The heliosphere is the region around the Sun that includes its magnetosphere, astrosphere, and outermost atmospheric layer. It forms a large, bubble-like shape with a tail, extending into space. In plasma physics, it is a cavity created by the Sun in the surrounding interstellar medium. This bubble is kept inflated by the solar wind, a stream of plasma from the Sun. Beyond the heliosphere, this solar plasma is replaced by interstellar plasma found throughout the Milky Way. As part of the interplanetary magnetic field, the heliosphere protects the Solar System from much of the cosmic ionizing radiation, though uncharged gamma rays are not blocked. The term "heliosphere" was likely first used by Alexander J. Dessler in scientific literature in 1967. The study of the heliosphere, called heliophysics, includes research on space weather and space climate.

The solar wind flows through the Solar System for billions of kilometers, reaching far beyond Pluto until it meets the "termination shock," where its movement slows due to pressure from the interstellar medium. Between the termination shock and the outer edge of the heliosphere, called the "heliopause," lies the "heliosheath," a wide transitional area. The overall shape of the heliosphere is similar to a comet, being roughly spherical on one side, about 100 astronomical units (AU) in size, and tail-shaped on the opposite side, known as the "heliotail," which extends 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 recorded a sudden increase in plasma density by forty times. 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 spaces between stars with different thicknesses. Over time, the heliosphere has changed significantly. Evidence shows that the heliosphere became smaller, reaching only the inner part of our solar system as recently as 3 million years ago. This change was caused by a nearby supernova, which allowed space between stars to reach Earth. This exposure may have affected Earth's climate and ecosystems.

Structure

The heliosphere is not a perfect sphere, even though its name suggests that. Its shape depends on three things: 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 affect the boundaries of the heliosphere more quickly, over short times like hours or 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 stronger or weaker at regular intervals, has a major influence on the heliosphere’s shape.

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 is pushed together into a nearly round shape, while the plasma moving in the opposite direction stretches out into a long tail, forming the heliotail.

Because scientists have limited information about these structures, many theories exist about their shapes. In 2020, Merav Opher and her team found that the heliosphere is shaped like a crescent, similar to a flattened 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 travel outward into space. Since the Sun rotates once every 25 days, the magnetic field carried by the solar wind becomes twisted into a spiral shape. The solar wind influences other 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 layer in the heliosphere caused by the Sun’s rotating magnetic field. It separates areas of the heliosphere with opposite magnetic polarities. This structure extends throughout the heliosphere and is the largest known structure in the Solar System. It is often compared to a "ballerina’s skirt" because of its appearance.

Edge structure

The outer part of the heliosphere is shaped by the movement of the solar wind and the gases in space between stars. The solar wind flows outward from the Sun in all directions at speeds of hundreds of kilometers per second near Earth. Far from the Sun, beyond Neptune’s orbit, this fast wind slows down as it meets gases in the space between stars. This happens in several steps:

• The solar wind moves at very high speeds inside the Solar System. When it reaches the termination shock, a type of shock wave, it slows to below the speed of sound and becomes slower than sound.
• Earlier, scientists thought the slower wind would form a rounded front and a long, tail-like shape behind it, called the heliosheath. However, observations in 2009 showed this was not correct. By 2011, scientists believed the heliosheath is filled with a bubble-like structure made of magnetic fields.
• The outer edge of the heliosheath, where the heliosphere meets space gases, is called the heliopause. This is the farthest point of the heliosphere. Observations in 2009 changed how scientists understand this boundary.
• The heliopause may create turbulence in space gases as the Sun moves around the center of the Milky Way. This turbulence happens because the pressure from the heliopause pushes against the space gases. However, the solar wind may not be fast enough to create a strong shock wave in this area.

The termination shock is the point where the solar wind slows to slower than sound speed because it meets the gases in space. This causes the solar wind to compress, heat up, and change its 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 gases is about 100 km/s. The exact speed depends on the density of the space gases, which changes. Even though space gases are very thin, they have a steady pressure. The pressure from the solar wind decreases as it moves farther from the Sun. When the solar wind is far enough from the Sun, its pressure can no longer keep up with the pressure from space gases, causing the solar wind to slow to below the speed of sound and create a shock wave. Beyond the termination shock is the heliopause, where the pressures from the solar wind and space gases balance, and the solar wind stops.

Other examples of termination shocks can be seen in everyday life. For instance, when water flows from a tap into a sink, it creates a sudden change in water flow called a hydraulic jump. The water spreads out quickly, forming a shallow, fast-moving disk (like the solar wind). Around the edge of this disk, a wall of water forms, where the water moves slower (like the space gases).

In 2005, scientists 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 began detecting returning particles in May 2006, when it was 76 astronomical units from the Sun. This suggests the heliosphere might be irregularly shaped, bulging in the Sun’s northern hemisphere and pushed inward in the south.

