Evolution of power in coherent structures across scales and heliocentric distance in solar wind turbulence

Solar Orbiter observations of homogeneous turbulence at various solar wind conditions are used to estimate the power that is carried by coherent structures above a threshold across the turbulent cascade [1]. Turbulence is a potential mechanism heating the solar wind. Both wave-wave interactions and coherent structures are mechanisms that may mediate the turbulent cascade. Coherent structures have been found to be sites of dissipation.

Following the method first proposed by Bendt & Chapman (2025) [2] a threshold is determined above which fluctuations may be coherent structures. We find that the percentage of power carried by coherent structures (LIM-P) is significant, increasing with increasing frequency and maximising at ~50% just below the scale break where the inertial range transitions to the kinetic range. At distances <0.4 AU the increase of this percentage follows a roughly linear trend. Beyond 0.4 AU, there are two subranges in the inertial range. In the kinetic range, the LIM-P decreases approximately linearly with increasing frequency. We generally find more power in coherent structures in parallel than perpendicular fluctuations. Within 0.4 AU this degree of anisotropy does not vary across inertial and kinetic ranges. Beyond 0.4 AU, there is successively more power in coherent structures perpendicular than parallel fluctuations.

If coherent structures do indeed dissipate to heat the solar wind, our results, that there is significant power in coherent structures support the idea that coherent structures are important for dissipating energy of the turbulent cascade and thus solar wind heating. The trend of the LIM-P across frequencies suggests that wave-wave interactions at larger scales are systematically supplanted by coherent structures on smaller scales.

[1] Bendt & Chapman (submitted to ApJLett) Fraction of energy carried by coherent structures in the turbulent cascade in the solar wind.

[2] Bendt & Chapman (2025) Ubiquitous threshold for coherent structures in solar wind turbulence. Phys. Rev. Research doi:10.1103/PhysRevResearch.7.023176