MIST

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° (△).

Global Morphology of Chorus Waves in the Outer Radiation Belt and the Effect of Geomagnetic Activity and fpe/fce

By Kaine Bunting (British Antarctic Survey)

Chorus waves are naturally occurring plasma waves often observed in the Earth’s outer radiation belt that strongly influence the behaviour of energetic electrons. These waves can both accelerate electrons to relativistic energies, which poses a threat to satellites, as well as scatter electrons into Earth's atmosphere, where they are consequently lost.

The ratio between the electron plasma frequency (fpe) and electron gyrofrequency (fce) holds information on both electron density and magnetic field strength and significantly influences the efficiency of these processes, with electron acceleration being most effective during periods of low fpe/fce.

Bunting et al. (2026) analyses a combined 24.5 years of wave data from three THEMIS satellites to investigate the effect of fpe/fce, geomagnetic activity and normalized frequency on the spatial distribution and intensity of chorus waves.

The strongest waves are generally observed on the dawn-side of the Earth during active geomagnetic conditions. Figure 1 shows global plots of the equatorial (|MLAT| < 9°) chorus wave intensity during active conditions (AE > 200nT). At intermediate relative frequencies (0.3fce < f < 0.4fce), chorus is largely independent of fpe/fce. However, at low frequencies (flhr < f < 0.1fce), strong waves are most often associated with high fpe/fce (>10) and at high frequencies (0.5fce < f < 0.7fce), chorus is strongest at low fpe/fce (<6).

Overall, this study highlights the critical role of fpe/fce on the spatial distribution and dynamic behaviour of chorus waves under varying geomagnetic conditions, as well as its influence on wave-particle interactions. During a geomagnetic storm fpe/fce outside of the plasmapause may gradually change from low to high values over the course of the recovery phase, suggesting that the role of chorus may change from efficient acceleration early in the recovery phase to little or no acceleration and even loss toward the end of the recovery phase.

See publication for details:

Bunting, K. A., Meredith, N. P., Bortnik, J., Ma, Q., Matsuura, R., & Shen, X.-C. (2026). Global morphology of chorus waves in the outer radiation belt and the effect of geomagnetic activity and fpe/fce. Journal of Geophysical Research: Space Physics, 131, e2025JA034737. https://doi.org/10.1029/2025JA034737

Figure 1 - Global maps of the average chorus wave intensity during active geomagnetic conditions (AE > 200nT) in the equatorial region (MLAT < 9°) as a function of  L* and magnetic local time for, from top to bottom, increasing relative frequency, and, from left to right, increasing fpe/fce. The maps extend linearly out to  L* = 10 with noon at the top and dawn to the right. The average intensities are shown in the large panels and the corresponding sampling distributions in the small panels to the bottom right of each large panel.