In an ordinary gas, such as Earth’s atmosphere, the only kind of waves that can be transmitted are sound waves. However, when a gas is heated to extraordinary temperatures, such as in the Sun’s atmosphere, it enters an electrified state known as a plasma and responds to magnetic fields. This allows waves, called Alfvén waves, to form in the magnetic field. These waves store energy and can efficiently carry it through a plasma.

A normal gas expresses its stored energy in the form of density, temperature and velocity. With a plasma, however, the magnetic field also stores energy. Both Solar Orbiter and Parker Solar Probe contain the necessary instruments to measure the properties of the plasma, including its magnetic field.

Although the two spacecraft are operating at different distances from the Sun, and in very different orbits, in February 2022, the spacecraft happened to align along the same stream of solar wind.

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

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

Because energy can neither be created nor destroyed, only converted from one form to another, the team compared the readings from Parker to those from Solar Orbiter. They did this comparison both with and without the magnetic energy term.

“We found that if we didn't include the wave energy flux at Parker, we don't quite match how much energy we have at Solar Orbiter,” says joint first author Samuel Badman, Center for Astrophysics, Harvard & Smithsonian, Massachusetts.

Close to the Sun, where Parker measured the stream, around 10% of the total energy was found in the magnetic field. At Solar Orbiter, this number had dropped to just 1% but the plasma had accelerated and had cooled more slowly than expected.

Comparing the numbers, the team concluded that the lost magnetic energy was powering the acceleration and slowing down the cooling of the plasma by providing some heating of its own.