets material from the galaxy, spacecraft that leave the heliosphere, like the two Voyagers, are now in interstellar space.

History

The heliosphere is influenced by events outside our solar system, such as nearby supernovas or moving through spaces between stars with different densities. Over time, the heliosphere has changed significantly. Evidence shows that the heliosphere was once smaller, reaching only the Inner Solar System as recently as 3 million years ago. This change was caused by a nearby supernova, which exposed Earth to material from space. This exposure may have affected Earth's climate and ecosystems.

Structure

The heliosphere is not shaped like a perfect sphere, despite its name. Its shape depends on three main factors: the interstellar medium (ISM), the solar wind, and the movement of the Sun and heliosphere through the ISM. Both the solar wind and the ISM are fluid, meaning the heliosphere's shape and size can change over time. Short-term changes in the solar wind, such as those lasting hours to a few years, have a greater effect on the boundaries of the heliosphere than changes in the ISM. The solar wind's pressure changes much more quickly than the pressure from the ISM. The 11-year solar cycle, which includes periods of high and low solar wind activity, has a major influence on these changes.

On a larger scale, the movement of the heliosphere through the ISM creates a shape similar to a comet. Solar wind plasma moving in the same direction as the Sun's motion through the galaxy is compressed into a nearly spherical shape. Plasma moving in the opposite direction stretches out farther, forming the long, trailing edge of the heliosphere, called the heliotail.

Because there is limited data and much of this structure is still unexplored, scientists have proposed many theories about its shape. In 2020, Merav Opher and her team found that the heliosphere is shaped like a crescent, similar to a deflated croissant.

The solar wind is made of particles, such as charged atoms from the Sun's corona, and magnetic fields produced by the Sun. These move outward into space. Since the Sun rotates once every 25 days, the magnetic field carried by the solar wind forms a spiral shape. The solar wind affects systems in the Solar System, such as Earth's magnetosphere, by carrying changes in the Sun's magnetic field outward and causing geomagnetic storms.

The heliospheric current sheet is a wave-like structure in the heliosphere caused by the Sun's rotating magnetic field. It separates areas of the heliosphere with opposite magnetic directions. This sheet extends throughout the heliosphere and is the largest structure in the Solar System. It is often compared to the shape of a "ballerina's skirt."

Edge structure

The outer part of the heliosphere is shaped by how the solar wind interacts with space between stars. The solar wind moves away from the Sun in all directions at speeds of several hundred kilometers per second near Earth. At a distance far beyond Neptune’s orbit, this fast-moving wind slows down when it meets gases in space between stars. This happens in stages:

• The solar wind moves at supersonic speeds inside the Solar System. At the termination shock, a shock wave, the wind slows to below the speed of sound and becomes subsonic.
• Scientists once thought that after slowing, the solar wind would form a blunt shape on one side and a long, comet-like tail behind it, called the heliosheath. However, observations in 2009 showed this model was incorrect. By 2011, it was believed the heliosheath is filled with a magnetic bubble "foam."
• The outer edge of the heliosheath, where the heliosphere meets space between stars, is called the heliopause. This is the edge of the entire heliosphere. Observations in 2009 changed how scientists understand this boundary.
• In theory, the heliopause causes turbulence in space between stars as the Sun moves around the center of the galaxy. This turbulence happens because the pressure of the heliopause pushes against space between stars. However, the solar wind’s speed relative to space between stars may be too slow to create a bow shock.

The termination shock is the point in the heliosphere where the solar wind slows to subsonic speed because of interactions with space between stars. This causes compression, heating, and changes to 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 happens because solar wind particles move at about 400 km/s, while the speed of sound in space between stars is about 100 km/s. The exact speed depends on the density, which changes often. Space between stars has very low density, but it has a steady pressure. The pressure from the solar wind decreases with the square of the distance from the Sun. When the solar wind’s pressure drops to where it can no longer move faster than the speed of sound in space between stars, it slows, creating a shock wave. Further from the Sun, the termination shock is followed by the heliopause, where the pressures of the solar wind and space between stars balance, and solar wind particles stop.

Other examples of termination shocks can be seen in everyday life. For instance, running a water tap into a sink creates a hydraulic jump. When water hits the sink’s bottom, it spreads out faster than the speed of waves in the water, forming a disk of shallow, fast-moving flow (like the solar wind). Around the disk’s edge, a wall of water forms; outside this wall, the water moves slower than the wave speed (like the subsonic space between stars).

