Third-Moment Studies of Cascade Dynamics in Solar Wind Turbulence (Invited)

Physics – Plasma Physics

Scientific paper

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[2134] Interplanetary Physics / Interplanetary Magnetic Fields, [2149] Interplanetary Physics / Mhd Waves And Turbulence, [4490] Nonlinear Geophysics / Turbulence, [7863] Space Plasma Physics / Turbulence

Scientific paper

Kolmogorov [1941] and Yaglom [1949] showed that the incompressible hydrodynamic equations governing fluid turbulence could be manipulated to yield a rigorous third-order structure function expression for the energy cascade at inertial range scales. In that derivation the structure function scales linearly with separation distance and the proportionality constant is a factor of the energy cascade rate. For decades it has been argued that the most commonly studied spatial scales for magnetic and velocity fluctuations in the solar wind form an inertial range in an MHD analogy to hydrodynamic turbulence. Politano and Pouquet [1998a,b] and Podesta [2008] derived third-moment expressions for the inertial range cascade in MHD in direct analogy with the earlier hydrodynamic results. We have been exploring the use of these expressions for both isotropic and anisotropic solar wind turbulence [MacBride 2005, 2008; Stawarz 2009, 2010; Smith 2009, 2010; Forman 2010a,b] and find (1) the measured third moments do scale linearly with separation and (2) the resulting estimate for the energy cascade rate accurately account for the energy cascade budget required for turbulence to heat the solar wind. In addition, the anisotropic formalism shows preferential cascade perpendicular to the mean magnetic field. Recent results show the unexpected backward transfer of energy associated with the dominant outward-propagating component when the cross-helicity < δ V \cdot δ B > is large. The latter behavior is thought to exist over only a limited range of heliocentric distances forming a transient turbulent dynamic near 1 AU. We will include some important comments about the need to monitor convergence and error analyses when using solar wind data.
Kolmogorov, 1941, Dokl. Akad. Nauk SSSR, 32, 16.
Forman, et al., 2010a, Physical Review Letters, 104, 189001.
Forman, et al., 2010b, Solar Wind 12, 176.
MacBride, et al., 2005, Solar Wind 11, 613.
MacBride, et al., 2008, The Astrophysical Journal, 679, 1644.
Podesta, et al., 2008, Journal of Fluid Mechanics, 609, 171.
Politano and Pouquet, 1998b, Physical Review E, 57, R21.
Politano and Pouquet, 1998a, Geophysical Research Letters, 25, 273.
Smith, et al., 2009, Physical Review Letters, 103, 201101.
Smith, et al., 2010, Physical Review Letters, 104, 169002.
Stawarz, et al., 2009, The Astrophysical Journal, 697, 1119.
Stawarz, et al., 2010, The Astrophysical Journal, 713, 920.
Yaglom, 1949, Dokl. Akad. Nauk SSSR, 69, 743.

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