Magnetic waves behind decades-long mystery of solar wind acceleration

A team of scientists compared data from NASA’s Parker Solar Probe and ESA’s Solar Orbiter and discovered that magnetic waves known as Alfvén waves, essentially a form of plasma wave, are responsible for pumping energy into the solar wind, causing it to accelerate and heat up as it travels away from the Sun.

• The study sought to answer a decades-old mystery regarding why the solar wind increases hotter and quicker than expected as it travels through space.
• By studying data from ESA’s Solar Orbiter and NASA’s Parker Solar Probe spacecraft as they went through the same stream of solar wind at various periods, researchers concluded that the observed acceleration and heating were caused by the dissipation of Alfvén waves.

The study addressed a decades-old solar problem that has perplexed scientists since the beginning of the Space Age and discovered that plasma waves, known as Alfvén waves, are responsible for pumping energy into the solar wind, causing it to speed and heat up as it exits the Sun’s outer atmosphere or corona.

This discovery is an essential step toward understanding why the solar wind, a stream of charged particles released by the Sun, becomes significantly hotter and faster as it goes away from the Sun than theoretical models predicted.

While the Parker Solar Probe collected data from around 9 million km (5.4 million miles) away from the Sun, ESA’s Solar Orbiter detected the same solar wind stream about two days later at a distance of around 89 million km (55 million miles), near the orbit of Venus.

It was the fortunate alignment of the spacecraft that enabled scientists to sample the same stream of solar wind at separate stages along its voyage from the Sun.

Parker Solar Probe first studied the stream’s attributes, including speed and temperature. When the Solar Orbiter encountered the same stream two days later, it discovered that the plasma had accelerated and increased in temperature beyond what was expected.

After comparing their data, the researchers concluded that the increased energy in the solar wind was due to the dissipation of Alfvén waves. These discoveries represented a substantial advance in understanding the physics of the solar wind and its interactions with the greater heliosphere.

The essential data for this discovery were gathered in late February 2022 during a rare alignment of the Solar Orbiter and Parker Solar Probe. The findings were released on August 29, 2024, following extensive study and cross-verification by the scientific team.

Understanding the solar wind is crucial because it influences space weather, which can seriously affect Earth. The solar wind interacts with the Earth’s magnetosphere, affecting satellite operations, GPS systems, and even power grids.

The acceleration and heating of the solar wind have long been poorly understood, but this study demonstrates that Alfvén waves, large-scale oscillations in the Sun’s magnetic field, are the primary cause of these phenomena. These waves explain why the solar wind does not cool as quickly as expected and why it increases in speed as it goes away from the Sun.

This finding sheds light on the Sun and has far-reaching implications for understanding other stars with similar winds throughout the cosmos. The researchers’ work constituted a significant step forward in heliophysics research.

“The observations show heating and acceleration of the plasma between the outer edge of the corona and near the orbit of Venus, along with the presence of large-amplitude Alfvén waves. We calculate that the damping and mechanical work performed by the Alfvén waves are sufficient to power the heating and acceleration of the fast solar wind in the inner heliosphere,” the researchers concluded.

References:

¹ In situ observations of large-amplitude Alfvén waves heating and accelerating the solar wind – Yeimy J. Rivera et al. – Science 385,962-966(2024) – August 29, 2024 – 10.1126/science.adk6953

² Solar Orbiter shows how solar wind gets a magnetic push – ESA – August 29, 2024

Harsha Borah

Harsha Borah is an experienced content writer with a proven track record in the industry. Harsha has worked with LitSpark Solutions and Whateveryourdose, honing skills in creating engaging content across various platforms. A gold medalist in a state-level writing competition organized by Assam Tourism, Harsha’s travelogue on Tezpur was widely appreciated. Harsha’s article, "The Dark Tale of the Only Judge in India to Be Hanged," ranks second on Google and has garnered over 11 000 views and 8 900 reads on Medium. Outside of writing, Harsha enjoys reading books and solving jigsaw puzzles.

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