Evidence from a 2005 meeting of the American Geophysical Union showed that Voyager 1 passed the termination shock in December 2004, when it was about 94 astronomical units from the Sun, based on changes in magnetic readings. Voyager 2 detected returning particles in May 2006, when it was 76 astronomical units from the Sun. This suggests the heliosphere may be irregularly shaped, bulging in the Sun’s northern hemisphere and pushed inward in the south.

The heliosheath is the region of the heliosphere beyond the termination shock. Here, the solar wind slows, compresses, and becomes turbulent as it interacts with space between stars. The inner edge of the heliosheath is about 80 to 100 astronomical units from the Sun. A proposed model suggests the heliosheath is shaped like a comet’s coma and extends several times that distance in the direction opposite to the Sun’s movement. The thickness of the heliosheath on its front side is estimated to be between 10 and 100 astronomical units. Scientists have found the heliosheath is not smooth but a "foamy zone" filled with magnetic bubbles, each about 1 astronomical unit wide. These bubbles form when the solar wind interacts with space between stars. Voyager 1 and Voyager 2 began detecting these bubbles in 2007 and 2008, respectively. The bubbles are created by magnetic reconnection between opposite parts of the Sun’s magnetic field as the solar wind slows. They are likely self-contained structures that have separated from the interplanetary magnetic field.

At about 113 astronomical units, Voyager 1 detected a "stagnation region" in the heliosheath. In this area, the solar wind stopped completely, the magnetic field strength doubled, and high-energy electrons from the galaxy increased 100 times. At about 122 astronomical units, the spacecraft entered a new region called the "magnetic highway," still under the Sun’s influence but with major differences.

The heliopause is the boundary where the Sun’s solar wind is stopped by space between stars. This is where the pressure of the solar wind balances the pressure from surrounding stars. Crossing the heliopause should show a sharp drop in the temperature of solar wind-charged particles, a change in the magnetic field’s direction, and an increase in galactic cosmic rays.

In May 2012, Voyager 1 detected a rapid increase in cosmic rays (a 9% rise in one month, following a slower increase of 25% from January 2009 to January 2012), suggesting it was near the heliopause. Between late August and early September 2012, Voyager 1 saw a sharp drop in protons from the Sun, from 25 particles per second to about 2 by early October. In September 2013, NASA announced Voyager 1 had crossed the heliopause on August 25, 2012, at 121 astronomical units from the Sun. Contrary to predictions, data showed the galaxy’s magnetic field is aligned with the Sun’s magnetic field.

On November 5, 2018, Voyager 2 detected a sudden drop in low-energy ions and an increase in cosmic rays, showing it had crossed the heliopause at 119 astronomical units from the Sun. Unlike Voyager 1, Voyager 2 did not detect interstellar flux tubes while passing through the heliosheath.

NASA collected data about the heliopause remotely during the

Outside structures

The heliopause is the last known boundary between the heliosphere and the space beyond it, which contains material, especially plasma, not from the Sun but from other stars. Outside the heliosphere, there is a transitional area, as discovered by the Voyager 1 spacecraft. In 2004, scientists found evidence of pressure from interstellar space, and some material from the Sun has been observed entering the interstellar medium. The heliosphere is believed to be located inside the Local Interstellar Cloud, which is part of the Local Bubble in the Orion Arm of the Milky Way Galaxy.

Beyond the heliosphere, the density of plasma increases by about 40 times. There is also a large drop in the number of certain solar particles detected, and a significant rise in galactic cosmic rays.

Scientists have measured the movement of the interstellar medium into the heliosphere using at least 11 spacecraft by 2013. By 2013, it was believed that the direction of this movement had changed over time. From Earth’s perspective, the flow of the interstellar medium comes from the constellation Scorpius, and its direction has likely shifted by several degrees since the 1970s.

A structure called the "hydrogen wall" may exist between the bow shock and the heliopause. This wall is made of interstellar material interacting with the edge of the heliosphere. A study published in 2013 explored the idea of a bow wave and hydrogen wall.

Another theory suggests the heliopause might be smaller on the side of the Solar System facing the Sun’s movement through the galaxy. Its size could also depend on the speed of the solar wind and the density of the interstellar medium. The heliopause is known to lie far beyond Neptune’s orbit. The Voyager 1 and 2 spacecraft are designed to study the termination shock, heliosheath, and heliopause. The IBEX mission aims to image the heliopause from Earth’s orbit, and early results from 2009 showed that previous ideas about the heliopause were incomplete.

