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.

Energetic Proton Dropouts During the Juno Flyby of Europa Strongly Depend on Magnetic Field Perturbations

By Hans Huybrighs (DIAS)

In September 2022 NASA's Juno spacecraft flew by Jupiter's ocean moon Europa. In this study we analyzed energetic ion dropouts measured near Europa. We care about energetic ions because they bombard Europa's icy surface . While they make the surface inhospitable, they might also help produce oxidants from the ice that could form a source of energy for life in the ocean.

The Juno measurements show what happens with the energetic protons at 350 km above the surface, in Europa’s wake (considering the general sense of motion of the protons). There, protons have disappeared, compared to further away where they are abundant. The cartoon below shows some classical ideas about what happens to the protons near Europa (panel 1-2).

Using particle tracing simulations of the ions we can show that the deflections by magnetic fields (3) are also important. In fact, at 100 keV magnetic deflections are the dominant factor that determine the ion abundance. At 1 MeV its a combination of magnetic deflections and the effect shown in panel 2. The simulations also show that the shape of the proton dropout region depends on the exact configuration of the magnetic field, which can vary depending on the atmosphere and electron beams in the wake. All this helps us better understand what happens with the energetic ions and shows us that our ideas of Europa's atmosphere and magnetic fields are reasonable but that improvements can be made.

See publication for details:

Huybrighs, H. L. F., Cervantes, S., Kollmann, P., Paranicas, C., Bowers, C. F., Cao, X., et al. (2025). Energetic proton dropouts during the Juno flyby of Europa strongly depend on magnetic field perturbations. Journal of Geophysical Research: Space Physics, 130, e2025JA034000. https://doi.org/10.1029/2025JA034000

Estimating Soft X-Ray Emission from Uranus's Magnetosheath

By Dan Naylor (Lancaster University)

Soft X-rays can be generated within planetary magnetosheaths due to charge exchange between neutrals and highly charged solar wind ions such as O^7+. Imaging of the soft X-rays is an emerging technology that aims to provide global and dynamic views of the magnetosheath and cusps, and their response to solar wind driving. The ESA-CAS SMILE mission will soon be launched with a soft X-ray imager (SXI) instrument onboard to investigate the terrestrial magnetosheath. We explore the viability of similar investigations at Uranus.
Uranus has one of the most unusual and complex environments in the solar system. A large obliquity combined with a highly tilted, offset magnetic axis result in an asymmetric and constantly varying magnetosphere where the plasma and neutral source rates from the moons are unconstrained. We impose a simple bullet shaped magnetopause and moon tori informed by Voyager 2 observations to predict soft X-ray emission from the Uranian magnetosheath. We estimate volumetric emission rates of soft X-rays are on the order of 10^-10 photon cm^-3 s^-1, being higher at equinox due to the orientation of the magnetosheath relative to the moon tori. Simple estimates of intensity and flux find that a SMILE-like instrument could detect ~100 photons in a quarter of a planetary rotation at a distance of 212 R_U, as shown in the figure. A hypothetical future imager, with improved FOV and effective area, would detect ~20,000 photons per planetary rotation at 100 R_U. These are promising initial results that suggest imaging of the magnetosheath is possible within key system timescales. Future studies will include magnetospheric cusps and a full range of solar wind ions, which are anticipated to increase emission rates.

Modelled intensity maps for a SMILE‐like SXI at 212 R_U from different viewing geometries at (top row) equinox, where the neutrals are edge-on to the Sun, and (bottom row) solstice, where the neutrals are ring on to the Sun: (a/d) front‐on, (b/e) top‐down and (c/f) side‐on. The different panels show that the amount of flux detected is dependent on viewing position, and an orbital mission should consider the implications of different possible imaging positions.

See publication for details:

Naylor, D., Ray, L. C., Dunn, W. R., Jasinski, J. M., & Paty, C. (2025). Estimating soft X-ray emission from Uranus's magnetosheath. Journal of Geophysical Research: Space Physics, 130, e2025JA034171. https://doi.org/10.1029/2025JA034171