Heliosphere

The heliosphere is the outer layer of the Sun, shaped like a huge, tailed bubble. It is formed by the solar wind, a stream of particles flowing from the Sun, and it stretches far beyond the region of Pluto. This bubble protects the Solar System by shielding it from harmful cosmic ionizing radiation, though it does not stop gamma rays.

Inside the heliosphere, there are special areas. The termination shock is where the solar wind slows down quickly because of the pressure from outside space. After that comes the heliosheath, a wide area between the termination shock and the heliopause, which is the very edge of the heliosphere. The whole shape looks a bit like a comet, with a round side and a long tail called the heliotail stretching for thousands of distances called astronomical units.

Two special spacecraft from the Voyager program traveled through these distant parts of the heliosphere. On 25 August 2012, Voyager 1 reached the heliopause and entered interstellar space. Later, on 5 November 2018, Voyager 2 also passed through the heliopause, becoming the second human-made object to explore the space between stars.

History

The heliosphere changes because of things outside our solar system, like explosions from stars called supernovas or moving through areas of space with different densities called interstellar mediums. Over time, the heliosphere has looked very different. About 3 million years ago, a nearby supernova made the heliosphere shrink to the inner part of our solar system. This change let materials from space reach Earth, which might have affected our planet's climate and living things.

Structure

The heliosphere isn’t a perfect sphere. Its shape changes because of three things: the space around us, the solar wind, and how the Sun moves through space. The solar wind changes quickly, causing the edges of the heliosphere to move around a lot. Every 11 years, the Sun goes through a cycle where its solar wind activity changes a lot.

Because the Sun is moving through space, the heliosphere looks more like a comet. The part of the solar wind going in the same direction as the Sun’s motion is squished together, but the part going the opposite way stretches out far behind, making a long tail.

Solar wind

Main articles: Solar wind and Interplanetary medium

The solar wind is made of tiny particles and magnetic fields that flow out from the Sun into space. As the Sun turns about every 25 days, the magnetic field gets twisted into a spiral shape. The solar wind affects Earth and other parts of our solar system, sometimes causing big changes in Earth’s magnetic field.

Heliospheric current sheet

Main article: Heliospheric current sheet

The heliospheric current sheet is a big ripple in the heliosphere made by the Sun’s rotating magnetic field. It marks where the magnetic field changes direction. This sheet stretches all through the heliosphere and is one of the largest structures in our solar system. It looks a bit like a ballerina’s skirt.

Edge structure

The outer part of the heliosphere is shaped by how the Sun's solar wind interacts with the gases in space far from the Sun. The solar wind moves away from the Sun in all directions at very high speeds. As it moves far beyond the planet Neptune, it slows down because it meets the gases in space.

There are a few important parts to this edge:

• The solar wind starts to slow down at a place called the termination shock, where it goes from moving faster than the speed of sound to slower than the speed of sound.
• After the termination shock is a region called the heliosheath. Here, the solar wind is slowed and mixed up by space gases. It was once thought to look like the tail of a comet, but we now know it is filled with magnetic bubbles.
• The very outer edge, where the heliosphere meets space, is called the heliopause. This is where the Sun's influence ends and space begins.

In 2012, the Voyager 1 spacecraft reached the heliopause, and in 2018, Voyager 2 did too. These spacecraft helped scientists learn more about the edges of our solar system.

Outside structures

The heliopause is the last known edge of the heliosphere, separating it from space filled with material from other stars. Just beyond the heliosphere, there is a change in space that has been noticed by the Voyager 1 spacecraft. Some of the Sun’s material slowly moves into this space.

Outside the heliosphere, the space around us changes a lot. There is much more material, fewer particles from the Sun, and more particles from far away stars. Scientists have measured how this space moves into the heliosphere using many spacecraft.

Hydrogen wall

The “hydrogen wall” might be a area of very hot hydrogen between the edge of the heliosphere and a shock wave. This wall is made when material from space meets the edge of the Sun’s area. Studies have looked at this idea, and the New Horizons spacecraft confirmed earlier findings in 2018.

Bow shock

Scientists once thought the Sun made a “shock wave” as it moves through space. This would happen if space material moved very fast toward the Sun. But in 2012, new measurements showed this shock wave probably does not exist. This was learned by measuring how fast space material moves compared to the Sun. This kind of shock wave has been seen around other stars, like the star Mira.

Observational methods

The exact distance and shape of the heliopause, the edge of the heliosphere, are still not known for sure. Spacecraft such as Pioneer 10, Pioneer 11, and New Horizons are moving outward through the Solar System and will eventually pass through the heliopause. We have lost contact with Pioneer 10 and 11.

