by Our Sun drives out a continuous flow of plasma, or ionized gas, called the solar wind, which covers our solar system. Outside of Earth’s protective magnetosphere, the fastest solar wind moves at speeds in excess of 310 miles (500 kilometers) per second. But researchers haven’t been able to figure out how the wind gets enough energy to reach that speed – until now.
Our team of helophysicists published a paper in August 2024 that indicates a new source of energy driving the solar wind.
Discovery of solar wind
Physicist Eugene Parker predicted the existence of the solar wind in 1958. The Mariner spacecraft, which went to Venus, confirmed its existence in 1962.
Since the 1940s, studies have shown that the Sun’s corona, or solar atmosphere, can heat up to extremely high temperatures – over 2 million degrees Fahrenheit (or more than 1 million degrees Celsius).
Parker’s work suggested that this extreme temperature could create a thermal outward pressure strong enough to overcome gravity and cause the outer layer of the Sun’s atmosphere to escape.
Gaps in solar wind science quickly emerged, however, as researchers made larger and more detailed measurements of the near-Earth solar wind. In particular, they found two problems with the fastest part of the solar wind.
In one case, the solar wind continued to heat up after leaving the hot corona unexplained. 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 so high.
These two observations meant that there was a need for an additional energy source outside of Parker’s models.
Alfven waves
The Sun and its solar wind are plasmas. Plasma is like gas, but all the particles in plasma have a charge and respond to magnetic fields.
Similar to how sound waves travel through air and carry energy on Earth, plasmas have something called Alfvén waves moving through them. For years, it was predicted that Alfvén waves would affect the dynamics of the solar wind and play an important role in transporting energy in the solar wind.
However, scientists could not say whether these waves interacted directly with the solar wind or generated enough energy to power it. 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.
The Solar Orbiter ventures between 1 astronomical unit, where Earth is, and 0.3 astronomical units, slightly closer to the Sun than Mercury. The Parker Solar Probe dives much deeper. It gets as close as five solar diameters from the Sun, within the outer edges of the corona. Each sun’s diameter is about 865,000 miles (1,400,000 kilometers).
With these two missions operating together, researchers like us can not only examine the solar wind close to the Sun, but we can also study how it varies from where Parker sees it and the point where the Solar Orbiter sees it.
Magnetic returns
In Parker’s first close approach to the Sun, he observed that the solar wind near the Sun was abundant with Alfvén waves.
The scientists used Parker to measure the magnetic field of the solar wind. At certain points they observed the field lines – or lines of magnetic force – waving at such high amplitude that they briefly reversed direction. The scientists called these phenomena a magnetic back switch. Together with Parker, they observed these plasma fluctuations in which energy is everywhere in the solar wind near the Sun.
Our research team wanted to find out if there was enough power in these switches to accelerate and heat the solar wind as it traveled away from the Sun. We also wanted to examine how the solar wind changed as the switches turned their energy back up. That would help us determine whether the energy of the returns was going in to heat the wind, accelerate it, or both.
To answer these questions, we identified a unique spacecraft configuration in which both spacecraft traversed the same part of the solar wind, but different distances from the Sun.
The secret of the switchback
Parker, close to the Sun, noted that about 10% of the solar wind’s energy lived in magnetic fluxes, while Solar Orbiter measured less than 1%. This difference, between Parker and the Solar Orbit, means that this wave energy has been converted into other forms of energy.
We did some modeling, like Eugene Parker. We constructed modern implementations of Parker’s original models and incorporated the effect of the observed wave energy into these original equations.
By comparing the data sets and the models, we could specifically see that this energy contributed to both acceleration and heating. We knew it contributed to acceleration because the wind was faster at Solar Orbit than Parker. And we knew that it helped with warming, because the wind was warmer at the Solar Orbiter than it would be if the waves weren’t present.
These measurements told us that the energy from the feedbacks was necessary and sufficient to explain the evolution of the solar wind as it traveled away from the Sun.
Not only does our measurement tell scientists about the physics of the solar wind and how the Sun can affect Earth, but it could also have implications across the globe.
Many other stars have stellar winds that carry their material out into space. Understanding the physics of the solar wind of our local star also helps us understand the stellar wind in other systems. Learning about a star’s wind could tell researchers more about exoplanet habitability.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Yeimy J. Rivera, Smithsonian Institution; Michael L. Stevens, Smithsonian Institutionand Samuel Badman, Smithsonian Institution
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Yeimy J. Rivera receives funding from NASA’s Parker Solar Probe project through subcontract SAO/SWEAP 975569.
Michael L. Stevens receives funding from NASA’s Parker Solar Probe project through subcontract SAO/SWEAP 975569.
Samuel Badman receives funding from NASA’s Parker Solar Probe project through subcontract SAO/SWEAP 975569.
