The Relative Prevalence of Wave Packets and Coherent Structures in the Inertial and Kinetic Ranges of Turbulence as Seen by Solar Orbiter

Abstract

The Solar Orbiter (SO) mission provides the opportunity to study the evolution of solar wind turbulence. We use SO observations of nine extended intervals of homogeneous turbulence to determine when turbulent magnetic field fluctuations may be characterized as: (i) wave packets and (ii) coherent structures (CSs). We perform the first systematic scale-by-scale decomposition of the magnetic field using two wavelets known to resolve wave packets and discontinuities, the Daubechies 10 (Db10) and Haar, respectively. The probability distribution functions (PDFs) of turbulent fluctuations on small scales exhibit stretched tails, becoming Gaussian at the outer scale of the cascade. Using quantile–quantile plots, we directly compare the wavelet fluctuations PDFs, revealing three distinct regimes of behavior. Deep within the inertial range (IR) both decompositions give essentially the same fluctuation PDFs. Deep within the kinetic range (KR) the PDFs are distinct as the Haar decompositions have larger variance and more extended tails. On intermediate scales, spanning the IR–KR break, the PDF is composed of two populations: a core of common functional form containing ∼97% of fluctuations, and tails that are more extended for the Haar decompositions than the Db10 decompositions. This establishes a crossover between wave-packet (core) and CS (tail) phenomenology in the IR and KR, respectively. The range of scales where the PDFs are two-component is narrow at 0.9 au (4–16 s) and broader (0.5–8 s) at 0.4 au. As CS and wave–wave interactions are both candidates to mediate the turbulent cascade, these results offer new insights into the distinct physics of the IR and KR.