Universal relations between parallel and perpendicular spectral power law exponents in non-axisymmetric magnetohydrodynamic turbulence

Abstract

Following a general heuristic approach, algebraic constraints are established between the parallel and perpendicular power-law exponents of non-axisymmetric, highly aligned magnetohydrodynamic turbulence, both with and without strong imbalance between the Elsässer variables. Such relations are universal both for the regimes of weak and strong turbulence and are useful to predict the corresponding turbulent power spectra. For scale-dependent alignment, a Boldyrev-type $k^{-3/2}$ perpendicular spectrum emerges transverse to the direction of alignment whereas a $k^{-5/3}$ spectrum is obtained for the same if the alignment becomes scale-independent. However, regardless of the nature of alignment, our analysis consistently yields a $k_{\parallel}^{-2}$ spectrum - commonly observed in both numerical simulations and in-situ data of solar wind. In appropriate limit, previously obtained algebraic relations and power spectra for axisymmetric MHD turbulence (Galtier, Pouquet and Mangeney, Physics of Plasmas, 2005) are successfully recovered. Finally, more realistic relations capturing weak Alfvénic turbulence (with constant $k_{\parallel}$) and the transition to strong turbulence are derived along with their corresponding power spectra.