NASA studies the edge of the Sun’s magnetic bubble


An illustration of the heliosphere bombarded with cosmic rays coming from outside our solar system. Credit: Goddard Space Flight Center / NASA Conceptual Imagery Lab

Our corner of the universe, the solar system, is nestled inside the Milky Way galaxy, which is home to more than 100 billion stars. The solar system is enclosed in a bubble called the heliosphere, which separates us from the vast galaxy beyond – and some of its harsh space radiation.

We are protected from this radiation by the heliosphere, which itself is created by another source of radiation: the Sun. The Sun is constantly spewing charged particles, called solar wind, from its surface. The solar wind blows about four times the distance of Neptune, taking with it the Sun’s magnetic field.

The heliosphere in the Milky Way galaxy. Credit: NASA Goddard Space Flight Center / Conceptual Image Lab / Walt Feimer

“Magnetic fields tend to push against each other, but not mix,” said Eric Christian, senior heliosphere researcher at ">Nasa‘s Goddard Space Flight Center in Greenbelt, Maryland. “Inside the heliosphere bubble are pretty much all of the Sun’s particles and magnetic fields. Outside there are those of the galaxy.

To understand the heliosphere, first separate the word, suggests David McComas, professor of astrophysical sciences at Princeton University in New Jersey. “Heliosphere” is the combination of two words: “Helios”, the Greek word for the Sun, and “sphere”, a large region of influence (although, to be clear, scientists are not sure of the exact form. of the heliosphere).

The heliosphere was discovered in the late 1950s, and many questions remain about it. As scientists study the heliosphere, they are learning more about how it reduces the exposure of astronauts and spacecraft to radiation and, more generally, how stars can influence their neighboring planets. .

A balloon in space

Radiation surrounds us every day. When we sunbathe, we are basking in the radiance of the Sun. We use radiation to heat up leftovers in our kitchen microwaves and depend on it for medical imaging.

Space radiation, however, is more similar to radiation released by radioactive elements like uranium. Space radiation that comes to us from other stars is called galactic cosmic radiation (GCR). Active areas of the galaxy – like supernovae, black holes, and neutron stars – can pull electrons from atoms and accelerate nuclei to near the speed of light, producing GCR.

Changes in the heliosphere

The heliosphere changes size throughout the solar cycle. Credit: NASA Goddard Space Flight Center / Science Visualization Studio / Tom Bridgman

On Earth, we have three layers of protection against radiation from space. The first is the heliosphere, which helps prevent the GCR from reaching the major planets in the solar system. Additionally, Earth’s magnetic field produces a shield called the magnetosphere, which pulls the GCR away from Earth and low-orbiting satellites like the International Space Station. Finally, gases in the Earth’s atmosphere absorb radiation.

When astronauts head to the moon or to ">March, they will not have the same protection that we have on Earth. They will only have the protection of the heliosphere, the size of which fluctuates throughout the Sun’s 11-year cycle.

In each solar cycle, the Sun goes through periods of intense activity and powerful solar winds, and periods of calm. Like a balloon, when the wind calms down, the heliosphere deflates. When it resumes, the heliosphere expands.

“The effect of the heliosphere on cosmic rays allows longer duration human exploration missions. In a way, it allows humans to reach Mars, ”said Arik Posner, a heliophysicist at NASA headquarters in Washington, DC“ The challenge for us is to better understand the interaction of cosmic rays with the heliosphere. and its limits. “

Anatomy of the heliosphere

There is a debate about the precise shape of the heliosphere. However, scientists agree that it has multiple layers. Let’s look at the layers from the inside out:

NASA's Voyager 2 probe enters interstellar space

This illustration shows the position of NASA’s Voyager 1 and Voyager 2 probes outside of the heliosphere, a protective bubble created by the Sun that extends well beyond Pluto’s orbit. Credit: NASA / JPL-Caltech

  • Termination shock: All the major planets in our solar system are located in the innermost layer of the heliosphere. Here, the solar wind emanates from the Sun at full speed, about a million miles per hour, for billions of miles, unaffected by the pressure of the galaxy. The outer limit of this central layer is called termination shock.
  • Heli-sheath: Beyond the termination shock is the heli-sheath. Here, the solar wind moves more slowly and deflects as it faces the pressure of the interstellar medium outside.
  • Heliopause: The heliopause marks the climax and final plasma border between the Sun and the rest of the galaxy. Here, the magnetic fields of solar and interstellar winds push against each other, and inner and outer pressures are in balance.
  • External heli sheath: The region just beyond the heliopause, which is further influenced by the presence of the heliosphere, is called the outer heliosheath.

How we study the far reaches of the heliosphere

Many NASA missions study the Sun and the most intimate parts of the heliosphere. But only two man-made objects crossed the border of the solar system and entered interstellar space.

In 1977, NASA launched Voyager 1 and Voyager 2. Each spacecraft is equipped with tools to measure the magnetic fields and particles it passes through directly. After passing the outer planets on a grand tour, they came out of the heliopause in 2012 and 2018, respectively, and are currently in the outer heliopause. They found that cosmic rays are about three times more intense outside the heliopause than deep in the heliosphere.

However, the picture the Voyagers paint is incomplete.

“Trying to understand the entire heliosphere from two points, Voyager 1 and 2, is like trying to determine the weather in the entire Pacific Ocean using two weather stations,” Christian said.

The Voyagers are working with the Interstellar Boundary Explorer (IBEX) to study the heliosphere. IBEX is a 176-pound, suitcase-sized satellite launched by NASA in 2008. Since then, IBEX has been orbiting the Earth, equipped with telescopes observing the outer edge of the heliosphere. IBEX captures and analyzes a class of particles called energetic neutral atoms, or ENAs, that cross its path. ENAs form where the interstellar medium and the solar wind meet. Some ENAs return to the center of the solar system – and IBEX.

“Anytime you pick up one of these ENAs, you know which direction it’s coming from,” said McComas, IBEX senior researcher. “By collecting a large number of these individual atoms, you are able to create this upside down image of our heliosphere.”

In 2025, NASA will launch the Interstellar Mapping and Acceleration (IMAP) probe. IMAP’s ENA cameras have a higher resolution and are more sensitive than IBEX’s.

Mysteries abound

In 2009, IBEX came to such a shocking conclusion that researchers first wondered if the instrument might have malfunctioned. This discovery became known as the IBEX tape – a strip in the sky where ENA emissions are two or three times brighter than the rest of the sky.

“The ribbon was totally unexpected and was not expected by any theory until we stole the mission,” McComas said. The cause is not yet entirely clear, but it is a clear example of the mysteries of the heliosphere that remain to be discovered.

IBEX tape

NASA’s Interstellar Boundary Explorer, or IBEX, studies the heliosphere from its orbit around Earth. IBEX’s very first sky chart showed a surprising feature dubbed the “IBEX Ribbon”. Credit: NASA / IBEX

“Our Sun is a star like billions of other stars in the universe. Some of these stars also have astrospheres, like the heliosphere, but it’s the only astrosphere we’re in that we can study closely, ”said Justyna Sokol, a researcher at the Southwest Research Institute in San Antonio, in. Texas. “We need to get out of our neighborhood to learn a lot more about the rest of the universe.”

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