The heliosphere is the Sun's magnetosphere, astrosphere, and outermost atmospheric layer. It forms a large, bubble-like region of space with a tail. In space science terms, it is the empty area created by the Sun in the space between stars. The "bubble" of the heliosphere is constantly expanded by plasma from the Sun, called the solar wind. Beyond the heliosphere, this solar plasma is replaced by plasma found throughout the Milky Way. As part of the interplanetary magnetic field, the heliosphere protects the Solar System from much of the harmful radiation from space. However, uncharged gamma rays are not blocked. The term "heliosphere" was likely first used by Alexander J. Dessler in a scientific paper in 1967. The study of the heliosphere is called heliophysics, which includes research on space weather and space climate.

The solar wind moves freely through the Solar System for billions of kilometers, extending far past Pluto until it reaches the "termination shock," where its movement slows suddenly because of pressure from the 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 is similar to a comet, being nearly round on one side, about 100 astronomical units (AU) wide, and tail-shaped on the other side, known as the "heliotail," which stretches for thousands of AU.

Two spacecraft from the Voyager program explored 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 forty-times increase in plasma density. Voyager 2 crossed the heliopause on November 5, 2018. The heliopause marks the boundary between material from the Sun and material from the rest of 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 space filled with different amounts of material. Over time, the heliosphere has changed a lot. Evidence shows that about 3 million years ago, a nearby supernova made the heliosphere smaller, reaching only the inner part of the solar system. This allowed material from space to reach Earth, which might have affected Earth's climate and ecosystems.

Structure

The heliosphere is not shaped like a perfect sphere, even though its name suggests it might be. Its shape depends on three main factors: the interstellar medium (ISM), the solar wind, and the movement of the Sun and heliosphere as they travel through the ISM. Both the solar wind and the ISM are fluid, so the heliosphere’s shape and size can change. However, changes in the solar wind have a stronger effect on the position of the boundaries over short timescales, such as hours to a few years. The solar wind’s pressure changes much faster than the pressure of the ISM at any given location. The 11-year solar cycle, which causes the solar wind to become more or less active, is believed to have 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. The solar wind plasma moving in the same direction as the Sun’s motion through the galaxy is compressed into a nearly spherical shape. In contrast, the plasma moving in the opposite direction stretches out into a long, trailing shape called the heliotail.

Because there is limited data about these structures, scientists have proposed many different theories about their shapes. In 2020, Merav Opher and her team of researchers concluded that the heliosphere is shaped like a crescent, similar to a deflated croissant.

The solar wind is made up of particles, such as charged atoms from the Sun’s outer layer, and magnetic fields that travel outward from the Sun. Since the Sun rotates once every about 25 days, the magnetic field carried by the solar wind becomes twisted into a spiral shape. The solar wind affects many systems in the Solar System, such as Earth’s magnetosphere, where changes in the Sun’s magnetic field can cause geomagnetic storms.

The heliospheric current sheet is a wavy structure in the heliosphere created by the Sun’s rotating magnetic field. It separates regions of the heliospheric magnetic field with opposite directions. This structure spans the entire heliosphere and is the largest known structure in the Solar System. It is often described as resembling a "ballerina’s skirt."

Edge structure

The outer part of the heliosphere is shaped by the interaction between the solar wind and the winds from space between stars. The solar wind moves away from the Sun in all directions at speeds of several hundred kilometers per second near Earth. Far from the Sun, beyond Neptune’s orbit, this fast-moving wind slows down as it meets gases in the space between stars. This happens in stages:

• The solar wind moves very fast within the Solar System. At a point called the termination shock, the wind slows to below the speed of sound and becomes slower than sound.
• Scientists once thought the slower wind would form a rounded shape on one side and a long tail behind it, called the heliosheath. However, observations in 2009 showed this idea was wrong. By 2011, it was believed the heliosheath is filled with a structure made of magnetic bubbles.
• The outer edge of the heliosheath, where the heliosphere meets space between stars, is called the heliopause. This is the farthest point of the heliosphere. Observations in 2009 changed how scientists understand this boundary.
• The heliopause may cause turbulence in space as the Sun moves around the center of the Milky Way. This turbulence happens because the pressure of the heliopause pushes against space between stars. However, the solar wind’s speed may be too slow to create a shock wave in this area.

