Structure in the Unstructured: New Phenomenon Discovered in the Solar Wind TU Braunschweig discovers new mechanism of plasma turbulence in the solar wind

Everyone is familiar with turbulence, whether it is milk being stirred into coffee or atmospheric turbulence that causes an aeroplane to shake. In space, however, turbulence behaves differently and still poses a mystery to Physics.  An international research team led by Technische Universität Braunschweig has now demonstrated, for the first time, a reversal phenomenon of magnetic field rotation in space turbulence. A structure emerges within the unstructured space plasma. in the findings have now been published in The Astrophysical Journal Letters.

The team from the Institute of Theoretical Physics at Technische Universität Braunschweig, in collaboration with researchers from a total of ten countries, has demonstrated a fundamentally new phenomenon. Using high-precision magnetic field and particle data from NASA’s Parker Solar Probe mission, the team showed directly for the first time that the turbulent magnetic fields exhibit a clear reversal of the magnetic field rotation sense over a certain wavelength. They even demonstrated that the reversal grows and shifts toward longer wavelengths in the radial direction away from the Sun. The reversal coincides with the transition from large-scale, fluid-like dynamics to small-scale, particle-like dynamics.

A small-scale phenomenon with major implications

The physics of turbulence in space differs from the phenomenon here on Earth. Matter – primarily gas and dust – exists there in what is known as a plasma state. This arises through ionisation: the gas contains so much energy that the electrons are stripped from the atoms or molecules. Firstly, space plasma is strongly coupled to the electromagnetic fields in space. Furthermore, it is collisionless, meaning that there are only a few particles spread out over a large volume within the plasma. As a result, the distances between these particles are very large and they do not collide with one another. These properties mean that matter behaves differently in turbulence, raising questions in Physics. How does the plasma evolve in turbulence? How is energy distributed in collisionless turbulence?

“On larger length scales, space plasma behaves like a fluid, and turbulence develops through excitation of vortices and electromagnetic waves. The fluctuations become random, that is, turbulent. However, the magnetic field rotation is not random. The fields preferentially rotate counterclockwise when seeing radially away from the Sun. The rotation, however, looks different on smaller length scales. The fields rotate clockwise when one looks at the turbulent plasma on smaller length scales. This scale just happens to be at the transition scale from flow-like behaviour in the plasma to particle-like behaviour. “We see a clear structure in the spectrum, even though the turbulence represents unstructured motions,” says Dr. Masatomi Iizawa, lead author of the study.

The results are of great significance for both plasma physics and astrophysics. The new finding is that this structure can be seen even at the scale where the plasma stops behaving like a fluid and individual charged particles take over. This scale lies beyond the reach of theories that treat plasma as a fluid and has received little theoretical attention, so the result sheds light on a part of turbulence that has remained largely unexplored. Plasma turbulence plays an important role in astrophysics, for example in the Earth’s magnetosphere, the solar wind, solar flares, as well as in interplanetary space and in accretion discs. Turbulence in space plasma can influence the shock waves and magnetic clouds driven by coronal mass ejections, the massive eruptions that often accompany solar flares. It is therefore a very important factor in predicting geomagnetic storms, which can have various effects ranging from the appearance of the Northern Lights to disruptions in electricity grids.

The project is currently being led by the Institute of Theoretical Physics at TU Braunschweig and is funded by the German Research Foundation (DFG).