scholarly journals Turbulence and dynamo interlinks

2012 ◽  
Vol 8 (S294) ◽  
pp. 337-348
Author(s):  
E. M. de Gouveia Dal Pino ◽  
R. Santos-Lima ◽  
G. Kowal ◽  
D. Falceta-Gonçalves
Keyword(s):  
Magnetic Fields ◽  
Magnetic Flux ◽  
Magnetic Reconnection ◽  
Small Scale ◽  
Low Density ◽  
Mhd Turbulence ◽  
Turbulent Dynamo ◽  
Dynamo Process

AbstractThe role of turbulence in astrophysical environments and its interplay with magnetic fields is still highly debated. In this lecture, we will discuss this issue in the framework of dynamo processes. We will first present a very brief summary of turbulent dynamo theories, then will focus on small scale turbulent dynamos and their particular relevance on the origin and maintenance of magnetic fields in the intra-cluster media (ICM) of galaxies. In these environments, the very low density of the flow requires a collisionless-MHD treatment. We will show the implications of this approach in the turbulent amplification of the magnetic fields in these environments. To finalize, we will also briefly address the connection between MHD turbulence and fast magnetic reconnection and its possible implications in the diffusion of magnetic flux in the dynamo process.

scholarly journals The Small-Scale Photospheric Magnetic Field as an Indicator of the Dynamo

1993 ◽  
Vol 141 ◽  
pp. 143-146
Author(s):  
K. Petrovay ◽  
G. Szakály
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Magnetic Flux ◽  
Observational Data ◽  
Convective Zone ◽  
Small Scale ◽  
Dynamo Theory ◽  
Flux Tubes ◽  
Magnetic Flux Tubes ◽  
Surface Fields

AbstractThe presently widely accepted view that the solar dynamo operates near the base of the convective zone makes it difficult to relate the magnetic fields observed in the solar atmosphere to the fields in the dynamo layer. The large amount of observational data concerning photospheric magnetic fields could in principle be used to impose constraints on dynamo theory, but in order to infer these constraints the above mentioned “missing link” between the dynamo and surface fields should be found. This paper proposes such a link by modeling the passive vertical transport of thin magnetic flux tubes through the convective zone.

scholarly journals Small-Scale Solar Magnetic Fields

1976 ◽  
Vol 71 ◽  
pp. 69-99 ◽  
Author(s):  
J. O. Stenflo
Keyword(s):  
Magnetic Fields ◽  
Magnetic Flux ◽  
Solar Surface ◽  
Life Time ◽  
Wave Number ◽  
Small Scale ◽  
Solar Magnetic Fields ◽  
Field Amplification ◽  
Wave Numbers ◽  
Diffuse Form

The observed properties of small-scale solar magnetic fields are reviewed. Most of the magnetic flux in the photosphere is in the form of strong fields of about 100–200 mT (1–2 kG), which have remarkably similar properties regardless of whether they occur in active or quiet regions. These fields are associated with strong atmospheric heating. Flux concentrations decay at a rate of about 107 Wb s-1, independent of the amount of flux in the decaying structure. The decay occurs by smaller flux fragments breaking loose from the larger ones, i.e. a transfer of magnetic flux from smaller to larger Fourier wave numbers, into the wave-number regime where ohmic diffusion becomes significant. This takes place in a time-scale much shorter than the length of the solar cycle.The field amplification occurs mainly below the solar surface, since very little magnetic flux appears in diffuse form in the photosphere, and the life-time of the smallest flux elements is very short. The observations further suggest that most of the magnetic flux in quiet regions is supplied directly from below the solar surface rather than being the result of turbulent diffusion of active-region magnetic fields.

