scholarly journals Global simulations of stellar dynamos

2019 ◽  
Vol 15 (S354) ◽  
pp. 65-85
Author(s):  
G. Guerrero
Keyword(s):  
Magnetic Field ◽  
Spherical Shell ◽  
Ab Initio ◽  
Open Problem ◽  
Magnetic Activity ◽  
Large Eddy Simulations ◽  
The Past ◽  
Large Eddy ◽  
Dynamo Mechanism ◽  
Solar Magnetic Activity

AbstractThe dynamo mechanism, responsible for the solar magnetic activity, is still an open problem in astrophysics. Different theories proposed to explain such phenomena have failed in reproducing the observational properties of the solar magnetism. Thus, ab-initio computational modeling of the convective dynamo in a spherical shell turns out as the best alternative to tackle this problem. In this work we review the efforts performed in global simulations over the past decades. Regarding the development and sustain of mean-flows, as well as mean magnetic field, we discuss the points of agreement and divergence between the different modeling strategies. Special attention is given to the implicit large-eddy simulations performed with the EULAG-MHD code.

Periodicity and aperiodicity in solar magnetic activity

1990 ◽  
Vol 330 (1615) ◽  
pp. 617-625 ◽  
Keyword(s):  
Magnetic Activity ◽  
Convective Zone ◽  
Magnetic Cycle ◽  
The Past ◽  
Proxy Records ◽  
Solar Magnetic Activity ◽  
Hydromagnetic Dynamo ◽  
Dynamo Waves ◽  
Plausible Theory ◽  
Sunspot Record

Solar activity varies irregularly with an 11-year period whereas the magnetic cycle has a period of 22 years. Similar cycles of activity are seen in other slowly rotating late-type stars. The only plausible theory for their origin ascribes them to a hydromagnetic dynamo operating at, or just below, the base of the convective zone. Linear (kinematic) dynamo models yield strictly periodic solutions with dynamo waves propagating towards or away from the equator. Nonlinear (magnetohydrodynamic) dynamo models allow transitions from periodic to quasi-periodic to chaotic behaviour, as well as loss of spatial symmetry followed by the development of complex spatial structure. Results from simple models can be compared with the observed sunspot record over the past 380 years and with proxy records extending over 9000 years, which show aperiodic modulation of the 11-year cycle.

Stochastic Resonance could explain Recurrence of Grand Minima

2020 ◽  
Author(s):  
Carlo Albert ◽  
Simone Ulzega
Keyword(s):  
Magnetic Field ◽  
Solar Activity ◽  
Stochastic Resonance ◽  
Intrinsic Noise ◽  
Magnetic Activity ◽  
Stable States ◽  
Frequent Switching ◽  
Solar Magnetic Activity ◽  
Stochastic Time ◽  
Grand Minima

<p>Proxies of solar activity have revealed repeated Grand Minima that occur with a certain regularity associated with the well-known Gleissberg and Süss/deVries cycles. These and other prominent cycles in the spectrum of solar activity are also seen in the spectrum of the planetary torque exerted on the solar tachocline, which has revived the hypothesis of a planetary influence on solar activity. It is not clear, however, how the extremely weak planetary forcing could influence the solar magnetic activity. Here, we suggest that stochastic resonance could explain the necessary amplification of the forcing and provide numerical evidence from stochastic time-delayed dynamo models. If the intrinsic noise of the solar dynamo allows for a frequent switching between active and quiescent stable states, tiny periodic forcings can be greatly amplified, provided the dynamo is poised close to a critical point. Such a forcing could be caused by a tidal modulation of the minimal magnetic field required for flux-tube buoyancy.</p>

scholarly journals Manifestations of Solar Magnetic Fields

1998 ◽  
Vol 11 (2) ◽  
pp. 857-860
Author(s):  
S.K. Solanki
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Solar Magnetic Field ◽  
Magnetic Activity ◽  
The Sun ◽  
Solar Magnetic Fields ◽  
Quiet Sun ◽  
Solar Magnetic Activity

AbstractThe magnetism of the Sun manifests itself in innumerable ways, many of which constitute what is referred to as solar magnetic activity, while others are counted among the phenomena of the quiet Sun. After a brief overview of the structure of the solar magnetic field, a few examples of its manifestations are pointed out.

