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Solar wind impression as a flat design sketch by Lucía Pérez Díaz (https://www.luciaperezdiaz.com) and made available via https://s-ink.org/solar-wind-sketch

Solar wind impression as a flat design sketch by Lucía Pérez Díaz (https://www.luciaperezdiaz.com) available via https://s-ink.org/solar-wind-sketch.

Published: 10 February 2025

New publications

by Fabio Crameri

From the Teams

Publication

Solar Wind

Heliosphere

Multi-spacecraft

Unlocking the Mysteries of the Solar Wind: Turbulence, Expansion, and Energy Dynamics

The solar wind is a remarkable stream of charged particles flowing outward from the Sun, extending beyond the planets into a vast region called the heliosphere. This flow of plasma holds the key to understanding the origin, evolution, and potential habitability of stellar systems. While spacecraft like NASA’s Parker Solar Probe and the European Space Agency’s Solar Orbiter have given us a front-row seat to observe the Sun’s activity, the puzzle of how the solar wind evolves as it expands into space remains a challenging and fascinating area of study.

A Tale of Two Studies

Recent research by members of the ISSI Team 591 offers fresh insights into the energetics of the solar wind. One study published in A&A by Andrea Verdini examined the turbulence and energy decay of the solar wind using data from the WIND spacecraft at Earth’s orbit (1 AU). Another, conducted by Rivera et al. and put into perspective in Science by Luca Sorriso-Valvo and his colleague, analysed simultaneous observations from the Parker Solar Probe and Solar Orbiter when they were aligned, tracking the same solar wind stream close to the Sun.

Together, these studies reveal the complex interplay between large-scale solar dynamics and small-scale turbulence as the solar wind moves outward.

What Happens as the Solar Wind Expands?

The solar wind originates in the Sun’s outer atmosphere, where magnetic reconnection and plasma waves inject energy into the stream. However, as it moves away from the Sun, the solar wind does not behave as a simple adiabatic system (one that neither gains nor loses heat). Instead, it continues to be heated, accelerated, and perturbed by multiple factors, including:

Magnetic Switchbacks: Sudden reversals of the magnetic field that inject energy.
Plasma Waves and Turbulence: Alfvén waves and cascading vortices that transfer energy across scales, eventually converting it into heat.
Thermal and Mechanical Expansion: The natural expansion of the solar wind slows down large-scale fluctuations while allowing small-scale turbulence to dominate.

Using advanced analysis techniques, researchers of the ISSI Team have found that turbulence in the solar wind operates like a two-phase system. At large scales, expansion dissipates energy, while at smaller scales, turbulence dissipates it into heat. This delicate balance shapes the solar wind’s structure and its ability to transport energy across the heliosphere.

The Importance of Multi-Spacecraft Observations

A major breakthrough in understanding solar wind energetics came when the Parker Solar Probe and Solar Orbiter were radially aligned, capturing the same stream of solar wind at different distances. These rare observations revealed how energy evolves as the wind moves farther from the Sun, confirming that plasma waves and turbulence contribute significantly to heating and maintaining the solar wind’s properties.

Illustration of NASA’s Parker Solar Probe flying close to the sun’s corona and collecting measurements.

Why It Matters

Studying the solar wind isn’t just about understanding our Sun—it’s about unlocking universal principles of plasma physics. The processes at work in the heliosphere are thought to occur in other astrophysical systems, such as protostellar disks, supernova remnants, and galactic jets. Moreover, the solar wind’s dynamics provide clues about the habitability of exoplanets in other stellar systems by shaping the space environments around stars.

Looking Ahead

As more spacecraft like Parker Solar Probe, Solar Orbiter, BepiColombo, and STEREO continue to collect data, collaborative international efforts will enable new discoveries. Future alignments of spacecraft will allow researchers to disentangle the complexities of solar wind energetics with greater precision.

Studying how the Sun drives the heliosphere helps us uncover fundamental processes in plasma physics and space weather, which influence planetary environments and technological systems.

Want to learn more? Stay tuned as the ISSI Team 591 is bringing us closer to understanding our dynamic Sun!

Credits ESA/Webb, NASA & CSA, A. Martel