1,054 research outputs found
Global bifurcations to subcritical magnetorotational dynamo action in Keplerian shear flow
Magnetorotational dynamo action in Keplerian shear flow is a three-dimensional, non-linear magnetohydrodynamic process whose study is relevant to the understanding of accretion processes and magnetic field generation in astrophysics. Transition to this form of dynamo action is subcritical and shares many characteristics of transition to turbulence in non-rotating hydrodynamic shear flows. This suggests that these different fluid systems become active through similar generic bifurcation mechanisms, which in both cases have eluded detailed understanding so far. In this paper, we build on recent work on the two problems to investigate numerically the bifurcation mechanisms at work in the incompressible Keplerian magnetorotational dynamo problem in the shearing box framework. Using numerical techniques imported from dynamical systems research, we show that the onset of chaotic dynamo action at magnetic Prandtl numbers larger than unity is primarily associated with global homoclinic and heteroclinic bifurcations of nonlinear magnetorotational dynamo cycles. These global bifurcations are found to be supplemented by local bifurcations of cycles marking the beginning of period-doubling cascades. The results suggest that nonlinear magnetorotational dynamo cycles provide the pathway to turbulent injection of both kinetic and magnetic energy in incompressible magnetohydrodynamic Keplerian shear flow in the absence of an externally imposed magnetic field. Studying the nonlinear physics and bifurcations of these cycles in different regimes and configurations may subsequently help to better understand the physical conditions of excitation of magnetohydrodynamic turbulence and instability-driven dynamos in a variety of astrophysical systems and laboratory experiments. The detailed characterization of global bifurcations provided for this three-dimensional subcritical fluid dynamics problem may also prove useful for the problem of transition to turbulence in hydrodynamic shear flows
Extragalactic jets with helical magnetic fields: relativistic MHD simulations
Extragalactic jets are inferred to harbor dynamically important, organized
magnetic fields which presumably aid in the collimation of the relativistic jet
flows. We here explore by means of grid-adaptive, high resolution numerical
simulations the morphology of AGN jets pervaded by helical field and flow
topologies. We concentrate on morphological features of the bow shock and the
jet beam behind the Mach disk, for various jet Lorentz factors and magnetic
field helicities. We investigate the influence of helical magnetic fields on
jet beam propagation in overdense external medium. We use the AMRVAC code,
employing a novel hybrid block-based AMR strategy, to compute ideal plasma
dynamics in special relativity. The helicity of the beam magnetic field is
effectively transported down the beam, with compression zones in between
diagonal internal cross-shocks showing stronger toroidal field regions. In
comparison with equivalent low-relativistic jets which get surrounded by
cocoons with vortical backflows filled by mainly toroidal field, the high speed
jets demonstrate only localized, strong toroidal field zones within the
backflow vortical structures. We find evidence for a more poloidal, straight
field layer, compressed between jet beam and backflows. This layer decreases
the destabilizing influence of the backflow on the jet beam. In all cases, the
jet beam contains rich cross-shock patterns, across which part of the kinetic
energy gets transferred. For the high speed reference jet considered here,
significant jet deceleration only occurs beyond distances exceeding , as the axial flow can reaccelerate downstream to the internal
cross-shocks. This reacceleration is magnetically aided, due to field
compression across the internal shocks which pinch the flow.Comment: 16 pages, Astronomy and Astrophysics accepted for publicatio
The predictability of advection-dominated flux-transport solar dynamo models
Space weather is a matter of practical importance in our modern society.
Predictions of forecoming solar cycles mean amplitude and duration are
currently being made based on flux-transport numerical models of the solar
dynamo. Interested in the forecast horizon of such studies, we quantify the
predictability window of a representative, advection-dominated, flux-transport
dynamo model by investigating its sensitivity to initial conditions and control
parameters through a perturbation analysis. We measure the rate associated with
the exponential growth of an initial perturbation of the model trajectory,
which yields a characteristic time scale known as the e-folding time .
The e-folding time is shown to decrease with the strength of the
-effect, and to increase with the magnitude of the imposed meridional
circulation. Comparing the e-folding time with the solar cycle periodicity, we
obtain an average estimate for equal to 2.76 solar cycle durations.
From a practical point of view, the perturbations analysed in this work can be
interpreted as uncertainties affecting either the observations or the physical
model itself. After reviewing these, we discuss their implications for solar
cycle prediction.Comment: 33 pages, 12 figure
On Predicting the Solar Cycle using Mean-Field Models
We discuss the difficulties of predicting the solar cycle using mean-field
models. Here we argue that these difficulties arise owing to the significant
modulation of the solar activity cycle, and that this modulation arises owing
to either stochastic or deterministic processes. We analyse the implications
for predictability in both of these situations by considering two separate
solar dynamo models. The first model represents a stochastically-perturbed flux
transport dynamo. Here even very weak stochastic perturbations can give rise to
significant modulation in the activity cycle. This modulation leads to a loss
of predictability. In the second model, we neglect stochastic effects and
assume that generation of magnetic field in the Sun can be described by a fully
deterministic nonlinear mean-field model -- this is a best case scenario for
prediction. We designate the output from this deterministic model (with
parameters chosen to produce chaotically modulated cycles) as a target
timeseries that subsequent deterministic mean-field models are required to
predict. Long-term prediction is impossible even if a model that is correct in
all details is utilised in the prediction. Furthermore, we show that even
short-term prediction is impossible if there is a small discrepancy in the
input parameters from the fiducial model. This is the case even if the
predicting model has been tuned to reproduce the output of previous cycles.
