Licentiate dissertation: Generation of magnetic fields on galactic scales
by
Fabio Del Sordo(NORDITA)
→
Europe/Stockholm
FB42
FB42
Description
This work is a computational and analytical study of some general features of
fluid motions and magnetic fields that are applied in the context of galactic
dynamics. Magnetic field generation is generally the result of gas motions in
electrically conducting media. Consequently we begin by studying some aspects of the nature
of the motion in the interstellar medium (ISM).
In Paper I, we argue that the driving of turbulence in galaxies is essentially irrotational. However, in a systematic investigation of the mechanisms
leading to a conversion of irrotational to vortical motions we are for the first
time able to quantify the relative importance of the various mechanisms.
The presence of vorticity is a necessary condition for having helicity, which is
proportional to the dot product of vorticity and velocity and can lead to an
α effect. This is a highly controversial subject, partly because it is so hard to
quantify the value of for a given flow. Therefore, we study in Paper II two
of the main methods for determining computationally - the imposed-field
method and the test-field method. It is now well known that the α effect produces magnetic helicity, which characterizes the mutual linkage of magnetic
flux and that is a conserved quantity in flows with large magnetic Reynolds
number. This picture helps understanding that the violation of magnetic
helicity conservation is connected with the difficulty of breaking interlocked
flux structures apart. However, in Paper III it is shown that this interpretation is too naive. In particular, using two similar flux configurations of triply
interlocked flux, it is shown that the relative field orientation also matters,
because in one case there is no net helicity while in the other there is.
