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Parker Solar Probe flying in front of the sun
The solar atmosphere, or corona, is far hotter than the Sun's surface, a paradox that has puzzled scientists for decades. Furthermore, the constant outflow of plasma and magnetic fields from the Sun, known as the solar wind, is accelerated to incredible speeds. Turbulent dissipation – the process by which mechanical energy is converted into heat – is believed to play a crucial role in both these phenomena. However, in the near-Sun environment, where plasma is largely collisionless, the exact mechanisms of this dissipation have remained elusive.
This new study leverages data from NASA's Parker Solar Probe, which has become the closest spacecraft to the Sun, flying directly through the solar atmosphere. This unprecedented proximity allowed researchers to directly explore this extreme environment for the first time, providing critical data to unravel these mysteries.
The paper presents compelling evidence that the "helicity barrier" is active and profoundly alters the nature of turbulent dissipation. This effect, previously theorised, creates a barrier to the turbulent cascade of energy at small scales, fundamentally changing how fluctuations dissipate and thus how the plasma is heated.
Jack McIntyre, PhD student and the lead author of the study from Queen Mary University of London, commented: "This result is exciting because, by confirming the presence of the 'helicity barrier', we can account for properties of the solar wind that were previously unexplained, including that its protons are typically hotter than its electrons. By improving our understanding of turbulent dissipation, it could also have important implications for other systems in astrophysics."
The research team also identified the specific conditions under which this barrier occurs. They found that the helicity barrier becomes fully developed when the magnetic field strength becomes large compared to the pressure in the plasma and becomes increasingly prominent when the imbalance between the oppositely propagating plasma waves that make up the turbulence is greater. Critically, these conditions are frequently met in the solar wind close to the Sun, where Parker Solar Probe is now exploring, meaning that this effect should be widespread.
Dr Christopher Chen, Reader in Space Plasma Physics at Queen Mary University of London and McIntyre's supervisor, added: "This paper is important as it provides clear evidence for the presence of the helicity barrier, which answers some long-standing questions about coronal heating and solar wind acceleration, such as the temperature signatures seen in the solar atmosphere, and the variability of different solar wind streams. This allows us to better understand the fundamental physics of turbulent dissipation, the connection between small-scale physics and the global properties of the heliosphere, and make better predictions for space weather."
The implications of this discovery extend beyond our own star, as many hot, diffuse astrophysical plasmas in the universe are also collisionless. Understanding how energy dissipates into heat in these environments has broad consequences for astrophysics. The direct observation of the helicity barrier in the solar wind provides a unique natural laboratory to study these complex processes.