In August 2018, the New Horizons spacecraft confirmed findings first detected in 1992 by the Voyager spacecraft about the hydrogen wall. Although the hydrogen is detected through extra ultraviolet light (which may come from another source), New Horizons’ measurements supported Voyager’s earlier discoveries with greater accuracy.

It was long believed that the Sun creates a "shock wave" as it moves through the interstellar medium. This would happen if the interstellar medium moves faster than sound toward the Sun, as the solar wind moves away from the Sun at supersonic speeds. When the interstellar wind hits the heliosphere, it slows down and creates turbulence. Scientists once thought a bow shock might form around 230 AU, but in 2012, new measurements showed it likely does not exist. Earlier studies by the Ulysses spacecraft measured the speed of the local interstellar medium relative to the Sun at 26.3 km/s, while IBEX measured it at 23.2 km/s.

This phenomenon has been observed around other stars outside the Solar System. NASA’s retired GALEX telescope found that the red giant star Mira in the constellation Cetus has both a tail of material ejected from the star and a distinct shock wave as it moves through space at over 130 kilometers per second.

Observational methods

The exact distance to and shape of the heliopause are still unknown. Spacecraft that travel between planets and stars, such as Pioneer 10, Pioneer 11, and New Horizons, are moving outward through the Solar System. These spacecraft will eventually cross the heliopause. Scientists have lost contact with Pioneer 10 and 11.

The heliosphere is not shaped like a comet, as earlier believed. Data from the Cassini spacecraft’s Ion and Neutral Camera (MIMI/INCA) suggest the heliosphere is more bubble-shaped. Instead of being shaped mainly by collisions between the solar wind and the interstellar medium, the interaction seems to be influenced more by the pressure of particles and the energy from magnetic fields.

The Interstellar Boundary Explorer (IBEX), launched in October 2008, found a narrow, bright feature in space called the IBEX ribbon. This discovery showed that the interstellar environment has a greater influence on the shape of the heliosphere than previously thought. Scientists do not yet know what causes the energetic neutral atoms (ENA) that form the ribbon.

The IBEX results surprised scientists because the maps of ENA do not match earlier models of the heliosphere. These findings will help scientists update their understanding of how the heliosphere interacts with the galaxy. In October 2010, changes in the IBEX ribbon were observed after six months of study. IBEX data did not support the existence of a bow shock, but one study suggested there might be a bow wave.

Missions that study the heliosphere include:

• Solar Anomalous and Magnetospheric Particle Explorer
• Solar and Heliospheric Observatory
• Solar Dynamics Observatory
• STEREO
• Ulysses spacecraft
• Parker Solar Probe
• Solar Orbiter

During a total solar eclipse, the high-temperature corona of the Sun is easier to observe from Earth-based observatories. During the Apollo program, the solar wind was measured on the Moon using the Solar Wind Composition Experiment. Examples of Earth-based solar observatories include the McMath–Pierce solar telescope, the GREGOR Solar Telescope, and the refurbished Big Bear Solar Observatory.

Exploration history

The heliosphere is the area influenced by the Sun. Its edge is shaped by two main factors: the Sun's magnetic field and the solar wind, which is a stream of charged particles from the Sun. The heliosphere has three main sections: the termination shock, the heliosheath, and the heliopause. Five spacecraft have studied the farthest parts of the heliosphere. These include Pioneer 10 (1972–1997; data to 67 AU), Pioneer 11 (1973–1995; 44 AU), Voyager 1 and Voyager 2 (launched in 1977, still active), and New Horizons (launched in 2006). Scientists have also observed energetic neutral atoms (ENAs) produced near the edges of the heliosphere.

Most of the heliosphere is filled with material from the Sun, except near obstacles like planets or comets. Cosmic rays, fast-moving neutral atoms, and cosmic dust can enter the heliosphere from space. Solar wind particles, which originate from the Sun's corona, escape at speeds of 300 to 800 km/s (671,000 to 1,790,000 mph). As the solar wind interacts with space beyond the solar system, its speed decreases. The point where the solar wind slows to the speed of sound is called the termination shock. The solar wind continues to slow as it moves through the heliosheath until it reaches the heliopause, where the pressure of the solar wind and the interstellar medium balance. Voyager 1 crossed the termination shock in 2004, and Voyager 2 crossed it in 2007.

