The relative prevalence of wave-packets and coherent structures in the inertial and kinetic ranges of turbulence as seen by Solar Orbiter

The Solar Orbiter (SO) mission provides a unique opportunity to study the evolution of turbulence in the solar wind across different distances from the sun and different plasma conditions. We use SO observations of extended intervals of homogeneous solar wind turbulence to investigate under what conditions the turbulent cascade in the solar wind is supported by either, or both of two distinct phenomenologies, (i) wave- wave interactions and (ii) coherent structure formation and interaction.

We identify nine Solar Orbiter observations of extended intervals of homogeneous solar wind turbulence where each interval is over 10 hours long without current-sheet crossings and other large events. We perform a systematic scale-by-scale decomposition of the observed magnetic field using two wavelets that are known to discriminate between wave-packets and discontinuities, the Daubechies 10 (Db10) and Haar respectively.

A characteristic of turbulence is that the probability distributions (pdfs) of fluctuations obtained on small scales exhibit extended supra-Gaussian tails, and as the scale is increased, the moments decrease and there is ultimately a cross-over to Gaussian pdfs at the outer scale of the turbulence. Using quantile quantile plots, we directly compare the fluctuations pdfs obtained from Haar and Db10 decompositions. This reveals three distinct regimes of behaviour. On larger scales, deep within the inertial range (IR), both the Haar and Db10 decompositions give essentially the same fluctuation pdfs. On the smallest scales, deep within the kinetic range (KR), the pdfs are distinct in that the Haar wavelet fluctuations have a much broader distribution, and the largest fluctuations are associated with coherent structures. On intermediate scales, that span the IR-KR scale break identified from the power spectra, the pdf is composed of two populations, a core with a common pdf functional form for the Haar and Db10 fluctuations, and extended tails where the Haar fluctuations dominate. This establishes a cross-over between wave-packet dominated phenomenology in the IR, to coherent structure dominated phenomenology in the KR. We find that the intermediate range of scales is quite narrow around 0.9 au so that the crossover from wave-packet to coherent structure dominated phenomenology is quite abrupt. At around 0.3au, the crossover occurs over a broader range of scales extending down to the 0.25s scale and up to 4s.

As coherent structures and wave-wave interactions have been proposed as candidates to mediate the turbulent cascade, these results offer new insights into the evolution of the turbulent cascade with distance from the sun.