Evolution of Magnetohydrodynamic Turbulence in the Expanding Solar Wind: Residual Energy and Intermittency

Shi_时, Chen_辰 (ORCID:0000000225827085); Sioulas, Nikos (ORCID:0000000211289685); Huang_黄, Zesen_泽森 (ORCID:0000000195705975); Velli, Marco (ORCID:0000000223813106); Tenerani, Anna (ORCID:0000000328806084); Réville, Victor (ORCID:0000000229163837)

doi:10.3847/1538-4357/ad9f38

Abstract We conduct 3D magnetohydrodynamic simulations of decaying turbulence in the context of the solar wind. To account for the spherical expansion of the solar wind, we implement the expanding box model. The initial turbulence comprises uncorrelated counterpropagating Alfvén waves and exhibits an isotropic power spectrum. Our findings reveal the consistent generation of negative residual energy whenever nonlinear interactions are present, independent of the normalized cross helicityσ_cand compressibility. The spherical expansion facilitates this process. The resulting residual energy is primarily distributed in the perpendicular direction, withS₂(b) − S₂(u) ∝ l_⊥or equivalently

- E_{r} \propto k_{⊥}^{- 2}

. HereS₂(b) andS₂(u) are second-order structure functions of magnetic field and velocity respectively. In most runs,S₂(b) develops a scaling relation

S_{2} (b) \propto l_{⊥}^{1 / 2}

(

E_{b} \propto k_{⊥}^{- 3 / 2}

). In contrast,S₂(u) is consistently shallower thanS₂(b), which aligns with in situ observations of the solar wind. We observe that the higher-order statistics of the turbulence, which act as a proxy for intermittency, depend on the initialσ_cand are strongly affected by the expansion effect. Generally, the intermittency is more pronounced when the expansion effect is present. Finally, we find that in our simulations, although the negative residual energy and intermittency grow simultaneously as the turbulence evolves, the causal relation between them seems to be weak, possibly because they are generated on different scales.

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