Our sun drives a constant outgoing stream of plasma, or ionized gas, called the solar wind, which envelops our solar system. Outside Earth’s protective magnetosphere, the fastest solar wind roars past at speeds of more than 310 miles (500 kilometers) per second. But researchers haven’t figured out how the wind gets enough energy to reach that speed.
Our team of heliophysicists published a paper in August 2024 pointing to a new energy source powering the solar wind.
Discovery of the solar wind
Physicist Eugene Parker predicted the existence of the solar wind in 1958. The Mariner spacecraft, en route to Venus, would confirm its existence in 1962.
Since the 1940s, studies have shown that the Sun’s corona, or atmosphere, can reach very high temperatures: more than 2 million degrees Fahrenheit (or more than 1 million degrees Celsius).
Parker’s research showed that this extreme temperature could create an outward thermal pressure strong enough to overcome gravity and cause the sun’s outer atmosphere to escape.
However, gaps in solar wind science soon emerged as researchers made increasingly detailed measurements of the solar wind near Earth. They discovered two problems in particular with the fastest part of the solar wind.
First, the solar wind continued to heat up after it unexplainedly left the hot corona. And even with this extra heat, the fastest wind still didn’t have enough energy for scientists to explain how it was able to accelerate to such high speeds.
Both observations implied that an additional energy source must exist beyond Parker’s models.
Alfvén waves
The sun and its solar wind are plasmas. Plasmas are like gases, but all particles in plasmas have a charge and respond to magnetic fields.
Similar to how sound waves travel through air and transport energy on Earth, plasmas have what are called Alfvén waves that travel through them. For decades, it was predicted that Alfvén waves would affect the dynamics of the solar wind and play a major role in transporting energy in the solar wind.
However, scientists could not determine whether these waves were actually interacting directly with the solar wind or whether they were generating enough energy to power the wind. To answer these questions, they would have to measure the solar wind very close to the sun.
In 2018 and 2020, NASA and the European Space Agency launched their respective flagship missions: the Parker Solar Probe and the Solar Orbiter. Both missions carried the right instruments to measure Alfvén waves near the Sun.
Solar Orbiter ventures between 1 astronomical unit, where Earth is, and 0.3 astronomical units, slightly closer to the sun than Mercury. Parker Solar Probe dives much deeper. It comes as close as five solar diameters from the sun, within the outer edges of the corona. Each solar diameter is about 865,000 miles (1,400,000 kilometers).
If both missions are flown together, researchers like us will be able to study not only the solar wind close to the Sun, but also how it changes between the point where Parker sees it and the point where Solar Orbiter sees it.
Magnetic hairpin bends
When Parker first approached the Sun closely, he saw that the solar wind near the Sun was indeed rich in Alfvén waves.
Scientists used Parker to measure the solar wind’s magnetic field. At some points, they noticed that the field lines—or lines of magnetic force—waved with such high amplitudes that they briefly reversed direction. Scientists called these phenomena magnetic switchbacks. Together with Parker, they observed these energetic plasma fluctuations throughout the nearby solar wind.
Our research team wanted to see if these switchbacks had enough energy to accelerate and heat the solar wind as it moved away from the sun. We also wanted to see how the solar wind changed as these switchbacks gave up their energy. That would help us determine whether the energy from the switchbacks was going to heating the wind, accelerating it, or both.
To answer these questions, we identified a unique spacecraft configuration, where both spacecraft crossed the same portion of the solar wind, but at different distances from the Sun.
The secret of the hairpin bends
Parker, close to the Sun, observed that about 10% of the solar wind energy was in magnetic switchbacks, while Solar Orbiter measured this as less than 1%. This difference means that between Parker and Solar Orbiter this wave energy was being transferred to other forms of energy.
We did some modeling, just as Eugene Parker had done. We built on modern implementations of Parker’s original models and incorporated the influence of the observed wave energy into these original equations.
By comparing both datasets and the models, we could see specifically that this energy was contributing to both acceleration and heating. We knew it was contributing to acceleration because the wind was faster at Solar Orbiter than it was at Parker. And we knew it was contributing to heating because the wind was hotter at Solar Orbiter than it would have been if the waves weren’t there.
These measurements showed us that the energy of the hairpin bends was both necessary and sufficient to explain the evolution of the solar wind as it moves away from the Sun.
Our measurements not only give scientists insight into the physics of the solar wind and how the Sun can affect Earth, but they could also have implications for the entire universe.
Many other stars have stellar winds that carry their material into space. Understanding the physics of our local star’s solar wind also helps us understand stellar winds in other systems. Learning more about stellar winds can tell researchers more about the habitability of exoplanets.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and reliable analysis to help you understand our complex world. It was written by: Yeimy J. Rivera, Smithsonian Institute; Michael L. Stevens, Smithsonian Instituteand Samuel Badman, Smithsonian Institute
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Yeimy J. Rivera receives funding from NASA’s Parker Solar Probe project through SAO/SWEAP subcontract 975569.
Michael L. Stevens receives funding from NASA’s Parker Solar Probe project through SAO/SWEAP subcontract 975569.
Samuel Badman receives funding from NASA’s Parker Solar Probe project through SAO/SWEAP subcontract 975569.