The termination shock is the point where the solar wind slows to below the speed of sound due to interactions with space between stars. This causes the wind to compress, heat up, and change the magnetic field. In the Solar System, the termination shock is about 75 to 90 astronomical units from the Sun. In 2004, the Voyager 1 spacecraft passed through the termination shock, followed by Voyager 2 in 2007.

The shock happens because the solar wind moves 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 of space, which changes over time. Even though space between stars has very low density, 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 is too weak to push against the pressure of space between stars, it slows to below the speed of sound, creating a shock wave. Farther from the Sun, the termination shock is followed by the heliopause, where the two pressures balance, and the solar wind stops.

Termination shocks can be seen in everyday situations, such as when water from a tap hits the sink. The water spreads out quickly, forming a shallow, fast-moving disk (like the solar wind). Around the edge of the disk, a wall of water forms, where the water moves slower than the wave speed (like the slower space between stars).

In May 2005, Ed Stone reported that Voyager 1 crossed 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 unevenly shaped, bulging in the Sun’s northern hemisphere and pushed inward in the south.

The heliosheath is the region beyond the termination shock. Here, the solar wind is slowed, compressed, 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. Scientists once thought the heliosheath looked like a comet’s tail, but recent data show it is a "foamy zone" filled with magnetic bubbles, each about 1 astronomical unit wide. These bubbles form when the solar wind and space between stars interact. Voyager 1 and 2 began detecting these bubbles in 2007 and 2008. The bubbles are created when magnetic fields from the Sun connect and separate as the solar wind slows. They are self-contained structures that have separated from the magnetic field in space.

At about 113 astronomical units, Voyager 1 found a region where the solar wind stopped moving, the magnetic field doubled in strength, and high-energy electrons from space increased 100 times. At 122 astronomical units, Voyager 1 entered a region called the "magnetic highway," where the Sun’s influence still exists but with major differences.

The heliopause is the boundary where the solar wind stops due to pressure from space between stars. This is where the pressure of the solar wind balances with the pressure from space between stars. Crossing the heliopause would be shown by a sharp drop in temperature of charged particles from the Sun, a change in the direction of the magnetic field, and an increase in galactic cosmic rays.

In May 2012, Voyager 1 detected a sudden increase in cosmic rays, suggesting it was near the heliopause. Between late August and early September 2012, Voyager 1 recorded a sharp drop in solar protons, from 25 per second to about 2 per second. In September 2013, NASA announced that Voyager 1 had crossed the heliopause on August 25, 2012, at 121 astronomical units from the Sun. Surprisingly, data showed the galaxy’s magnetic field 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, confirming it had crossed the heliopause at 119 astronomical units from the Sun. Unlike Voyager 1, Voyager 2 did not detect magnetic structures in the heliosheath.

NASA also studied the heliopause using the SHIELDS mission in 2021.

The heliotail is the long tail of the heliosphere, stretching thousands of astronomical units. It is similar to a comet’s tail but always points away from the Sun. In this region, the solar wind slows and escapes the heliosphere, gradually fading due to interactions with space between stars. Observations by NASA’s IBEX mission showed the heliotail has a shape like a four-leaf clover. Since the tail’s particles do not emit light, it cannot be seen with regular telescopes. IBEX made the first observations of this structure.

Outside structures

The heliopause is the last known boundary between the heliosphere and the space outside the solar system. This space contains material, especially plasma, from other stars, not from the Sun. Even so, just outside the heliosphere (also called the "solar bubble"), there is a transition area, as found by Voyager 1. In 2004, scientists detected pressure from space outside the solar system. Some material from the Sun also moves into this space. The heliosphere is believed to be inside the Local Interstellar Cloud, which is part of the Local Bubble. This bubble is located in the Orion Arm of the Milky Way Galaxy.

Outside the heliosphere, the density of plasma increases 40 times. There is also a big decrease in the number of certain particles from the Sun, and more cosmic rays from galaxies are detected.

