Solar Orbiter shows how solar wind gets a magnetic boost

ESA’s Solar Orbiter spacecraft has provided crucial data to answer the question of where the energy that heats and accelerates the solar wind comes from. Working with NASA’s Parker Solar Probe, Solar Orbiter reveals that the energy needed to power this flow comes from strong fluctuations in the Sun’s magnetic field.

The solar wind is a constant stream of charged particles that escape from the sun’s atmosphere (called the corona) and flow toward Earth. It is the collision of the solar wind with our planet’s atmosphere that triggers the colorful northern lights in our sky.

The “fast” solar wind travels at speeds greater than 500 km/s, or 1.8 million km/h. Oddly enough, this wind exits the sun’s corona at a slower speed, so it accelerates as it moves away. Million-degree wind naturally cools as it expands and becomes less dense, much like the air on Earth when you climb a mountain. And yet, it cools more slowly than you would expect from this effect alone.

So what provides the energy needed to accelerate and heat the fastest parts of the solar wind? In a new study published in ScienceThe researchers used data from ESA’s Solar Orbiter and NASA’s Parker Solar Probe to provide conclusive evidence that the answer lies in large-scale oscillations in the Sun’s magnetic field, known as Alfvén waves.

“Before this work, Alfvén waves had been suggested as a potential energy source, but we didn’t have definitive proof,” says co-lead author Yeimy Rivera of the Harvard and Smithsonian Center for Astrophysics in Massachusetts.

In an ordinary gas, such as the Earth’s atmosphere, the only transmittable waves are sound waves. However, when a gas is heated to extraordinary temperatures, such as in the solar atmosphere, it enters an electrified state called plasma and reacts to magnetic fields. This allows waves, called Alfvén waves, to form in the magnetic field. These waves store energy and can efficiently transport it through a plasma.

A normal gas expresses its stored energy as density, temperature, and velocity. In the case of a plasma, the magnetic field also stores energy. Both the Solar Orbiter and the Parker Solar Probe contain the instruments needed to measure the properties of the plasma, including its magnetic field.

Although the two spacecraft operate at different distances from the Sun and in very different orbits, in February 2022 they aligned along the same solar wind stream.

Parker, which was operating 13.3 solar radii (about 9 million kilometers) from the Sun, at the outer reaches of the Sun’s corona, crossed the stream first. Solar Orbiter, operating 128 solar radii (89 million kilometers), then crossed the stream a day or two later. “This work was only possible because of the very special alignment of the two spacecraft, which sampled the same stream of solar wind at different stages of its path from the Sun,” Yeimy says.

Taking full advantage of this rare alignment, the team compared measurements of the same plasma flow at two different locations. They first transformed the measurements into four key energy quantities, including a measure of the energy stored in the magnetic field, called the wave energy flux.

Since energy can neither be created nor destroyed, but only converted from one form to another, the team compared Parker’s measurements to those from Solar Orbiter. They made this comparison with and without the term magnetic energy.

“We found that if we don’t include the wave energy flux at Parker, we can’t match the amount of energy we have at Solar Orbiter,” says Samuel Badman, co-first author, Center for Astrophysics, Harvard & Smithsonian, Massachusetts.

Near the Sun, where Parker measured the flux, about 10 percent of the total energy was in the magnetic field. At Solar Orbiter, that figure had dropped to just 1 percent, but the plasma had accelerated and cooled more slowly than expected.

By comparing the numbers, the team concluded that the lost magnetic energy was fueling the acceleration and slowing the cooling of the plasma by providing some heating itself.

The data also show the importance of magnetic patterns called “switchbacks” for wind acceleration. Switchbacks are large deflections of the Sun’s magnetic field lines and are examples of Alfvén waves. They have been observed since the first solar probes in the 1970s, but their detection rate has increased dramatically since Parker Solar Probe became the first spacecraft to pass through the Sun’s corona in 2021 and detected that the switchbacks come together in patches.

This new work confirms that these lace patches contain enough energy to be responsible for the missing part of the acceleration and heating of the fast solar wind.

“This new work expertly puts together some important pieces of the solar puzzle. Increasingly, the combination of data collected by Solar Orbiter, Parker Solar Probe and other missions shows us that different solar phenomena actually work together to create this extraordinary magnetic environment,” says Daniel Müller, Solar Orbiter project scientist at ESA.

And it doesn’t just tell us about our solar system. “Our Sun is the only star in the universe whose wind we can directly measure. So what we’ve learned about our Sun potentially applies at least to other solar-type stars, and perhaps to other types of stars that have winds,” Samuel says.

The team is now working to extend their analysis to apply it to slower forms of the solar wind, to see if energy from the sun’s magnetic field also plays a role in speeding up and warming them.