The heliosheath is the area of the heliosphere beyond the termination shock. Here, the solar wind is slowed, compressed, and becomes turbulent as it interacts with space gases. The inner edge of the heliosheath is about 80 to 100 astronomical units from the Sun. Scientists once thought the heliosheath might look like a comet’s tail, stretching several times the distance of the Sun’s path through space. However, Voyager missions 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 and space gases interact. Voyager 1 and Voyager 2 began detecting these bubbles in 2007 and 2008. The bubbles are created by magnetic reconnection between opposite parts of the Sun’s magnetic field as the solar wind slows.

At about 113 astronomical units, Voyager 1 found a "stagnation region" in the heliosheath. Here, the solar wind stopped, the magnetic field doubled in strength, and high-energy electrons from the galaxy increased 100 times. At about 122 astronomical units, Voyager 1 entered a new area called the "magnetic highway," still influenced by the Sun but with major differences.

The heliopause is the boundary where the solar wind is stopped by space gases. This is where the pressure from the solar wind and space gases balance. Crossing the heliopause should cause a sharp drop in the temperature of charged particles from the solar wind, a change in the direction of the magnetic field, and an increase in galactic cosmic rays.

In May 2012, Voyager 1 detected a rapid increase in cosmic rays, 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 particles per second by October. In September 2013, NASA announced that Voyager 1 crossed the heliopause on August 25, 2012, at 121 astronomical units from the Sun. Surprisingly, 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 also studied the heliopause from the SHIELDS mission in 2021.

The heliotail is the long, several thousand astronomical units long tail of the heliosphere and the Solar System. It is similar to a comet’s tail but always points away from the Sun. In the heliotail, the solar wind slows and escapes the heliosphere, slowly disappearing due to interactions with space gases. NASA’s IBEX mission found the heliotail has a shape like a four-leaf clover. The tail’s particles do not emit light, so 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 interstellar space filled with material, especially plasma, from other stars, not from the Sun. Just outside the heliosphere, which is sometimes called the "solar bubble," there is a transitional region discovered by Voyager 1. In 2004, scientists found evidence of pressure from interstellar material, and some of the Sun's material also enters the interstellar space. The heliosphere is believed to be located within the Local Interstellar Cloud, which is part of the Local Bubble in the Orion Arm of the Milky Way Galaxy.

Outside the heliosphere, the density of plasma increases by 40 times. Scientists also notice fewer particles from the Sun and more galactic cosmic rays in this region.

Measurements from at least 11 spacecraft by 2013 showed the flow of the interstellar medium (ISM) into the heliosphere. By 2013, scientists suspected the direction of this flow had changed over time. From Earth's perspective, the flow originally came from the constellation Scorpius, but it may have 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 2013 study explored 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 depend on the speed of the solar wind and the density of the interstellar medium. The heliopause is known to be far beyond Neptune's orbit. The Voyager 1 and 2 spacecraft were sent to study the termination shock, heliosheath, and heliopause. Meanwhile, the IBEX mission aimed 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 were incomplete.

In August 2018, the New Horizons spacecraft confirmed findings first detected in 1992 by the Voyager missions about the hydrogen wall. Although the hydrogen was detected using extra ultraviolet light (which might come from another source), New Horizons confirmed Voyager's results with greater accuracy.

Scientists long believed 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 and causes turbulence. A bow shock was thought to occur around 230 AU, but in 2012, scientists determined it likely does not exist. This conclusion came from new measurements: earlier studies by Ulysses measured the speed of the local interstellar medium (LISM) 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, such as the red giant star Mira in the constellation Cetus. Mira moves through space at over 130 kilometers per second and has both a tail of ejected material and a clear shock wave in the direction of its movement. These observations were made by NASA's retired GALEX telescope.

Observational methods

Scientists are still unsure about the exact distance and shape of the heliopause. 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, communication with Pioneer 10 and 11 has been lost.

The heliosphere is shaped more like a bubble than a comet, according to data from Cassini’s Ion and Neutral Camera (MIMI/INCA). Instead of being shaped mainly by collisions between the solar wind and the interstellar medium, the INCA (ENA) maps suggest that the interaction is influenced more by the pressure of particles and the energy of magnetic fields.

Early data from the Interstellar Boundary Explorer (IBEX), launched in October 2008, showed a narrow, bright area in space that is two to three times brighter than other areas, now called the IBEX ribbon. These findings suggest that the environment outside the Solar System has a greater influence on the shape of the heliosphere than previously thought. Scientists do not yet know what causes the ENA (energetic neutral atoms) ribbon.