The movement of the interstellar medium into the heliosphere has been measured by at least 11 spacecraft as of 2013. By 2013, scientists thought the direction of this movement had changed over time. From Earth’s perspective, the flow of material comes from the constellation Scorpius. It is likely that this direction has 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 examined the idea of a bow wave and hydrogen wall.

Another idea suggests the heliopause might be smaller on the side of the Solar System facing the Sun’s movement through the galaxy. Its size may also depend on the speed of the solar wind and the density of the interstellar medium. The heliopause lies far beyond Neptune’s orbit. The Voyager 1 and 2 spacecraft are designed to study the termination shock, heliosheath, and heliopause. Meanwhile, the IBEX mission aims to image the heliopause from Earth orbit within two years of its 2008 launch. Early results from IBEX in 2009 showed that earlier ideas about the heliopause may not fully explain its complexity.

In August 2018, long-term studies by the New Horizons spacecraft confirmed findings first detected in 1992 by the Voyager spacecraft. Although the hydrogen is detected using extra ultraviolet light (which may come from another source), New Horizons’ observations support the earlier Voyager results 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 faster than sound. When the interstellar wind meets the heliosphere, it slows down and creates turbulence. A bow shock was thought to occur around 230 AU, but in 2012, scientists concluded it likely does not exist. This decision was based on new measurements: The speed of the local interstellar medium relative to the Sun was previously measured at 26.3 km/s by Ulysses, but IBEX measured it at 23.2 km/s.

This phenomenon has been observed outside the Solar System, around other stars, by NASA’s retired GALEX telescope. The red giant star Mira in the constellation Cetus has been found to have both a tail of material ejected from the star and a clear shock wave in the direction of its movement through space (over 130 kilometers per second).

Observational methods

The exact distance to and shape of the heliopause are not yet known. 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 reach the heliopause. However, scientists have lost contact with Pioneer 10 and 11.

The heliosphere is not shaped like a comet, as earlier believed. Data from the Cassini mission’s Ion and Neutral Camera (MIMI/INCA) suggests the heliosphere is more bubble-shaped. Instead of being shaped mainly by collisions between the solar wind and the space between stars, the INCA (ENA) maps show that the interaction is influenced more by the pressure of particles and the strength of magnetic fields.

Early data from the Interstellar Boundary Explorer (IBEX), launched in October 2008, revealed a narrow, bright area in space that is two to three times brighter than other areas. This area is now called the IBEX ribbon. Scientists believe that the space outside the Solar System has a greater influence on the shape of the heliosphere than previously thought. The cause of the ENA (energetic neutral atoms) ribbon is still unknown.

The IBEX results are very interesting. The maps show patterns that do not match earlier models of the heliosphere. Scientists are excited to study these maps and update their understanding of the heliosphere and its interaction with the galaxy. In October 2010, changes were observed in the IBEX ribbon after six months, based on new IBEX data. The IBEX data did not support the existence of a bow shock, but some studies suggest there may be a "bow wave."

Examples of missions that have collected or continue to collect data about 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 hot 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 newer GREGOR Solar Telescope, and the refurbished Big Bear Solar Observatory.

Exploration history

The heliosphere is the area around the Sun that is influenced by the Sun’s energy. Two main factors help scientists determine the edge of the heliosphere: the Sun’s magnetic field and the solar wind, which is a stream of charged particles that flows outward from the Sun. The heliosphere has three major sections: the termination shock, the heliosheath, and the heliopause. Five spacecraft have collected important data about the heliosphere’s outer regions. 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 particles called energetic neutral atoms (ENAs) coming from the edges of the heliosphere.

Most of the material in the heliosphere comes from the Sun, except near objects like planets or comets. However, cosmic rays, fast-moving neutral atoms, and cosmic dust can enter the heliosphere from space. Solar wind particles begin at the Sun’s corona, a very hot outer layer, and move outward at speeds of 300 to 800 km/s (671,000 to 1.79 million mph). When the solar wind interacts with the space between stars, its speed slows until it stops. The point where the solar wind slows to the speed of sound is called the termination shock. After passing through the heliosheath, the solar wind stops at the heliopause, where the pressure from the solar wind and the pressure from the space between stars balance. Voyager 1 crossed the termination shock in 2004, and Voyager 2 crossed it in 2007.