scholarly journals Large eddy simulations in 2030 and beyond

2014 ◽  
Vol 372 (2022) ◽  
pp. 20130320 ◽  
Author(s):  
U Piomelli
Keyword(s):  
Stokes Equations ◽  
Academic Community ◽  
Large Eddy Simulations ◽  
Navier Stokes ◽  
Academic Environment ◽  
Flow Conditions ◽  
Local Flow ◽  
Navier Stokes Equations ◽  
The Past ◽  
Large Eddy

Since its introduction, in the early 1970s, large eddy simulations (LES) have advanced considerably, and their application is transitioning from the academic environment to industry. Several landmark developments can be identified over the past 40 years, such as the wall-resolved simulations of wall-bounded flows, the development of advanced models for the unresolved scales that adapt to the local flow conditions and the hybridization of LES with the solution of the Reynolds-averaged Navier–Stokes equations. Thanks to these advancements, LES is now in widespread use in the academic community and is an option available in most commercial flow-solvers. This paper will try to predict what algorithmic and modelling advancements are needed to make it even more robust and inexpensive, and which areas show the most promise.

scholarly journals Exploring shallow sunspot formation by using Implicit Large-eddy simulations

2016 ◽  
Vol 12 (S328) ◽  
pp. 117-119
Author(s):  
F. J. Camacho ◽  
G. Guerrero ◽  
P. K. Smolarkiewicz ◽  
A. G. Kosovichev ◽  
N. N. Mansour
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Active Regions ◽  
Homogeneous Magnetic Field ◽  
Large Eddy Simulations ◽  
The Sun ◽  
Surface Layers ◽  
Near Surface ◽  
Magnetic Structures ◽  
Large Eddy

AbstractThe mechanism by which sunspots are generated at the surface of the sun remains unclear. In the current literature two types of explanations can be found. The first one is related to the buoyant emergence of toroidal magnetic fields generated at the tachocline. The second one states that active regions are formed, from initially diffused magnetic flux, by MHD instabilities that develop in the near-surface layers of the Sun. Using the anelastic MHD code EULAG we address the problem of sunspot formation by performing implicit large-eddy simulations of stratified magneto-convection in a domain that resembles the near-surface layers of the Sun. The development of magnetic structures is explored as well as their effect on the convection dynamics. By applying a homogeneous magnetic field over an initially stationary hydrodynamic convective state, we investigate the formation of self-organized magnetic structures in the range of the initial magnetic field strength, 0.01 < B0/Beq < 0.5, where Beq is the characteristic equipartition field strength.

A detailed analysis of a dynamo mechanism in a rapidly rotating spherical shell

2012 ◽  
Vol 701 ◽  
pp. 228-250 ◽  
Author(s):  
F. Takahashi ◽  
H. Shimizu
Keyword(s):  
Magnetic Field ◽  
Spherical Shell ◽  
Large Scale ◽  
Magnetic Energy ◽  
Thin Sheet ◽  
Dynamo Model ◽  
Current Loop ◽  
Flow Acceleration ◽  
Dynamo Mechanism ◽  
The Magnetic Field

AbstractMechanisms of magnetic field intensification by flows of an electrically conducting fluid in a rapidly rotating spherical shell are investigated using a numerical dynamo model with an Ekman number of $1{0}^{\ensuremath{-} 5} $. A strong dipolar solution with a magnetic energy 55 times larger than the kinetic energy of thermal convection is obtained. In a regime of small viscosity and inertia with the strong magnetic field, the convection structure consists of a few large-scale retrograde flows in the azimuthal direction and localized thin sheet-like plumes. A detailed term-by-term analysis of the magnetic field amplification processes shows that the magnetic field is amplified through stretching of magnetic lines, which occurs typically through four types of flow: the retrograde azimuthal flow near the outer boundary, the downwelling flow of the sheet plume, the prograde azimuthal flow near the rim of the tangent cylinder, and the cylindrical-radially alternating flows of the plume cluster. The current loop structure emerges as a result of stretching the magnetic lines along the magnetic field by the flow acceleration. The most remarkable effects of the generated magnetic field on the flow come from the strong azimuthal (toroidal) magnetic field. Similarities of the present model in the convection and magnetic field structures to previous studies at larger and even smaller Ekman numbers suggest universality of the dynamo mechanism in rotating spherical dynamos.

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