Researchers confirm “helicity barrier”, unravelling solar mysteries
Groundbreaking research, led by a PhD student and academic at Queen Mary’s School of Physical and Chemical Sciences, has directly confirmed the existence of the “helicity barrier”. This key discovery advances our understanding of both coronal heating and solar wind acceleration, two of the most enduring challenges in solar physics.
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Using data from NASA’s Parker Solar Probe, Jack McIntyre, a PhD student in the School, and his supervisor Dr Christopher Chen, Reader in Space Plasma Physics, have published the first direct evidence of this phenomenon in Physical Review X. Their discovery helps explain why the Sun’s outer atmosphere, the corona, is millions of degrees hotter than its surface, and how the solar wind, a stream of charged particles streaming from the Sun, is accelerated to supersonic speeds.
For decades, scientists have been puzzled by how energy in the Sun’s turbulent atmosphere transforms into heat, particularly in regions where the plasma is so diffuse that particles rarely collide. One leading theory involves turbulent dissipation, where energy cascades from large-scale motions down to smaller scales and is converted into heat. However, until now, the precise mechanisms at play in this process remained unclear.
McIntyre and Chen’s research changes that. By analysing Parker Solar Probe data, the closest ever measurements of the solar atmosphere, they identified direct evidence of the “helicity barrier”, a previously theoretical effect that blocks the cascade of turbulent energy at small scales. This blockage alters how energy dissipates, influencing how the plasma is heated and explaining several long-standing solar mysteries.
Jack McIntyre said:
"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 study also uncovered the specific physical conditions that allow the helicity barrier to emerge: when magnetic field strength dominates over plasma pressure, and when there’s a strong imbalance between oppositely moving waves within the turbulent plasma. These conditions are often present in the near-Sun environment, where the Parker Solar Probe is currently operating, suggesting that the helicity barrier is a widespread feature of the solar wind.
Dr Christopher Chen 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."
Beyond the Sun, the findings could influence our understanding of other hot, diffuse, and largely collisionless plasmas across the universe. By confirming this barrier using direct observations, our team of researchers has provided a powerful tool for exploring how energy dissipates in some of the most extreme environments in astrophysics.