The heliosphere is the magnetosphere, astrosphere, and outermost atmospheric layer of the Sun. It forms a large, bubble-like region of space with a tail shape. In space science, it is the empty space created by the Sun in the surrounding 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 found throughout the Milky Way. As part of the interplanetary magnetic field, the heliosphere protects the Solar System from most cosmic ionizing radiation. 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 travels through the Solar System for billions of kilometers and extends beyond Pluto until it reaches the "termination shock," where its movement slows suddenly due to pressure from the interstellar medium. 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 extends 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 recorded a sudden increase in plasma density, which was forty times higher than before. 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, such as the two Voyagers, are now in interstellar space.
History
The heliosphere, which is the protective bubble created by the sun, is influenced by events outside our solar system, such as nearby supernovas or movement through space regions with different densities. Over time, the heliosphere has changed significantly. Evidence shows that as recently as 3 million years ago, the heliosphere became smaller, reaching only the inner planets of our solar system. This change occurred because of a powerful explosion from a star near our solar system, which allowed space material from outside our solar system to reach Earth. This event may have affected Earth's weather and living conditions.
Structure
The heliosphere is not shaped like a perfect sphere, even though its name suggests otherwise. Its shape is influenced by 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, meaning the heliosphere’s shape and size can change over time. However, changes in the solar wind have a stronger effect on the position of the heliosphere’s boundaries over short periods, such as hours to a few years. The solar wind’s pressure changes much more quickly than the pressure from the ISM at any given location. The 11-year solar cycle, which causes the solar wind’s activity to reach high and low points, is believed to significantly affect 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 much farther before meeting the ISM, forming the long, trailing shape of the heliotail.
Because there is limited data and much of this structure is still unexplored, scientists have proposed many different theories about its shape. In 2020, Merav Opher and her research team concluded that the heliosphere is shaped like a crescent, similar to a deflated croissant.
The solar wind is made up of particles, such as ionized 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 many systems in the Solar System, including the Earth’s magnetosphere, where changes in the Sun’s magnetic field create geomagnetic storms.
The heliospheric current sheet is a ripple in the heliosphere caused by the Sun’s rotating magnetic field. It separates regions of the heliospheric magnetic field with opposite polarity. This structure spans the entire heliosphere and is considered 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 the interaction between the solar wind and the winds from interstellar space. The solar wind flows outward 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 interstellar medium. This happens in several steps:
The termination shock is the point in the heliosphere where the solar wind slows to a speed slower than the speed of sound (relative to the Sun) because of its interaction with the interstellar medium. This causes the solar wind to become compressed, heated, and the magnetic field to change. In our Solar System, the termination shock is thought to be 75 to 90 astronomical units from the Sun. In 2004, Voyager 1 passed through the Sun’s termination shock, followed by Voyager 2 in 2007.
The shock forms because the solar wind moves 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 of the medium, which changes over time. The interstellar medium has very low density but maintains a steady pressure. The pressure from the solar wind decreases with the square of the distance from the Sun. When the solar wind is far enough from the Sun, its pressure becomes too weak to keep moving faster than the speed of sound, causing a shock wave. Beyond the termination shock lies the heliopause, where the pressures of the solar wind and the interstellar medium balance, and the solar wind is stopped by the interstellar medium.
Termination shocks can also be seen in everyday life. For example, when water flows from a tap into a sink, it creates a hydraulic jump. The water spreads out at a speed faster than the local wave speed, forming a shallow, wide disk. Around the edge of this disk, a wall of water forms, where the water moves slower than the local wave speed.
Evidence from a 2005 meeting of the American Geophysical Union showed that Voyager 1 crossed the termination shock in December 2004, 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 may be irregularly shaped, bulging in the Sun’s northern hemisphere and flattened in the south.
The heliosheath is the region of the heliosphere beyond the termination shock. Here, the solar wind is slowed, compressed, and becomes turbulent as it interacts with the interstellar medium. The inner edge of the heliosheath is about 80 to 100 astronomical units from the Sun. Some models suggest the heliosheath is shaped like a comet’s coma, stretching several times the distance of the Sun’s path through space. The thickness of the heliosheath on its windward side is estimated to be between 10 and 100 astronomical units. Scientists describe the heliosheath as a "foamy zone" filled with magnetic bubbles, each about 1 astronomical unit wide. These bubbles form when the solar wind interacts with the interstellar medium. Voyager 1 and Voyager 2 began detecting these bubbles in 2007 and 2008, respectively. The bubbles are likely 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, where the solar wind stopped moving, the magnetic field strength doubled, and high-energy electrons from the galaxy increased 100 times. At about 122 astronomical units, Voyager 1 entered a region called the "magnetic highway," where the Sun’s influence still exists but with notable differences.
The heliopause is the boundary where the solar wind is stopped by the interstellar medium. This is where the pressures of the solar wind and the interstellar medium balance. Crossing the heliopause should be marked by a sharp drop in the temperature of solar wind-charged particles, 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 recorded a sharp drop in protons from the Sun, from 25 particles per second to about 2 particles per second. In September 2013, NASA announced that Voyager 1 had crossed the heliopause on August 25, 2012, at a distance of 121 astronomical units from the Sun. Unlike predictions, data showed the galaxy’s magnetic field was 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, indicating 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 Earth using the SHIELDS mission in 2021.
