ESA's Solar Orbiter spacecraft, working in conjunction with NASA's Parker Solar Probe, has shed light on the long-standing mystery of what drives the energy required to heat and accelerate the solar wind. The findings suggest that large fluctuations in the sun's magnetic field, known as Alfvén waves, play a crucial role in powering this high-speed phenomenon.

The solar wind is a continuous stream of charged particles that escapes from the sun's corona, extending beyond Earth's atmosphere. It is the interaction between the solar wind and our planet's atmosphere that gives rise to the mesmerizing phenomenon of auroras in the sky.

While the solar wind initially exits the sun's corona at lower speeds, it gains momentum as it moves farther away. This acceleration contradicts expectations, as the wind would naturally cool down as it expands into a larger volume and becomes less dense.

To uncover the energy source responsible for the acceleration and heating of the fastest parts of the solar wind, scientists utilized data collected by ESA's Solar Orbiter and NASA's Parker Solar Probe. In a new study published in the journal Science, researchers presented conclusive evidence that Alfvén waves, generated by large-scale oscillations in the sun's magnetic field, provide the necessary energy boost.

Unlike ordinary gases, which transmit only sound waves, a plasma – such as the sun's superheated atmosphere – responds to magnetic fields and allows the formation of Alfvén waves. These waves store and efficiently transport energy through the plasma. Both Solar Orbiter and Parker Solar Probe carry instruments capable of measuring key properties of the plasma, including its magnetic field.

A fortuitous alignment in February 2022 allowed both spacecraft to sample the same stream of solar wind, despite operating at different distances from the sun. Comparisons of the measurements taken by the two probes revealed that the energy stored in the magnetic field, known as the wave energy flux, played a vital role in driving the acceleration of the solar wind.

Close to the sun, Parker Solar Probe detected that approximately 10% of the total energy was contained in the magnetic field. As the solar wind reached Solar Orbiter, this percentage dropped to just 1%. However, the plasma continued to accelerate and cool more slowly than predicted. This discrepancy led the team to conclude that the lost magnetic energy was responsible for both the acceleration and the slower cooling of the plasma.

Furthermore, the data highlighted the significance of magnetic structures called switchbacks in the acceleration of the wind. These switchbacks, characterized by large deflections in the sun's magnetic field lines, are examples of Alfvén waves. The detection rate of switchbacks has significantly increased since Parker Solar Probe became the first spacecraft to fly through the sun's corona in 2021. The researchers found that these patches of switchbacks contain enough energy to account for the missing portion of the acceleration and heating of the fast solar wind.

Daniel Müller, ESA Project Scientist for Solar Orbiter, expressed the significance of this study in unraveling the complex nature of the sun's magnetic environment. He noted that the combination of data from Solar Orbiter, Parker Solar Probe, and other missions is providing valuable insights into various solar phenomena, potentially applicable to other stars with similar characteristics.

The research team intends to extend their analysis to explore whether the sun's magnetic field energy influences the acceleration and heating of slower forms of the solar wind. As we continue to uncover the secrets of our own star, the implications could extend beyond our solar system, illuminating the behavior of other stars and their winds.

The study, titled "In situ observations of large-amplitude Alfvén waves heating and accelerating the solar wind," was published in Science.

(Note: The content of this article is based on information provided by the European Space Agency and is for informational purposes only.