Solar Dynamo

2020 ◽  
Author(s):  
Robert Cameron
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Magnetic Flux ◽  
Differential Rotation ◽  
Large Scale ◽  
Toroidal Field ◽  
Solar Dynamo ◽  
Small Scale ◽  
The Sun ◽  
The Magnetic Field

The solar dynamo is the action of flows inside the Sun to maintain its magnetic field against Ohmic decay. On small scales the magnetic field is seen at the solar surface as a ubiquitous “salt-and-pepper” disorganized field that may be generated directly by the turbulent convection. On large scales, the magnetic field is remarkably organized, with an 11-year activity cycle. During each cycle the field emerging in each hemisphere has a specific East–West alignment (known as Hale’s law) that alternates from cycle to cycle, and a statistical tendency for a North-South alignment (Joy’s law). The polar fields reverse sign during the period of maximum activity of each cycle. The relevant flows for the large-scale dynamo are those of convection, the bulk rotation of the Sun, and motions driven by magnetic fields, as well as flows produced by the interaction of these. Particularly important are the Sun’s large-scale differential rotation (for example, the equator rotates faster than the poles), and small-scale helical motions resulting from the Coriolis force acting on convective motions or on the motions associated with buoyantly rising magnetic flux. These two types of motions result in a magnetic cycle. In one phase of the cycle, differential rotation winds up a poloidal magnetic field to produce a toroidal field. Subsequently, helical motions are thought to bend the toroidal field to create new poloidal magnetic flux that reverses and replaces the poloidal field that was present at the start of the cycle. It is now clear that both small- and large-scale dynamo action are in principle possible, and the challenge is to understand which combination of flows and driving mechanisms are responsible for the time-dependent magnetic fields seen on the Sun.

scholarly journals Numerical Simulation of the Explosive Events in the Solar Atmosphere

1993 ◽  
Vol 141 ◽  
pp. 134-137
Author(s):  
Shu-Ping Jiu
Keyword(s):  
Numerical Simulation ◽  
Magnetic Flux ◽  
Magnetic Reconnection ◽  
Solar Atmosphere ◽  
Plasma Flow ◽  
Small Scale ◽  
Flare Activity ◽  
Specific Location ◽  
Mhd Simulations ◽  
Explosive Events

AbstractExplosive events are the earliest indicators of flare activity and potentially predict the imminent occurrence of a flare at a specific location. They are highly energetic small-scale phenomena which are frequently detected throughout the quiet and active sun. The observations show that explosive events are related to emerging magnetic flux and tend to occur on the edges of high photospheric magnentic field regions. The cancellation of photospheric magnetic flux are the manifestation of explosive events, so that they are identified as the magnetic reconnection of flux elements. We assume that emerging flux are convected to the network boundaries with the typical velocity of intranetwork elements. Two-dimension (2D) compressible MHD simulations are performed to explore the reconnection process between emerging intranework flux and network field. The numerical results clearly show the cancellation of magnetic flux and the acceleration of the plasma flow.

scholarly journals Dynamo and Fossil Magnetic Field in Young Stars

1993 ◽  
Vol 157 ◽  
pp. 171-175
Author(s):  
A.E. Dudorov
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Main Sequence ◽  
Young Stars ◽  
Small Scale ◽  
Scaling Relations ◽  
Main Sequence Stars ◽  
Nonlinear Stabilization ◽  
Turbulent Dynamo ◽  
Solar Type

The theory of fossil magnetic fields shows that new born stars may have internal magnetic fields of more than 1 million gauss. Convection inside young solar type stars will tangle any strong fossil magnetic field. The small scale magnetic field rises to the surface and determines the young stars activity attenuating with their age. When a fossil field is diminished a turbulent dynamo may begin to work in the condition of nonlinear stabilization. The scaling relations for the turbulent αω dynamo show that the strength of the generated “fossil” magnetic field inside the main sequence stars is stabilized on the level one tenth — 10 millions gauss, depending on the mass of the stars.

scholarly journals MHD turbulence-Star Formation Connection: from pc to kpc scales

