Physics – Plasma Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsh53a..05v&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SH53A-05
Physics
Plasma Physics
[7800] Space Plasma Physics, [7815] Space Plasma Physics / Electrostatic Structures, [7863] Space Plasma Physics / Turbulence, [7867] Space Plasma Physics / Wave/Particle Interactions
Scientific paper
The cooling of the expanding solar wind is less efficient than expected. Scientists pointed out that the reason of this empirical evidence is related to the turbulent character of the solar wind plasma. The identification of the physical mechanism replacing "energy dissipation" in a collisionless magnetized plasma and establishing the link between macroscopic and microscopic scales would open a new scenario of broad importance in the field of turbulence and would have far-reaching implications in the problem of space plasma heating. Turbulent heating consists both in a progressive energy degradation and disorder increasing, going from large to small scales. The increase of disorder results into the production, through nonlinear interaction, of small-scale fluctuations involving not only the kinetic energy, as in the case of heat, but also the potential energy associated with electric and magnetic field fluctuations. To numerically analyze the kinetic effects on the evolution of the turbulent spectra in the solar wind, we make use of a recently developed numerical hybrid-Vlasov code [1], where the Vlasov equation is solved for the proton distribution function, while the electron response is taken into account through a generalized Ohm law that retains Hall effects and electron inertia terms. By performing multi-dimensional phase space simulations (1D or 2D in space and 3D in velocity) [2-5] on the last generation of supercomputers, we show that newly identified electrostatic (acoustic-like) modes, in longitudinal propagation with respect to the average magnetic field and driven by particle distribution functions far from local thermodynamic equilibrium, represent a privileged channel for turbulence to carry the energy towards small disordered scales. The system dynamics is analyzed for different electron to ion temperature ratios. Our numerical results provide a qualitative interpretation of a complex phenomenology ubiquitously recovered in many solar-wind spacecraft observations: the presence of significant electrostatic activity at high frequency identified long ago as consisting of longitudinal ion-acoustic waves [6-8], the generation of beams of accelerated protons in the direction of the ambient magnetic field [9], moving with mean velocity of the order of the local Alfvén speed. Alternative mechanisms for the generation of proton-beam distributions have been recently studied in [10] by means of PIC hybrid simulations in low plasma beta regimes. [1] Valentini, F., Travnicek, P., Califano, F., Hellinger, P., Mangeney, A., J. Comput. Phys. 225, 753 (2007). [2] Valentini, F., Veltri, P., Califano, F., Mangeney, A., Phys. Rev. Lett. 101, 025006 (2008). [3] Valentini, F., Veltri, P., Phys. Rev. Lett. 102, 225001 (2009). [4] Valentini, F., Califano, F., Veltri, P., Planetary and Space Science, doi:10.1016/j.pss.2009.11.007 (in press). [5] Valentini, F., Califano, F., Veltri, Phys. Rev. Lett. 104, 205002 (2010). [6] D. A. Gurnett and R. R. Anderson, J. Geophys. Res. 82, 632 (1977). [7] D. A. Gurnett and L. A. Frank, J. Geophys. Res. 83, 58 (1978). [8] D. A. Gurnett et al., J. Geophys. Res. 84, 2029 (1979). [9] E. Marsch et al., J. Geophys. Res. 87, 52 (1982). [10] J. A. Araneda, E. Marsch, and A. F. Vinas, Phys. Rev. Lett. 100, 125003 (2008).
Califano Francesco
Valentini Francesco
Veltri Pierluigi
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