Co-existence of Discrete Modes and Turbulence in Direct MHD Simulations

Details Co-existence of Discrete Modes and Turbulence in Direct MHD Simulations Co-existence of Discrete Modes and Turbulence in Direct MHD Simulations

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsh13a0384g&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SH13A-0384

Physics

2149 Mhd Waves And Turbulence (2752, 6050, 7836), 7522 Helioseismology, 7839 Nonlinear Phenomena (4400, 6944), 7863 Turbulence (4490), 7868 Wave/Wave Interactions

Scientific paper

Motivated by measurements of discrete, reproducible modes in the solar wind and energetic particle fluxes that are in the frequency ranges that correspond to solar internal pressure (p) and gravity (g) modes, a controversy persists in the magnetohydrodynamic (MHD) turbulence community whether discrete modes (independent of their source) can persist in the presence of a turbulent cascade. We perform direct numerical simulations of a compressible 3-D MHD system to find evidence for such coexistence. We find two scenarios: First, in the presence of a DC magnetic field, turbulent cascades are known to be suppressed in the field-parallel direction. In this case, a discrete mode can persist for several nonlinear times without interacting strongly with the turbulence if its wavevector component parallel to the DC magnetic field is significantly larger than the turbulent cascade's k-space variation in the field-parallel direction. Second, for sufficiently weak forcing a driven mode can persist as a discrete mode despite being deeply embedded inside a turbulence cascade. In this case, the mode does not display spectral broadening, nor does the spectrum of the adjacent turbulence depart from a Kolmogorov-like cascade. Projection effects of the entire k-space onto a 1-D reduced spectrum direction, as typical of single-spacecraft observations, suggest the discrete mode is more likely to be observed when the angle between solar-wind flow direction and the DC magnetic-field direction is small. This study complements driven reduced MHD simulations of the solar corona [1] that support similar conclusions based on time-domain (frequency) analysis. [1] P. Dmitruk et al., GRL, 31, 21805 (2004).

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