After identifying the source of the solar wind, the European Solar Orbiter probe has now uncovered the origin of the energy that heats and accelerates this flow, as reported in a newly published study. This significant discovery was made in collaboration with observations from NASA’s Parker probe, which showed that the energy needed to power the solar wind comes from large fluctuations in the Sun’s magnetic field.
Behavior of the Solar Wind
The solar wind is a constant stream of charged particles that escapes from the solar atmosphere, also known as the corona, and flows beyond Earth. There are two types of solar wind: the “fast” wind, composed of particles moving at speeds greater than 500 km/s (1.8 million km/h), which bursts out from coronal holes — regions where magnetic field lines are only connected to the Sun at one end. Then, there is the “slow” solar wind, which does not exceed 500 km/s and is thought to be linked to the Sun’s active regions where sunspots form.
Surprisingly, the fast wind emerges from the solar corona at lower speeds, indicating that it undergoes acceleration as it moves away from the Sun. Despite its high temperature, close to a million degrees, the solar wind naturally cools as it expands and becomes less dense. However, it cools more slowly than expected due to this effect alone, suggesting the presence of poorly understood energy processes.
Alfvén Waves and Magnetic Energy
Data collected by Solar Orbiter and Parker Solar Probe provided conclusive evidence that the acceleration and heating of the fast solar wind appear to be fueled by oscillations in the magnetic field, known as Alfvén waves. Before this research, these waves had been suggested as potential energy sources, but without definitive proof. Yeimy Rivera from the Center for Astrophysics, Harvard & Smithsonian (Massachusetts) and co-lead author of the study, stated that “we didn’t have definitive proof before this work.”
These waves, which only manifest in highly electrified plasmas, can efficiently store and transport energy within the magnetic field.
A Unique Probe Alignment for Precise Measurements
Although both probes operate at different distances from the Sun and on distinct orbits, they aligned in February 2022 to measure the same stream of solar wind. Parker Solar Probe, operating at around 9 million kilometers from the Sun, was the first to pass through this flow, followed a day or two later by Solar Orbiter at a distance of 89 million kilometers. This rare alignment allowed researchers to measure the plasma’s properties and magnetic field fluctuations, quantifying the energy stored in the magnetic field and its influence on the solar wind.
Data analysis revealed that magnetic energy plays a crucial role in the acceleration and behavior of the plasma. “Switchbacks,” which are sudden reversals in magnetic field lines, were also identified as having a significant impact on this acceleration.
The results show that near the Sun, when Parker measured the flow, about 10% of the total energy was contained within the magnetic field. In contrast, at Solar Orbiter, this figure had dropped to only 1%, even though the plasma still accelerated and cooled more slowly than expected. This suggests that the lost magnetic energy contributed to the acceleration and slower cooling of the plasma by providing additional heating.
Implications of These Results for Astrophysics
This study offers new insights into the astrophysical and magnetospheric processes related to the Sun and reinforces the idea that different solar phenomena interact to create a complex environment. As Samuel Badman from the Center for Astrophysics, Harvard & Smithsonian and co-author of the study, explains, “our Sun is the only star in the Universe whose wind we can directly measure. What we have learned about our Sun could apply to other Sun-like stars, and perhaps to other types of stars that have winds.”
The work of the Solar Orbiter and Parker Solar Probe teams highlights the importance of international collaboration in astrophysics, providing a valuable framework for future exploration of our star and its effects within the Solar System.