NASA’s Parker Solar Probe has collected data confirming the existence of the long theorized “helicity barrier,” a threshold where the turbulent dissipation process that converts mechanical energy into heat behaves fundamentally differently near the Sun.
This change to turbulent dissipation alters how plasma heats up, accounting for the long-standing riddle of why the Sun’s corona is even hotter than its surface.
The helicity barrier also affects the behavior of plasma outflows and magnetic fields, contributing to the high speeds of the solar wind, a force that can impact sensitive electrical systems both on Earth and in orbit.
NASA’s Parker Solar Probe
Launched in 2018, NASA’s Parker Solar Probe was designed to unlock the secrets of the Sun. It became not only the fastest object ever built by humans but also, in 2024, the closest any spacecraft has come to the Sun. By flying through the Sun’s atmosphere, the probe provided researchers with unprecedented data about the Sun’s outermost layer, the corona.
The research team analyzed data from the probe’s first 10 orbits, combining multiple datasets to capture a broader spectral range. Over its seven-year mission, Parker completed 24 orbits, each drawing closer to the Sun than the last.
The Helicity Barrier
“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,” said lead author Jack McIntyre, a PhD student at Queen Mary University of London.
“By improving our understanding of turbulent dissipation, it could also have important implications for other systems in astrophysics,” McIntyre added.
In the corona, plasma becomes nearly collisionless due to the helicity barrier, meaning that energy is dissipated through wave-particle interactions rather than particle collisions. This makes the turbulent cascade, which converts energy into heat, highly complex and uneven. The study reveals that specific conditions must be met for the helicity barrier to emerge, primarily a high ratio of magnetic field strength to plasma pressure. A greater imbalance between oppositely propagating plasma waves further enhances the effect.
According to the researchers, these conditions are common in the corona, meaning the helicity barrier is widespread. As solar winds pass through this region, they are heated and accelerated by the influence of the barrier.
“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,” said co-author Dr Christopher Chen, Reader in Space Plasma Physics at Queen Mary University of London.
Astronomical Applications
While understanding the behavior of our nearest star is of obvious importance to Earthlings, given the potential danger to our infrastructure posed by solar winds, the findings present much broader implications for astronomers.
Collisionless behavior has been observed in other hot, diffuse astrophysical plasmas throughout the universe. With this newly confirmed evidence of the helicity barrier, the Parker Solar Probe offers a valuable natural laboratory for studying turbulent dissipation under extreme conditions.
“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,” Chen concluded.
The recent paper, “Evidence for the Helicity Barrier from Measurements of the Turbulence Transition Range in the Solar Wind,” appeared on July 8, 2025, in Physical Review X.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.