The Sun is currently releasing a stream of electrically charged particles. This is the solar wind. Just how this flow can reach incredible velocities was a mystery that endured for decades. It took two spacecraft and international cooperation to get an answer: The magnetic field near the Sun sometimes gets kinky.
The “fast” solar wind moves at about 500 kilometers (310 miles) per second. But that’s not the speed at which it escapes the Sun’s corona – the extremely hot atmosphere of our star. The wind, like the corona, has a temperature of millions of degrees and it is expected to cool down as it expands, like a spray can cooling down when used. And yet for the solar wind, the cooling happens more slowly.
An animation shows how these kinky waves propagate from the Sun.
Image Credit: NASA’s Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez
It has long been proposed that these two peculiar and unexplained features were connected. There was no clear certainty, but an intriguing proposal was the possibility that Alfvén waves were behind the acceleration. These are large-scale oscillations in the Sun’s magnetic field and there is finally conclusive evidence that this is indeed the case.
“Our study addresses a huge open question about how the solar wind is energized and helps us understand how the Sun affects its environment and, ultimately, the Earth,” Dr Yeimy Rivera, co-leader of the study from the Smithsonian Astrophysical Observatory, said in a statement. “If this process happens in our local star, it’s highly likely that this powers winds from other stars across the Milky Way galaxy and beyond and could have implications for the habitability of exoplanets.”
The discovery was possible thanks to the combined efforts of two incredible probes: NASA’s Parker Solar Probe and the European Space Agency’s Solar Orbiter. The first flyby past the Sun was closer than any other craft before. The latter is looking at our star in a completely different way. Together, they are providing groundbreaking insight into the solar environment.
“We didn’t initially realize that Parker and Solar Orbiter were measuring the same thing at all. Parker saw this slower plasma near the Sun that was full of switchback waves, and then Solar Orbiter recorded a fast stream which had received heat and with very little wave activity,” added Samuel Badman, an astrophysicist at the Center for Astrophysics and the other co-lead of the study. “When we connected the two, that was a real eureka moment.”
Parker measured the energy of the stream and found that about 10 percent of it was in the magnetic field. But when the now fast wind reached Solar Orbiter, the magnetic field energy was just 1 percent. The conclusion was obvious: The magnetic energy was transferred to the solar wind. The data show that a phenomenon called switchbacks, an example of Alfvén waves, was responsible for this exchange.
This infographic shows how the switchbacks help accelerate the solar wind.
Image Credit: ESA
“This new work expertly brings together some large pieces of the solar puzzle. More and more, the combination of data collected by Solar Orbiter, Parker Solar Probe and other missions is showing us that different solar phenomena actually work together to build this extraordinary magnetic environment,” added Daniel Müller, ESA Project Scientist for Solar Orbiter, said in a statement.
The work of the teams on both missions continues as they further explore the connection between magnetic fields and solar wind.
A paper discussing the results is published in Science.