Given the inherent uncertainties in determining the transport coefficients and
nonlinear responses for mean-field models, we argue that this makes predicting
the solar cycle using the output from such models impossible.Comment: 22 Pages, 5 Figures, Preprint accepted for publication in Ap
Consciousness in the Universe is Scale Invariant and Implies an Event Horizon of the Human Brain
Our brain is not a "stand alone" information processing organ: it acts as a central part of our integral nervous system with recurrent information exchange with the entire organism and the cosmos. In this study, the brain is conceived to be embedded in a holographic structured field that interacts with resonant sensitive structures in the various cell types in our body. In order to explain earlier reported ultra-rapid brain responses and effective operation of the meta-stable neural system, a field-receptive mental workspace is proposed to be communicating with the brain. Our integral nervous system is seen as a dedicated neural transmission and multi-cavity network that, in a non-dual manner, interacts with the proposed supervening meta-cognitive domain. Among others, it is integrating discrete patterns of eigen-frequencies of photonic/solitonic waves, thereby continuously updating a time-symmetric global memory space of the individual. Its toroidal organization allows the coupling of gravitational, dark energy, zero-point energy field (ZPE) as well as earth magnetic fields energies and transmits wave information into brain tissue, that thereby is instrumental in high speed conscious and sub-conscious information processing. We propose that the supposed field-receptive workspace, in a mutual interaction with the whole nervous system, generates self-consciousness and is conceived as operating from a 4th spatial dimension (hyper-sphere). Its functional structure is adequately defined by the geometry of the torus, that is envisioned as a basic unit (operator) of space-time. The latter is instrumental in collecting the pattern of discrete soliton frequencies that provided an algorithm for coherent life processes, as earlier identified by us. It is postulated that consciousness in the entire universe arises through, scale invariant, nested toroidal coupling of various energy fields, that may include quantum error correction. In the brain of the human species, this takes the form of the proposed holographic workspace, that collects active information in a "brain event horizon", representing an internal and fully integral model of the self. This brain-supervening workspace is equipped to convert integrated coherent wave energies into attractor type/standing waves that guide the related cortical template to a higher coordination of reflection and action as well as network synchronicity, as required for conscious states. In relation to its scale-invariant global character, we find support for a universal information matrix, that was extensively described earlier, as a supposed implicate order as well as in a spectrum of space-time theories in current physics. The presence of a field-receptive resonant workspace, associated with, but not reducible to, our brain, may provide an interpretation framework for widely reported, but poorly understood transpersonal conscious states and algorithmic origin of life. It also points out the deep connection of mankind with the cosmos and our major responsibility for the future of our planet.</p
Quasars: a supermassive rotating toroidal black hole interpretation
A supermassive rotating toroidal black hole (TBH) is proposed as the
fundamental structure of quasars and other jet-producing active galactic
nuclei. Rotating protogalaxies gather matter from the central gaseous region
leading to the birth of massive toroidal stars whose internal nuclear reactions
proceed very rapidly. Once the nuclear fuel is spent, gravitational collapse
produces a slender ring-shaped TBH remnant. These events are typically the
first supernovae of the host galaxies. Given time the TBH mass increases
through continued accretion by several orders of magnitude, the event horizon
swells whilst the central aperture shrinks. The difference in angular
velocities between the accreting matter and the TBH induces a magnetic field
that is strongest in the region of the central aperture and innermost
ergoregion. Due to the presence of negative energy states when such a
gravitational vortex is immersed in an electromagnetic field, circumstances are
near ideal for energy extraction via non-thermal radiation including the
Penrose process and superradiant scattering. This establishes a self-sustaining
mechanism whereby the transport of angular momentum away from the quasar by
relativistic bi-directional jets reinforces both the modulating magnetic field
and the TBH/accretion disk angular velocity differential. Quasar behaviour is
extinguished once the BH topology becomes spheroidal. Similar mechanisms may be
operating in microquasars, SNe and GRBs when neutron density or BH tori arise.
In certain circumstances, long-term TBH stability can be maintained by a
negative cosmological constant, otherwise the classical topology theorems must
somehow be circumvented. Preliminary evidence is presented that Planck-scale
quantum effects may be responsible.Comment: 26 pages, 14 figs, various corrections and enhancements, final
versio
On long-term modulation of the Sun’s magnetic cycle
We utilize reconstructions based on cosmogenic radionuclides as well as direct observations of solar magnetic activity, to argue that the solar dynamo has operated similarly to the present day for at least the past 10 000 yr. The persistence of the 87-yr Gleissberg cycle throughout supermodulation events suggests that the Hale and Schwabe cycles continue independently of the modulational mechanism for activity. We further analyse behaviour of solar activity during the Spörer and Maunder Minima. Such grand minima recur with the characteristic de Vries period of approximately 208 yr but their incidence is modulated by the Hallstatt cycle with a characteristic period of around 2300 yr.We ascribe the latter to supermodulation, caused by breaking the symmetry of the dynamo pattern. Finally, we emphasize the need for further calculations in order to determine the effects of changes in solar field morphology and symmetry on the solar wind and on cosmic ray deflection
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