Data from Cassini shows that the heliosphere looks more like a bubble than a comet. This is based on information from its special camera. The way the heliosphere interacts with space around it seems to depend more on pressure and magnetic fields than on collisions.

The Interstellar Boundary Explorer (IBEX), launched in October 2008, found something surprising: a very narrow, bright ribbon in the sky, now called the IBEX ribbon. This discovery shows that the space around us has a bigger effect on the heliosphere than we thought. Scientists are excited to study these findings and learn more about how our heliosphere connects with the galaxy.

Some 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 eclipse, we can see the Sun's hot corona more clearly from Earth. During the Apollo program, the Solar wind was measured on the Moon. Some Earth-based telescopes that study the Sun include the McMath–Pierce solar telescope, the GREGOR Solar Telescope, and the Big Bear Solar Observatory.

Exploration history

The heliosphere is the area influenced by the Sun. Its edge is mainly determined by the heliospheric magnetic field and the solar wind from the Sun. It has three main parts from the start to its edge: the termination shock, the heliosheath, and the heliopause.

Five spacecraft have given us much of the information about the farthest reaches of the heliosphere. These include Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and New Horizons. We also see a special kind of particle called an energetic neutral atom coming from its edges.

Except near objects like planets or comets, the heliosphere is mostly made of material from the Sun. However, things like cosmic rays, fast-moving neutral atoms, and cosmic dust can come into the heliosphere from outside. The solar wind starts at the Sun's very hot surface, the corona, and moves outward at speeds from 300 to 800 kilometers per second. It slows down when it meets the interstellar medium. The place where it slows to the speed of sound is called the termination shock. It continues to slow down as it moves through the heliosheath until it reaches the heliopause, where the pressures from the interstellar medium and the solar wind balance out.

Voyager 1 passed the termination shock in 2004, and Voyager 2 did so in 2007. Scientists once thought there was a bow shock beyond the heliopause, but later data suggested it might be more like a gentle "bow wave" instead. Voyager data also led to new ideas about "magnetic bubbles" in the heliosheath and a area where the solar wind stops.

In 2012, Voyager 1 noticed a big increase in cosmic rays, showing it was getting close to the heliopause. By August 2012, Voyager 1 entered interstellar space. In 2018, Voyager 2 also passed through the heliopause. These two Voyagers are the only human-made objects to enter interstellar space. They are still moving, but have not yet left the edge of our Solar System, which is thought to be the outer edge of the Oort Cloud. In 2025, NASA launched the Interstellar Mapping and Acceleration Probe (IMAP) to use the Voyager findings to learn more.

Timeline of exploration and detection

Here is a simple timeline of important discoveries and missions related to the heliosphere:

• 1904: Astronomers using the Potsdam Great Refractor with a spectrograph found clues about space between stars while studying a star called Mintaka in Orion.
• 1958: Eugene Parker wrote a paper predicting the solar wind, though many scientists did not believe it at first.
• January 1959: The spacecraft Luna 1 became the first to observe the solar wind.
• 1962: Mariner 2 detected the solar wind.
• 1972–1973: The spacecraft Pioneer 10 was the first to travel through the heliosphere beyond Mars, flying by Jupiter and sending data from far away.
• February 1992: After flying by Jupiter, the Ulysses spacecraft explored different areas of the heliosphere.
• 1992: Pioneer and Voyager probes found special light signals from hydrogen in the heliosphere.
• 2004: Voyager 1 became the first spacecraft to reach a major boundary of the heliosphere.
• 2005: Observations from SOHO showed that the shape of the heliosphere is not perfectly round, likely because of the magnetic field from our galaxy.
• 2009: Scientists using the IBEX project discovered and mapped a bright area of energy coming from the edge of the heliosphere.
• October 2009: Scientists suggested the heliosphere might look like a bubble, not a comet.
• October 2010: Changes were seen in the bright area after more observations with IBEX.
• May 2012: Data from IBEX suggested there might not be a bow shock in front of the heliosphere.
• June 2012: At a great distance, Voyager 1 noticed an increase in particles from outside our solar system.
• 25 August 2012: Voyager 1 passed through the edge of the heliosphere, becoming the first human-made object to leave it.
• August 2018: The New Horizons spacecraft confirmed earlier findings about the heliosphere made by the Voyager probes.
• 5 November 2018: Voyager 2 also passed through the edge of the heliosphere.