The IBEX results show that the maps of the region do not match earlier predictions. Scientists are excited to study these maps and update their understanding of the heliosphere and how it interacts with the galaxy. In October 2010, changes in the ribbon were observed after six months, based on new IBEX data. IBEX data did not support the existence of a bow shock, but one study suggested there may be a "bow wave."

Examples of missions that have 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 can be more easily observed 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 influenced by the Sun. Its edge is determined by two main factors: 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 main sections: the termination shock, the heliosheath, and the heliopause. These regions mark the Sun’s influence as it interacts with space beyond the solar system. Data about the heliosphere’s farthest reaches has been collected by five spacecraft: Pioneer 10 (1972–1997; data to 67 AU), Pioneer 11 (1973–1995; 44 AU), Voyager 1 and Voyager 2 (launched 1977; ongoing), and New Horizons (launched 2006). Scientists have also observed particles called energetic neutral atoms (ENAs) near the edges of the heliosphere.

Most of the heliosphere is filled with material from the Sun, except near planets or comets. Cosmic rays, fast-moving neutral atoms, and cosmic dust can enter the heliosphere from space. Solar wind particles originate from the Sun’s corona, a very hot layer of the Sun’s atmosphere. These particles escape the Sun at speeds of 300 to 800 km/s (671,000 to 1.79 million mph) and travel outward. As the solar wind interacts with the interstellar medium (material between stars), its speed decreases. The point where the solar wind slows to the speed of sound is called the termination shock. Beyond this, the solar wind continues to slow as it moves through the heliosheath, a region that ends at the heliopause. At the heliopause, the pressure from the solar wind and the interstellar medium balance. Voyager 1 crossed the termination shock in 2004, and Voyager 2 did so in 2007.

Scientists once believed there was a bow shock beyond the heliopause, but data from the Interstellar Boundary Explorer mission suggested the Sun’s movement through space is too slow to create one. Instead, it may form a gentler "bow wave." Voyager data also led to a new theory that the heliosheath contains "magnetic bubbles" and a stagnation zone. In 2010, Voyager 1 detected a stagnation region where the solar wind’s speed drops to zero, 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 nearing the heliopause. NASA announced in 2013 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 notable differences.

Pioneer 10, launched in March 1972, passed the Moon within 10 hours. Over its mission, it made many discoveries about the heliosphere and Jupiter’s magnetosphere. It detected sodium and aluminum ions in the solar wind and helium in the inner solar system. Pioneer 10 crossed Jupiter’s magnetosphere 17 times, mapping its interaction with the solar wind. It returned data until 1997, including solar wind measurements up to 67 AU. In 2003, it was contacted at 82 AU, but no new solar wind data was collected.

Voyager 1 surpassed Pioneer 10’s distance from the Sun in 1998, traveling faster and gaining about 1.02 AU per year. In 2023, Voyager 2 became the second farthest human-made object from the Sun. Pioneer 11, launched a year after Pioneer 10, collected similar data up to 44.7 AU in 1995. Both Pioneer and Voyager spacecraft traveled different paths, providing data on the heliosphere from different directions. Their findings helped confirm the existence of a hydrogen wall.

Voyager 1 is believed to have crossed the heliopause in 2012, and Voyager 2 did so in 2018. These spacecraft are the only human-made objects to enter interstellar space. However, they have not yet left the Solar System, which is considered to extend to the outer edge of the Oort Cloud. When Voyager 2 crossed the heliopause in 2018, its instruments recorded a sharp drop in solar wind speed and no further signs of it. Other instruments also detected 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 discoveries.

Timeline of exploration and detection

• 1904: Astronomers used the Potsdam Great Refractor with a spectrograph to find evidence of the space between stars while observing the binary star Mintaka in Orion.
• 1958: Eugene Parker published a paper that predicted the solar wind; his theory was not accepted at first by scientists.
• January 1959: Luna 1 became the first spacecraft to observe the solar wind.
• 1962: Mariner 2 detected the solar wind.
• 1972–1973: Pioneer 10 became the first spacecraft to explore the heliosphere beyond Mars, flew by Jupiter on December 4, 1973, and continued to send solar wind data up to 67 AU.
• February 1992: After flying by Jupiter, the Ulysses spacecraft became the first to study the middle and upper parts of the heliosphere.
• 1992: 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.
• 2005: Observations from SOHO showed the heliosphere is not symmetrical around a central point, but is likely distorted by the local galactic magnetic field.
• 2009: Scientists from the IBEX project discovered and mapped a ribbon-shaped area where many energetic neutral atoms are emitted. These atoms are believed to come from the heliopause.
• October 2009: The heliosphere may be shaped like a bubble, not a comet.
• October 2010: Important changes were found in the ribbon 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 about the hydrogen wall confirmed results first found in 1992 by the Voyager spacecraft.
• November 5, 2018: Voyager 2 crossed the heliopause, leaving the heliosphere.