Scientists once thought a bow shock existed beyond the heliopause, but data from the Interstellar Boundary Explorer suggested the Sun's speed through space is too slow for a bow shock to form. Instead, it may be a gentler "bow wave." Voyager data led to a new theory that the heliosheath contains "magnetic bubbles" and a stagnation zone. In 2010, Voyager 1 detected a "stagnation region" in the heliosheath, where the solar wind stops, the magnetic field strength doubles, and high-energy electrons from the galaxy increase 100 times.

In May 2012, Voyager 1 detected a sudden rise in cosmic rays, suggesting it was approaching the heliopause. In 2013, NASA announced that Voyager 1 had entered interstellar space on August 25, 2012. In December 2012, NASA reported that Voyager 1 had entered a new region called the "magnetic highway," where the Sun's influence still exists but with significant differences.

Pioneer 10 was launched in March 1972 and passed the Moon within 10 hours. Over 35 years, it made many discoveries about the heliosphere and Jupiter's magnetosphere. It was the first spacecraft to detect sodium and aluminum ions in the solar wind and helium in the inner solar system. Pioneer 10 passed through Jupiter's magnetosphere 17 times, studying its interaction with the solar wind. It stopped sending data in 1997 but was contacted again in 2003 at 82 AU from Earth.

Voyager 1 surpassed Pioneer 10's distance from the Sun in 1998, reaching 69.4 AU. Voyager 2 became the second farthest human-made object from the Sun in 2023. Pioneer 11, launched in 1973, collected similar data to Pioneer 10 up to 44.7 AU in 1995. Both Pioneer and Voyager spacecraft traveled different paths, providing data on the heliosphere from different directions. Their data helped confirm the presence of a hydrogen wall.

In 2012, Voyager 1 was thought to have crossed the heliopause, and Voyager 2 did so in 2018. The twin Voyagers are the only human-made objects to enter interstellar space. However, they have not yet left the Solar System, which is considered to end at the Oort Cloud. When Voyager 2 crossed the heliopause, its Plasma Science Experiment detected a sharp drop in solar wind speed, and other instruments confirmed the transition. These observations support data from NASA's IBEX mission. In 2025, NASA launched the Interstellar Mapping and Acceleration Probe (IMAP) to build on Voyager's findings.

Timeline of exploration and detection

• 1904: Astronomers used the Potsdam Great Refractor with a spectrograph to study the binary star Mintaka in Orion. They found evidence of the interstellar medium, which is the material between stars.
• 1958: Eugene Parker wrote a paper that predicted the existence of solar wind. At first, scientists did not believe his theory.
• January 1959: The spacecraft Luna 1 became the first to observe solar wind.
• 1962: The spacecraft Mariner 2 detected solar wind.
• 1972–1973: Pioneer 10 became the first spacecraft to explore the heliosphere beyond Mars. It flew by Jupiter on December 4, 1973, and continued to send solar wind data as far as 67 AU from the Sun.
• February 1992: After passing Jupiter, the Ulysses spacecraft became the first to study the mid and high latitudes of the heliosphere.
• 1992: The Pioneer and Voyager probes detected Ly-α radiation scattered by hydrogen in the heliosphere.
• 2004: Voyager 1 became the first spacecraft to reach the termination shock, a boundary where the solar wind slows down.
• 2005: Observations from SOHO showed that the heliosphere is not perfectly symmetrical. It is likely distorted by the local galactic magnetic field.
• 2009: Scientists from the IBEX project discovered and mapped a ribbon-shaped area with high levels of energetic neutral atoms. These atoms are believed to come from the heliopause, the edge of the heliosphere.
• October 2009: Studies suggested the heliosphere may be shaped like a bubble, not a comet.
• October 2010: IBEX observations showed changes in the ribbon after six months.
• May 2012: IBEX data suggested there is probably no bow "shock" at the edge of the heliosphere.
• June 2012: Voyager 1 detected an increase in cosmic rays at a distance of 119 AU from the Sun.
• August 25, 2012: Voyager 1 crossed the heliopause, becoming the first human-made object to leave the heliosphere.
• August 2018: Long-term studies by the New Horizons spacecraft confirmed findings first observed in 1992 by the Voyager probes about the hydrogen wall.
• November 5, 2018: Voyager 2 crossed the heliopause, leaving the heliosphere.