2010 ◽  
Vol 6 (S274) ◽  
pp. 333-339 ◽  
Author(s):  
E. M. de Gouveia Dal Pino ◽  
R. Santos-Lima ◽  
A. Lazarian ◽  
M. R. M. Leão ◽  
D. Falceta-Gonçalves ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Flux ◽  
Numerical Simulations ◽  
Magnetic Reconnection ◽  
Turbulent Flows ◽  
Molecular Clouds ◽  
Three Dimensional ◽  
Mhd Turbulence ◽  
Star Forming

AbstractThe transport of magnetic flux to outside of collapsing molecular clouds is a required step to allow the formation of stars. Although ambipolar diffusion is often regarded as a key mechanism for that, it has been recently argued that it may not be efficient enough. In this review, we discuss the role that MHD turbulence plays in the transport of magnetic flux in star forming flows. In particular, based on recent advances in the theory of fast magnetic reconnection in turbulent flows, we will show results of three-dimensional numerical simulations that indicate that the diffusion of magnetic field induced by turbulent reconnection can be a very efficient mechanism, especially in the early stages of cloud collapse and star formation. To conclude, we will also briefly discuss the turbulence-star formation connection and feedback in different astrophysical environments: from galactic to cluster of galaxy scales.

Renormalization group analysis of the magnetohydrodynamic turbulence and dynamo

2021 ◽  
Vol 926 ◽  
Author(s):  
Krzysztof A. Mizerski
Keyword(s):  
Renormalization Group ◽  
Magnetic Fields ◽  
Large Scale ◽  
Magnetic Energy ◽  
Group Analysis ◽  
Small Scale ◽  
Mhd Turbulence ◽  
Dynamical Equations ◽  
Renormalization Group Analysis ◽  
Magnetic Diffusivity

The magnetohydrodynamic (MHD) turbulence appears in engineering laboratory flows and is a common phenomenon in natural systems, e.g. stellar and planetary interiors and atmospheres and the interstellar medium. The applications in engineering are particularly interesting due to the recent advancement of tokamak devices, reaching very high plasma temperatures, thus giving hope for the production of thermonuclear fusion power. In the case of astrophysical applications, perhaps the main feature of the MHD turbulence is its ability to generate and sustain large-scale and small-scale magnetic fields. However, a crucial effect of the MHD turbulence is also the enhancement of large-scale diffusion via interactions of small-scale pulsations, i.e. the generation of the so-called turbulent viscosity and turbulent magnetic diffusivity, which typically exceed by orders of magnitude their molecular counterparts. The enhanced resistivity plays an important role in the turbulent dynamo process. Estimates of the turbulent electromotive force (EMF), including the so-called $\alpha$ -effect responsible for amplification of the magnetic energy and the turbulent magnetic diffusion are desired. Here, we apply the renormalization group technique to extract the final expression for the turbulent EMF from the fully nonlinear dynamical equations (Navier–Stokes, induction equation). The simplified renormalized set of dynamical equations, including the equations for the means and fluctuations, is derived and the effective turbulent coefficients such as the viscosity, resistivity, the $\alpha$ -coefficient and the Lorentz-force coefficients are explicitly calculated. The results are also used to demonstrate the influence of magnetic fields on energy and helicity spectra of strongly turbulent flows, in particular the magnetic energy spectrum.

scholarly journals SIGNATURES OF MAGNETIC RECONNECTION AT BOUNDARIES OF INTERPLANETARY SMALL-SCALE MAGNETIC FLUX ROPES

2010 ◽  
Vol 720 (1) ◽  
pp. 454-464 ◽  
Author(s):  
Hui Tian ◽  
Shuo Yao ◽  
Qiugang Zong ◽  
Jiansen He ◽  
Yu Qi
Keyword(s):  
Magnetic Flux ◽  
Magnetic Reconnection ◽  
Small Scale ◽  
Magnetic Flux Ropes ◽  
Flux Ropes

Magnetohydrodynamic Turbulence Decay Under the Influence of Uniform or Random Magnetic Fields

