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"Turbulent Cascades in MHD"
Dr. Alexandros Alexakis
National Center for Atmospheric Research, USA

When:  Friday, 7 October 2005 at 1:00pm
Where: 2nd Floor Seminar Room

Most astrophysical and planetary systems, e.g. solar/stellar winds,
accretion disks and interstellar medium, are in a turbulent state and
coupled to magnetic fields. In MHD flows, the two fields (velocity and
magnetic) and two associated energies involved in the dynamical
processes allow for many possibilities for the energy to transfer
between smaller or larger scales, making the dynamics more complex to
address in both theory and modeling.  We investigate the transfer of
energy from large scales to small scales in fully developed forced
three-dimensional MHD-turbulence by analyzing the results of direct
numerical simulations in the absence of an externally imposed uniform
magnetic field. Our results show that the transfer of kinetic energy
from the large scales to kinetic energy at smaller scales, and the
transfer of magnetic energy from the large scales to magnetic energy
at smaller scales, are local, as is also found in the case of neutral
fluids, and in a way that is compatible with Kolmogorov (1941) theory
of turbulence. However, the transfer of energy from the velocity field
to the magnetic field is a highly non-local process in Fourier
space. Energy from the velocity field at large scales can be
transfered directly into small scale magnetic fields without the
participation of intermediate scales. Some implications of our results
to MHD turbulence modeling are also discussed.

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Alexandros obtained his undergraduate degree in Physics from the
Univ. of Crete and his Ph.D. in Physics from the Univ. of Chicago,
USA.  He is currently a research fellow at the National Center for
Atmospheric Research, in Boulder, CO, USA. For more info on Alexandros
you may visit his web page at: http://www.asp.ucar.edu/~alexakis/