Reflection-driven MHD turbulence in the solar atmosphere and solar wind

Benjamin D. G. Chandran; Jean C. Perez

doi:10.1017/S0022377819000540

paper

Reflection-driven MHD turbulence in the solar atmosphere and solar wind

arXiv:1908.00880 · doi:10.1017/S0022377819000540

Abstract

We present 3D numerical simulations and an analytic model of reflection-driven MHD turbulence in the solar wind. Our simulations describe transverse, non-compressive MHD fluctuations within a narrow magnetic flux tube that extends from the photosphere out to a heliocentric distance $r$ of 21 solar radii $(R_{s})$ . We launch outward-propagating " $z^{+}$ fluctuations" into the simulation domain by imposing a randomly evolving photospheric velocity field. As these fluctuations propagate away from the Sun, they undergo partial reflection, producing inward-propagating " $z^{-}$ fluctuations." Counter-propagating fluctuations subsequently interact, causing fluctuation energy to cascade to small scales and dissipate. Our analytic model incorporates alignment, allows for strongly or weakly turbulent nonlinear interactions, and divides the $z^{+}$ fluctuations into two populations with different characteristic radial correlation lengths. The inertial-range power spectra in our simulations evolve toward a $k_{⊥}^{- 3/2}$ scaling at $r > 10 R_{s}$ , where $k_{⊥}$ is the wave-vector component perpendicular to the background magnetic field. In two of our simulations, the $z^{+}$ power spectra are much flatter between the coronal base and $r ≃ 4 R_{s}$ . We argue that these spectral scalings are caused by: (1) high-pass filtering in the upper chromosphere; (2) the anomalous coherence of inertial-range $z^{-}$ fluctuations in a reference frame propagating outwards with the $z^{+}$ fluctuations; and (3) the change in the sign of the radial derivative of the Alfvén speed at $r = r_{m} ≃ 1.7 R_{s}$ , which disrupts this anomalous coherence between $r = r_{m}$ and $r ≃ 2 r_{m}$ . At $r > 1.3 R_{s}$ , the turbulent heating rate in our simulations is comparable to the heating rate in a previously developed solar-wind model that agreed with a number of observational constraints.

arXiv comment40 pages, 7 figures, accepted for publication in the Journal of Plasma Physics (JPP). Includes proof corrections

Reflection-driven MHD turbulence in the solar atmosphere and solar wind

Abstract

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