The European Space Agency’s Solar Orbiter spacecraft has made groundbreaking strides in understanding the enigmatic forces that propel and heat the solar wind. For decades, scientists have puzzled over the origins of the energy that accelerates this stream of charged particles as it escapes the sun’s corona, the sun’s outer atmosphere. Now, thanks to the combined efforts of Solar Orbiter and NASA’s Parker Solar Probe, researchers have pinpointed the source of this energy: large-scale fluctuations in the sun’s magnetic field, specifically in the form of Alfvén waves.
The Mystery of Solar Wind Acceleration
The solar wind is a continuous flow of charged particles, primarily electrons and protons that emanate from the sun and travel through space, occasionally interacting with Earth’s atmosphere to create stunning auroras. However, a long-standing question has been how this wind accelerates from its initial lower speeds in the corona to the astonishing velocities observed in space—often exceeding 1.8 million km/h (500 km/s). Furthermore, the solar wind cools more slowly than expected as it expands, suggesting an additional heating source.
Alfvén Waves: The Missing Piece
In a recent study published in Science, researchers have provided conclusive evidence that Alfvén waves—oscillations in the sun’s magnetic field—are responsible for both the acceleration and heating of the solar wind. These magnetic waves carry significant energy and can efficiently transfer it through the sun’s plasma, a state of matter formed when gas is heated to extreme temperatures and becomes electrified.
Yeimy Rivera, co-author of the study from the Center for Astrophysics, Harvard & Smithsonian, explains that while Alfvén waves were previously suspected as the energy source, definitive proof was lacking until now. Both Solar Orbiter and Parker Solar Probe possess the sophisticated instruments necessary to measure the properties of the solar plasma, including its magnetic field, allowing for this groundbreaking discovery.
A Rare Alignment of Spacecraft
In February 2022, a fortuitous alignment of the Solar Orbiter and Parker Solar Probe allowed scientists to compare measurements from the same solar wind stream at different stages of its journey. Parker, located at the outer edges of the sun’s corona, measured the solar wind at 9 million kilometres from the sun, while Solar Orbiter, farther out at 89 million kilometres, captured data from the same stream a day or two later.
This unique alignment enabled researchers to calculate the energy in the solar wind’s magnetic field at both locations. By comparing the energy measured by Parker to that recorded by Solar Orbiter, they discovered that around 10% of the energy close to the sun was stored in the magnetic field. This energy decreased to just 1% when the solar wind reached Solar Orbiter’s position, indicating that the “lost” magnetic energy was being converted into the wind’s acceleration and heating.
The Role of Switchbacks
A key aspect of this discovery is the role of “switchbacks,” sharp deflections in the sun’s magnetic field lines. These switchbacks, observed since the early solar probes of the 1970s, are now understood to be manifestations of Alfvén waves. Parker Solar Probe’s recent observations revealed that these switchbacks are more common and influential than previously thought, clustering in patches containing enough energy to account for the solar wind’s acceleration and heating.
Implications for Stellar Physics
This research sheds light on the processes driving our solar system’s solar wind and offers insights into the behaviour of winds from other sun-like stars. According to Samuel Badman, co-author of the study, the findings from our sun could apply to other stars in the universe, enhancing our understanding of stellar winds more broadly.
As the team continues to analyze data, they aim to explore whether the sun’s magnetic field energy also influences the slower forms of the solar wind, potentially revealing a universal mechanism for solar and stellar wind acceleration.
This new understanding represents a significant leap forward in solar physics, unravelling one of the sun’s enduring mysteries and paving the way for future explorations of our star’s dynamic environment.
Reference:
Yeimy J. Rivera et al., In situ observations of large-amplitude Alfvén waves heating and accelerating the solar wind, Science (2024). DOI: 10.1126/science.adk6953
