Reflectiondriven magnetohydrodynamic turbulence in the solar atmosphere and solar wind
Abstract
We present threedimensional direct numerical simulations and an analytic model of reflectiondriven magnetohydrodynamic (MHD) turbulence in the solar wind. Our simulations describe transverse, noncompressive MHD fluctuations within a narrow magnetic flux tube that extends from the photosphere, through the chromosphere and corona and out to a heliocentric distance r of 21 solar radii (R_{☉ }) . We launch outwardpropagating '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 inwardpropagating 'z^{} fluctuations'. Counterpropagating fluctuations subsequently interact, causing fluctuation energy to cascade to small scales and dissipate. Our analytic model incorporates dynamic alignment, allows for strongly or weakly turbulent nonlinear interactions and divides the z^{+} fluctuations into two populations with different characteristic radial correlation lengths. The inertialrange power spectra of z^{+} and z^{} fluctuations in our simulations evolve toward a k_{\bot }^{3/2} scaling at r>10R_{☉} , where k_{\bot } is the wavevector 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≃ 4R_{☉ } . We argue that these spectral scalings are caused by: (i) highpass filtering in the upper chromosphere; (ii) the anomalous coherence of inertialrange z^{} fluctuations in a reference frame propagating outwards with the z^{+} fluctuations; and (iii) the change in the sign of the radial derivative of the Alfvén speed at r=r_{m}≃ 1.7R_{☉ } , which disrupts this anomalous coherence between r=r_{m} and r≃ 2r_{m} . At r>1.3R_{☉ } , the turbulent heating rate in our simulations is comparable to the turbulent heating rate in a previously developed solarwind model that agreed with a number of observational constraints, consistent with the hypothesis that MHD turbulence accounts for much of the heating of the fast solar wind.
 Publication:

Journal of Plasma Physics
 Pub Date:
 August 2019
 DOI:
 10.1017/S0022377819000540
 arXiv:
 arXiv:1908.00880
 Bibcode:
 2019JPlPh..85d9009C
 Keywords:

 astrophysical plasmas;
 plasma nonlinear phenomena;
 space plasma physics;
 Physics  Space Physics;
 Astrophysics  Solar and Stellar Astrophysics;
 Physics  Plasma Physics
 EPrint:
 40 pages, 7 figures, accepted for publication in the Journal of Plasma Physics (JPP). Includes proof corrections