The Turbulent Cascade at 1 AU: Energy Transfer and the Third-Order Scaling for MHD

Details The Turbulent Cascade at 1 AU: Energy Transfer and the Third-Order Scaling for MHD The Turbulent Cascade at 1 AU: Energy Transfer and the Third-Order Scaling for MHD

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsh33b..05m&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SH33B-05

Physics

2134 Interplanetary Magnetic Fields, 2149 Mhd Waves And Turbulence (2752, 6050, 7836), 2159 Plasma Waves And Turbulence, 2164 Solar Wind Plasma

Scientific paper

We perform a test of MHD turbulent cascade theory in the solar wind and directly evaluate the contribution of local turbulence to heating the solar wind at 1 AU. We look at turbulent fluctuations in the solar wind velocity V, and magnetic field B, using the vector Elsasser variables Z± \equiv V ± B / \sqrt{4 π ρ} measured at the ACE spacecraft stationed at Earth L1. We combine the fluctuations δ Z± over time lags in the inertial range, from 64 seconds to several hours, to form components of the mixed vector third moments that Politano and Pouquet (1998a,b) show obey an exact law, similar to the Kolmogorov 4/5 law, but valid in anisotropic MHD turbulence. This effort is vital to studies of dissipation processes because it provides both the rate that energy is delivered to the dissipation process, but also the form in that the cascade in directions parallel and perpendicular to the mean magnetic field can be measured separately. We demonstrate that the scaling is reasonably linear as expected for the inertial range. The total turbulent energy injection/dissipation rate we derive this way agrees with the in situ heating of the solar wind inferred from the temperature gradient, while methods using the power spectra only seldom agree with heating rates derived from gradients of the thermal proton distribution. We derive expressions of the third-order moments that are applicable to the spectral cascade parallel and perpendicular to the mean magnetic field. We apply these expressions to fast- and slow-wind subsets of the data with additional subsetting for mean field direction. We find that both the fast wind and the slow wind exhibit an active energy cascade over inertial range scales. Furthermore, we find that the energy flux in the parallel cascade is consistently smaller than in the perpendicular cascade. This is especially true of high-speed wind conditions where we see that the turbulence is moving away from the pre-existing field-aligned geometry of Dasso et al. [2005]. This work does not assume a particular MHD theory for the power spectrum such as Iroshnikov [1964], Kraichnan [1965], Goldreich and Sridhar [1995], or Boldyrev [2005, 2006]. Although this is a study of fluctuations within the inertial range of interplanetary turbulence, it has direct bearing on the rate and manner that energy is injected into the dissipation range.

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