189 research outputs found
Effects of anisotropy in geostrophic turbulence
The Boussinesq model of convection in a flat layer with heating from below is
considered. We analyze the effects of anisotropy caused by rapid rotation in
physical and wave spaces and demonstrate the suppression of energy transfer by
rotation. We also examine the structure of the wave triangle in nonlinear
interaction. The range of parameters is adapted to the models of convection in
the geodynamo
Why dynamos are prone to reversals
In a recent paper (Phys. Rev. Lett. 94 (2005), 184506; physics/0411050) it
was shown that a simple mean-field dynamo model with a spherically symmetric
helical turbulence parameter alpha can exhibit a number of features which are
typical for Earth's magnetic field reversals. In particular, the model produces
asymmetric reversals, a positive correlation of field strength and interval
length, and a bimodal field distribution. All these features are attributable
to the magnetic field dynamics in the vicinity of an exceptional point of the
spectrum of the non-selfadjoint dynamo operator. The negative slope of the
growth rate curve between the nearby local maximum and the exceptional point
makes the system unstable and drives it to the exceptional point and beyond
into the oscillatory branch where the sign change happens. A weakness of this
reversal model is the apparent necessity to fine-tune the magnetic Reynolds
number and/or the radial profile of alpha. In the present paper, it is shown
that this fine-tuning is not necessary in the case of higher supercriticality
of the dynamo. Numerical examples and physical arguments are compiled to show
that, with increasing magnetic Reynolds number, there is strong tendency for
the exceptional point and the associated local maximum to move close to the
zero growth rate line. Although exemplified again by the spherically symmetric
alpha^2 dynamo model, the main idea of this ''self-tuning'' mechanism of
saturated dynamos into a reversal-prone state seems well transferable to other
dynamos. As a consequence, reversing dynamos might be much more typical and may
occur much more frequently in nature than what could be expected from a purely
kinematic perspective.Comment: 11 pages, 10 figure
Turbulent 3D MHD dynamo model in spherical shells: regular oscillations of the dipolar field
We report the results of three-dimensional numerical simulations of convection-driven dynamos in relatively thin rotating spherical shells that show a transition from an strong non-oscillatory dipolar magnetic field to a weaker regularly oscillating dipolar field. The transition is induced primarily by the effects a stress-free boundary condition. The variation of the inner to outer radius ratio is found to have a less important effect
Performance benchmarks for a next generation numerical dynamo model
Numerical simulations of the geodynamo have successfully represented many observable characteristics of the geomagnetic field, yielding insight into the fundamental processes that generate magnetic fields in the Earth's core. Because of limited spatial resolution, however, the diffusivities in numerical dynamo models are much larger than those in the Earth's core, and consequently, questions remain about how realistic these models are. The typical strategy used to address this issue has been to continue to increase the resolution of these quasi-laminar models with increasing computational resources, thus pushing them toward more realistic parameter regimes. We assess which methods are most promising for the next generation of supercomputers, which will offer access to O(106) processor cores for large problems. Here we report performance and accuracy benchmarks from 15 dynamo codes that employ a range of numerical and parallelization methods. Computational performance is assessed on the basis of weak and strong scaling behavior up to 16,384 processor cores. Extrapolations of our weak-scaling results indicate that dynamo codes that employ two-dimensional or three-dimensional domain decompositions can perform efficiently on up to ∼106 processor cores, paving the way for more realistic simulations in the next model generation
Detecting the oldest geodynamo and attendant shielding from the solar wind: Implications for habitability
The onset and nature of the earliest geomagnetic field is important for
understanding the evolution of the core, atmosphere and life on Earth. A record
of the early geodynamo is preserved in ancient silicate crystals containing
minute magnetic inclusions. These data indicate the presence of a geodynamo
during the Paleoarchean, between 3.4 and 3.45 billion years ago. While the
magnetic field sheltered Earth's atmosphere from erosion at this time, standoff
of the solar wind was greatly reduced, and similar to that during modern
extreme solar storms. These conditions suggest that intense radiation from the
young Sun may have modified the atmosphere of the young Earth by promoting loss
of volatiles, including water. Such effects would have been more pronounced if
the field were absent or very weak prior to 3.45 billion years ago, as
suggested by some models of lower mantle evolution. The frontier is thus trying
to obtain geomagnetic field records that are >>3.45 billion-years-old, as well
as constraining solar wind pressure for these times. In this review we suggest
pathways for constraining these parameters and the attendant history of Earth's
deep interior, hydrosphere and atmosphere. In particular, we discuss new
estimates for solar wind pressure for the first 700 million years of Earth
history, the competing effects of magnetic shielding versus solar ion
collection, and bounds on the detection level of a geodynamo imposed by the
presence of external fields. We also discuss the prospects for constraining
Hadean-Paleoarchean magnetic field strength using paleointensity analyses of
zircons.Comment: 78 pages, 8 figures, Supplementary Content: Reconstructing the Past
Sun + table of solar parameters from ZAMS to present through geological tim
Direct and inverse cascades in the geodynamo
The rapid rotation of planets causes cyclonic thermal turbulence in their
cores which may generate the large-scale magnetic fields observed outside the
planets. We consider the model which enables us reproduce the typical features
of small-scale geostrophic flows in physical and wave spaces. We present
estimates of kinetic and magnetic energy fluxes as a function of the wave
number. The joint existence of forward and inverse cascades are demonstrated.
We also consider the mechanism of magnetic field saturation at the end of the
kinematic dynamo regime
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