Speaker
Dr
Shiva Kumar Malapaka
Description
Large-scale magnetic structure formation in the universe is
one of the problems in modern astrophysics, which lacks a
clear solution. A possible explanation for the formation of
such structures could be offered from a property of 3D-
magnetohydrodynamic (MHD) turbulence, namely inverse cascade
of magnetic helicity. Magnetic helicity is defined as the
volume integral of the dot product of the magnetic field and
the magnetic vector potential. Inverse cascade means the
transfer of that quantity spectrally from small scales to
large scales, with a constant flux. We report some of the
important results form the spectral and structural study of
this property using high resolution Direct Numerical
Simulations (DNS) in two setups namely, forced 3D-MHD
turbulence and decaying 3D-MHD turbulence. These results
include self-similar behavior in various quantities of the
turbulent flow in their spectra; which were hitherto
unknown. A new relation involving magnetic helicity,
magnetic energy, kinetic helicity and kinetic energy; has
been obtained from the dimensional analysis of the eddy
damped quasi normal Markovian (EDQNM) approximation equation
for magnetic helicity. This relation is satisfied by the
obtained power laws of these quantities in both the forced
and decaying turbulence cases . Further, this relation can
also be written in a form to represent the alpha effect. We
also propose a new mechanism involving both forced and
decaying turbulences to explain large-scale magnetic
structures. Statistical analysis of structures in the the
turbulent fields in both the cases will also be presented.
Reference: A Study of Magnetic Helicity in Forced and
Decaying 3D-MHD Turbulence, Ph.d thesis, Shiva Kumar.
Malapaka
(http://edoc.mpg.de/display.epl?mode=doc&id=464051&col=33&grp=1311)