The heliotail is the long, several-thousand astronomical unit tail of the heliosphere, and thus the Solar System’s tail. It is similar to a comet’s tail, though a comet’s tail always points away from the Sun. The heliotail is a region where the solar wind slows and escapes the heliosphere, gradually disappearing due to charge exchange. Observations from NASA’s Interstellar Boundary Explorer (IBEX) show the heliotail has a four-leaf clover shape. The tail contains fast and slow particles, with slow particles on the edges and fast particles in the center. This shape may be linked to the Sun sending faster solar winds from its poles and slower winds from its equator. The clover-shaped tail moves farther from the Sun, causing charged particles to shift orientation.
In 2009, data from the Cassini and IBEX missions challenged the "heliotail" theory. In July 2013, IBEX results revealed the heliosphere has a four-lobed tail.
Outside structures
The heliopause is the last known boundary between the heliosphere, a bubble of space created by the Sun, and the interstellar space beyond it. This space is filled with material, mostly plasma, that comes from other stars, not the Sun. Just outside the heliosphere, there is a transitional area, as discovered by the Voyager 1 spacecraft. Scientists first detected some pressure from interstellar space in 2004, and some material from the Sun has been found entering the interstellar medium. The heliosphere is believed to be 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. Scientists also notice fewer particles from the Sun and more galactic cosmic rays. The movement of the interstellar medium into the heliosphere has been measured by 11 spacecraft as of 2013. By 2013, it was thought that the direction of this movement may have changed over time. From Earth’s perspective, the flow of material comes from the constellation Scorpius, and it is likely that the 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 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 direction of 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 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. 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 suggested that earlier ideas about the heliopause were not fully accurate.
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 using extra ultraviolet light (which might come from another source), New Horizons’ observations support Voyager’s earlier discoveries with greater precision.
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 the speed of sound toward the Sun, as the solar wind moves away from the Sun at supersonic speeds. When the interstellar wind meets the heliosphere, it slows down and creates turbulence. A bow shock was once thought to occur at about 230 AU, but in 2012, new measurements showed it likely does not exist. This conclusion came from updated data: The speed of the local interstellar medium relative to the Sun was previously measured at 26.3 km/s by the Ulysses spacecraft, 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. For example, 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 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 not known exactly. Spacecraft that travel between planets and stars, such as Pioneer 10, Pioneer 11, and New Horizons, are moving outward through the Solar System and will eventually cross the heliopause. Scientists have not been able to communicate with Pioneer 10 and 11 anymore.
Instead of a shape like a comet, the heliosphere seems to be bubble-shaped, based on data from Cassini’s Ion and Neutral Camera (MIMI/INCA). Rather than being shaped mainly by the collisions between the solar wind and the material between stars, maps of ENA (energetic neutral atoms) suggest that the interaction is more influenced 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 band in space that is two to three times brighter than other areas, now called the IBEX ribbon. Early findings suggest that the environment beyond 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 ribbon.
Scientists found the IBEX results very surprising. The maps do not match any earlier models of this region. Scientists are excited to study these ENA maps to better understand the heliosphere and its interaction with the galaxy. In October 2010, changes in the ribbon were noticed after six months, based on the second set of IBEX observations. IBEX data did not support the existence of a bow shock, but one study suggested there might be a "bow wave."
Examples of missions that have collected or continue to collect data about the heliosphere include:
During a total solar eclipse, the hot corona of the Sun can be more easily seen 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 where the Sun's influence is strongest. The edge of the heliosphere 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. 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 special particles called energetic neutral atoms (ENA) at 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 move outward at speeds of 300 to 800 km/s (671,000 to 1.79 million mph). When the solar wind meets 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. As the solar wind continues to slow through the heliosheath, it reaches the heliopause, where 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.
Earlier, scientists 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, scientists now think there may be a gentler "bow wave" in that region.
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, located about 113 AU from the Sun. In this area, the solar wind stops completely, the magnetic field becomes twice as strong, and high-energy electrons from the galaxy increase 100 times.
In May 2012, Voyager 1 detected a sudden increase in cosmic rays at about 120 AU from the Sun, 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" at about 122 AU from the Sun. This area is still influenced by the Sun but has unique characteristics.
Pioneer 10 was launched in March 1972 and passed the Moon within 10 hours. Over the next 35 years, it made many discoveries about the heliosphere and Jupiter's influence 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 measurements up to about 67 AU. It was contacted again in 2003 when it was 82 AU from Earth, but no new solar wind data was sent back.
Voyager 1 surpassed Pioneer 10's distance from the Sun on February 17, 1998, because it traveled faster. On July 18, 2023, Voyager 2 became the second farthest human-made object from the Sun, overtaking Pioneer 10. Pioneer 11, launched a year after Pioneer 10, collected similar data up to 44.7 AU in 1995. Pioneer 11 had a magnetometer, a tool for measuring magnetic fields. Pioneer and Voyager spacecraft traveled different paths, so their data helped scientists confirm the existence of a hydrogen wall.
In 2012, Voyager 1 is 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 outer edge of the Oort Cloud. When Voyager 2 passed the heliopause, its Plasma Science Experiment (PLS) recorded a sharp drop in solar wind speed on November 5, 2018. Other instruments on Voyager 2 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 study the space beyond the heliosphere using data from the Voyagers.