2011 ◽  
Vol 133 (8) ◽  
Author(s):  
Jacques C. Richard ◽  
Benjamin M. Riley ◽  
Sharath S. Girimaji
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Lorentz Force ◽  
Magnetic Energy ◽  
Small Scale ◽  
Magnetohydrodynamic Turbulence ◽  
Turbulence Decay ◽  
Structure Orientation ◽  
The Magnetic Field

We perform direct numerical simulations of decaying magnetohydrodynamic turbulence subject to initially uniform or random magnetic fields. We investigate the following features: (i) kinetic–magnetic energy exchange and velocity field anisotropy, (ii) action of Lorentz force, (iii) enstrophy and helicity behavior, and (iv) internal structure of the small scales. While tendency toward kinetic–magnetic energy equi-partition is observed in both uniform and random magnetic field simulations, the manner of approach to that state is very different in the two cases. Overall, the role of the Lorentz force is merely to bring about the equi-partition. No significant variance anisotropy of velocity fluctuations is observed in any of the simulations. The mechanism of enstrophy generation changes with the strength of the magnetic field, and helicity shows no significant growth in any of the cases. The small-scale structure (orientation between vorticity and strain-rate eigenvectors) does not appear to be influenced by the magnetic field.

scholarly journals Mass and energy supply of a cool coronal loop near its apex

2018 ◽  
Vol 611 ◽  
pp. A49 ◽  
Author(s):  
Limei Yan ◽  
Hardi Peter ◽  
Jiansen He ◽  
Lidong Xia ◽  
Linghua Wang
Keyword(s):  
Magnetic Field ◽  
Magnetic Flux ◽  
Magnetic Reconnection ◽  
Transition Region ◽  
Doppler Velocity ◽  
Small Scale ◽  
Solar Dynamics Observatory ◽  
Plasma Properties ◽  
Flux Tubes ◽  
Magnetic Flux Tubes

Context. Different models for the heating of solar corona assume or predict different locations of the energy input: concentrated at the footpoints, at the apex, or uniformly distributed. The brightening of a loop could be due to the increase in electron density ne, the temperature T, or a mixture of both.Aim. We investigate possible reasons for the brightening of a cool loop at transition region temperatures through imaging and spectral observation.Methods. We observed a loop with the Interface Region Imaging Spectrograph (IRIS) and used the slit-jaw images together with spectra taken at a fixed slit position to study the evolution of plasma properties in and below the loop. We used spectra of Si iv, which forms at around 80 000 K in equilibrium, to identify plasma motions and derive electron densities from the ratio of inter-combination lines of O IV. Additional observations from the Solar Dynamics Observatory (SDO) were employed to study the response at coronal temperatures (Atmospheric Imaging Assembly, AIA) and to investigate the surface magnetic field below the loop (Helioseismic and Magnetic Imager, HMI).Results. The loop first appears at transition region temperatures and later also at coronal temperatures, indicating a heating of the plasma in the loop. The appearance of hot plasma in the loop coincides with a possible accelerating upflow seen in Si IV, with the Doppler velocity shifting continuously from ~−70 km s−1 to ~−265 km s−1. The 3D magnetic field lines extrapolated from the HMI magnetogram indicate possible magnetic reconnection between small-scale magnetic flux tubes below or near the loop apex. At the same time, an additional intensity enhancement near the loop apex is visible in the IRIS slit-jaw images at 1400 Å. These observations suggest that the loop is probably heated by the interaction between the loop and the upflows, which are accelerated by the magnetic reconnection between small-scale magnetic flux tubes at lower altitudes. Before and after the possible heating phase, the intensity changes in the optically thin (Si IV) and optical thick line (C II) are mainly contributed by the density variation without significant heating.Conclusions. We therefore provide evidence for the heating of an envelope loop that is affected by accelerating upflows, which are probably launched by magnetic reconnection between small-scale magnetic flux tubes underneath the envelope loop. This study emphasizes that in the complex upper atmosphere of the Sun, the dynamics of the 3D coupled magnetic field and flow field plays a key role in thermalizing 1D structures such as coronal loops.

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