Fraction of energy carried by coherent structures in the turbulent cascade in the solar wind
By Alina Bendt (SERENE, School of Engineering, University of Birmingham)
Turbulence is a highly disordered state of flow. It is ubiquitous in astrophysical plasma flows. Turbulence is a proposed mechanism to heat the solar wind, though to what extent turbulence can heat and drive the solar wind is yet an open question. Coherent structures are known to be sites of enhanced dissipation. We use the method proposed by Bendt & Chapman (2025) to distinguish between wave-packets and coherent structures in magnetic field observations by Solar Orbiter and to determine the power that is carried by coherent structures across the inertial (MHD, intermediate scales) and kinetic (small scales) ranges.
We find that coherent structures carry up to a maximum of 50% of the total power in magnetic field fluctuations. In the inertial range, from large to small scales, the percentage of power carried in coherent structures increases roughly linearly at distances less than 0.4 au from the Sun. At larger distances, there are two subranges in the inertial range. In the kinetic range, the percentage of power in coherent structures decreases approximately linearly towards smaller scales.
Our result of a significant percentage of the total power being carried in coherent structures supports the idea that coherent structures are important for turbulent heating of the solar wind. We also provide first insight into the recently discovered behaviour of two subranges in the inertial range.
Reference: Bendt & Chapman 2026 ApJL doi: https://doi.org/10.3847/2041-8213/ae3820
Bendt & Chapman 2025 PhysRevRes doi: https://doi.org/10.1103/PhysRevResearch.7.023176
See publication for details:
A. Bendt and S. C. Chapman 2026 Fraction of Energy Carried by Coherent Structures in the Turbulent Cascade in the Solar Wind ApJL https://iopscience.iop.org/article/10.3847/2041-8213/ae3820
Power in coherent structures as a function of frequency. Results are plotted for the magnetic field component B⟂(BxVsw). Left to right, the panels group the intervals by heliocentric distance: panels (a), (d) R < 0.4 au; panels (b), (e) 0.4 ≤ R < 0.8 au; and panels (c), (f) R ≥ 0.8 au. Upper panels plot the percentage of power in coherent structures LIM-P(fn) and lower panels overplot the power spectral density of coherent structures (purple ×, grey shading) on the total power (purple ⋆) for one of these intervals. On all panels, black vertical lines denote the 1 hr, 1 minute, and 1 s timescales. On upper panels, the vertical grey shading indicates the range of frequencies of the ion-gyro radius of all intervals. The of the single interval shown in the lower panels is indicated by a black vertical line. For the different intervals in the upper panels, the colours denote plasma beta, β < 0.5 (blue), 0.5 ≤ β < 2 (red), and β≥2 (black). Field-alignment angle value (range 0°–90° obtained by folding in angles ≥90°): θ < 20° (+), 20°–60° (∘), and θ ≥ 60° (△).