Scientists once thought there was a bow shock 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 idea that the heliosheath contains "magnetic bubbles" and a stagnation zone. In 2010, Voyager 1 detected a "stagnation region" within the heliosheath, about 113 AU from the Sun. There, the solar wind’s speed drops to zero, the magnetic field becomes twice as strong, and high-energy electrons from the galaxy increase 100 times.

In May 2012, at about 120 AU from the Sun, 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 changes.

Pioneer 10 was launched in March 1972 and passed the Moon within 10 hours. Over the next 35 years, it made many firsts in studying the heliosphere and Jupiter’s effects on it. Pioneer 10 was the first spacecraft to detect sodium and aluminum ions in the solar wind and helium in the inner Solar System. In November 1972, it encountered Jupiter’s large magnetosphere and passed in and out of it 17 times, mapping its interaction with the solar wind. Pioneer 10 collected data until March 1997, including solar wind data up to about 67 AU. It was contacted in 2003 when it was 7.6 billion miles (82 AU) from Earth, but no solar wind data was returned at that time.

Voyager 1 surpassed Pioneer 10’s distance from the Sun on February 17, 1998, because it traveled faster, gaining about 1.02 AU per year. On July 18, 2023, Voyager 2 became the second farthest human-made object from the Sun, overtaking Pioneer 10. Pioneer 11, launched in 1973, collected similar data to Pioneer 10 up to 44.7 AU in 1995. Pioneer 11 had the same instruments as Pioneer 10 but also included a flux-gate magnetometer. The Pioneer and Voyager spacecraft followed different paths, so they studied the heliosphere from different directions. Data from these missions helped confirm the existence of a hydrogen wall.

In 2012, scientists believe Voyager 1 crossed the heliopause, and Voyager 2 did the same in 2018. The twin Voyager spacecraft 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 outer edge of the Oort Cloud. When Voyager 2 crossed the heliopause on November 5, 2018, its Plasma Science Experiment (PLS) recorded a sharp drop in the speed of solar wind particles, and this change has not been seen since. Other instruments on Voyager 2 also measured the transition, and these findings support data from NASA’s IBEX mission. In 2025, NASA launched the Interstellar Mapping and Acceleration Probe (IMAP) to build on Voyager’s discoveries.

Timeline of exploration and detection

• 1904: Astronomers used the Potsdam Great Refractor with a spectrograph to find evidence of the interstellar medium while observing the binary star Mintaka in Orion.
• 1958: Eugene Parker wrote a paper that predicted solar wind; scientists did not believe it at first.
• January 1959: Luna 1 was the first spacecraft to observe the solar wind.
• 1962: Mariner 2 found the solar wind.
• 1972–1973: Pioneer 10 studied the heliosphere beyond Mars, passed by Jupiter on December 4, 1973, and sent back solar wind data up to 67 AU.
• February 1992: After passing Jupiter, the Ulysses spacecraft became the first to explore the mid and high latitudes of the heliosphere.
• 1992: Pioneer and Voyager probes found Ly-α radiation resonantly scattered by heliospheric hydrogen.
• 2004: Voyager 1 reached the termination shock.
• 2005: SOHO observations showed the heliosphere is not symmetrical but distorted, likely due to the local galactic magnetic field.
• 2009: IBEX project scientists discovered and mapped a ribbon-shaped region of intense energetic neutral atom emission. These neutral atoms are thought to come from the heliopause.
• October 2009: The heliosphere may be bubble-shaped, not comet-shaped.
• October 2010: Changes in the ribbon were found after six months, based on the second set of IBEX observations.
• May 2012: IBEX data suggests there is probably not a bow "shock."
• June 2012: At 119 AU, Voyager 1 detected an increase in cosmic rays.
• 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 seen in 1992 by the two Voyager spacecraft about the hydrogen wall.
• November 5, 2018: Voyager 2 crossed the heliopause, leaving the heliosphere.