236 research outputs found
PEMBERDAYAAN MASYARAKAT DALAM PEMBUATAN AKSESORIS MANIK-MANIK KHAS TORAJA UNTUK MENUNJANG PENGEMBANGAN OBJEK WISATA BARU DI DESA KOLESAWANGAN â TANA TORAJA
PEMBERDAYAAN MASYARAKAT DALAM PEMBUATAN AKSESORIS MANIK-MANIKKHAS TORAJA UNTUK MENUNJANG PENGEMBANGAN OBJEK WISATA BARU DIDESA KOLESAWANGAN â TANA TORAJ
Regulation of citrate synthase activity in methylotrophs by reduced nicotinamide-adenine dinucleotide, adenine nucleotides and 2-oxoglutarate
Axial invariance of rapidly varying diffusionless motions in the Earth's core interior
Geostrophic jets propagating as Alfv\'en waves are shown to arise ina rapidly
rotating spherical shell permeated by a magnetic field among the transient
motions set up by an impulsive rotation of the inner core. These axially
invariant motions evolve on a time-scale which is short compared to the
magnetic diffusion time. The numerical study is taken as illustrative of a more
general point: on such a fast time-scale the dimensionless number appropriate
to compare the rotation and magnetic forces is independent of the magnetic
diffusivity in contrast with the often used Elsasser number. Extension of the
analysis to non-axisymmetrical motions is supported by published studies of
dynamo models and magnetic instabilities
Visco-magnetic torque at the core mantle boundary
A magneto-hydrodynamic model of boundary layers at the Core-Mantle Boundary
(CMB) is derived and used to compute the viscous and electromagnetic torques
generated by the Earth's nutation forcing. The predicted electromagnetic torque
alone cannot account for the dissipation estimated from the observations of the
free core nutation. The presence of a viscous boundary layer in the
electromagnetic skin layer at the CMB, with its additional dissipative torques,
may explain the geodetic data. An apparent Ekman number at the top of the core
between 3 and is inferred depending on the electrical conductivity
of the mantle
Can core-surface flow models be used to improve the forecast of the Earth's main magnetic field?
[1] Geomagnetic main field models used for navigation are updated every 5 years and contain a forecast of the geomagnetic secular variation for the upcoming epoch. Forecasting secular variation is a difficult task. The change of the main magnetic field is thought to be principally due to advection of the field by flow at the surface of the outer core on short timescales and when large length scales are considered. With accurate secular variation (SV) and secular acceleration (SA) models now available from new satellite missions, inverting for the flow and advecting it forward could lead to a more accurate prediction of the main field. However, this scheme faces two significant challenges. The first arises from the truncation of the observable main field at spherical harmonic degree 13. This can however be handled if the true core flow is large scale and has a rapidly decaying energy spectrum. The second is that even at a given single epoch the instantaneous SV and SA cannot simultaneously be explained by a steady flow. Nevertheless, we find that it may be feasible to use flow models for an improved temporal extrapolation of the main field. A medium-term (â10 years) hindcast of the field using a steady flow model outperforms the usual extrapolation using the presently observed SV and SA. On the other hand, our accelerated, toroidal flow model, which explains a larger portion of the observed average SA over the 2000â2005 period, fails to improve both the short-term and medium-term hindcasts of the field. This somewhat paradoxical result is related to the occurrence of so-called geomagnetic jerks, the still poorly known dynamical nature of which remains the main obstacle to improved geomagnetic field forecasts
On the peculiar nature of turbulence in planetary dynamos
Under the combined constraints of rapid rotation, sphericity, and magnetic
field, motions in planetary cores get organized in a peculiar way. Classical
hydrodynamic turbulence is not present, but turbulent motions can take place
under the action of the buoyancy and Laplace forces. Laboratory experiments,
such as the rotating spherical magnetic Couette DTS experiment in Grenoble,
help us understand what motions take place in planetary core conditions.Comment: in press in Compte Rendu de l'Academie des Science
The transition to Earth-like torsional oscillations in magnetoconvection simulations
Evidence for torsional oscillations (TOs) operating within the Earth's fluid outer core has been found in the secular variation of the geomagnetic field. These waves arise via disturbances to the predominant (magnetostrophic) force balance believed to exist in the core. The coupling of the core and mantle allow TOs to affect the length-of-day of the Earth via angular momentum conservation. Encouraged by previous work, where we were able to observe TOs in geodynamo simulations, we perform 3-D magnetoconvection simulations in a spherical shell in order to reach more Earth-like parameter regimes that proved hitherto elusive. At large Ekman numbers we find that TOs can be present but are typically only a small fraction of the overall dynamics and are often driven by Reynolds forcing at various locations throughout the domain. However, as the Ekman number is reduced to more Earth-like values, TOs become more apparent and can make up the dominant portion of the short timescale flow. This coincides with a transition to regimes where excitation is found only at the tangent cylinder, is delivered by the Lorentz force and gives rise to a periodic Earth-like wave pattern, approximately operating on a 4 to 5 year timescale. The core travel times of our waves also become independent of rotation at low Ekman number with many converging to Earth-like values of around 4 years
Can core-surface flow models be used to improve the forecast of the Earth's main magnetic field?
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