53,203 research outputs found
The initial temporal evolution of a feedback dynamo for Mercury
Various possibilities are currently under discussion to explain the observed
weakness of the intrinsic magnetic field of planet Mercury. One of the possible
dynamo scenarios is a dynamo with feedback from the magnetosphere. Due to its
weak magnetic field Mercury exhibits a small magnetosphere whose subsolar
magnetopause distance is only about 1.7 Hermean radii. We consider the magnetic
field due to magnetopause currents in the dynamo region. Since the external
field of magnetospheric origin is antiparallel to the dipole component of the
dynamo field, a negative feedback results. For an alpha-omega-dynamo two
stationary solutions of such a feedback dynamo emerge, one with a weak and the
other with a strong magnetic field. The question, however, is how these
solutions can be realized. To address this problem, we discuss various
scenarios for a simple dynamo model and the conditions under which a steady
weak magnetic field can be reached. We find that the feedback mechanism
quenches the overall field to a low value of about 100 to 150 nT if the dynamo
is not driven too strongly
Growth rate of small-scale dynamo at low magnetic Prandtl numbers
In this study we discuss two key issues related to a small-scale dynamo
instability at low magnetic Prandtl numbers and large magnetic Reynolds
numbers, namely: (i) the scaling for the growth rate of small-scale dynamo
instability in the vicinity of the dynamo threshold; (ii) the existence of the
Golitsyn spectrum of magnetic fluctuations in small-scale dynamos. There are
two different asymptotics for the small-scale dynamo growth rate: in the
vicinity of the threshold of the excitation of the small-scale dynamo
instability, , and when the
magnetic Reynolds number is much larger than the threshold of the excitation of
the small-scale dynamo instability, , where
is the small-scale dynamo instability threshold in the
magnetic Reynolds number . We demonstrated that the existence of the
Golitsyn spectrum of magnetic fluctuations requires a finite correlation time
of the random velocity field. On the other hand, the influence of the Golitsyn
spectrum on the small-scale dynamo instability is minor. This is the reason why
it is so difficult to observe this spectrum in direct numerical simulations for
the small-scale dynamo with low magnetic Prandtl numbers.Comment: 14 pages, 1 figure, revised versio
Galactic and Accretion Disk Dynamos
Dynamos in astrophysical disks are usually explained in terms of the standard
alpha-omega mean field dynamo model where the local helicity generates a radial
field component from an azimuthal field. The subsequent shearing of the radial
field gives rise to exponentially growing dynamo modes. There are several
problems with this model. The exponentiation time for the galactic dynamo is
hard to calculate, but is probably uncomfortably long. Moreover, numerical
simulations of magnetic fields in shearing flows indicate that the presence of
a dynamo does not depend on a non-zero average helicity. However, these
difficulties can be overcome by including a fluctuating helicity driven by
hydrodynamic or magnetic instabilities. Unlike traditional disk dynamo models,
this `incoherent' dynamo does not depend on the presence of systematic fluid
helicity or any kind of vertical symmetry breaking. It will depend on geometry,
in the sense that the dynamo growth rate becomes smaller for very thin disks,
in agreement with constraints taken from the study of X-ray novae. In this
picture the galactic dynamo will operate efficiently, but the resulting field
will have a radial coherence length which is a fraction of the galactic radius.Comment: 16 pages, in Proceedings of the Chapman Conference on Magnetic
Helicit
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