scholarly journals Interstellar and intergalactic dynamos

2012 ◽  
Vol 8 (S294) ◽  
pp. 225-236
Author(s):  
M. Hanasz ◽  
D. Woltanski ◽  
K. Kowalik
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Numerical Models ◽  
Rotation Period ◽  
Cosmic Ray ◽  
Small Scale ◽  
Galactic Rotation ◽  
Galaxy Mergers ◽  
Wide Range

AbstractWe review recent developments of amplification models of galactic and intergalactic magnetic field. The most popular scenarios involve variety of physical mechanisms, including turbulence generation on a wide range of physical scales, effects of supernovae, buoyancy as well as the magnetorotational instability. Other models rely on galaxy interaction, which generate galactic and intergalactic magnetic fields during galaxy mergers. We present also global galactic-scale numerical models of the Cosmic Ray (CR) driven dynamo, which was originally proposed by Parker (1992). We conduct a series of direct CR+MHD numerical simulations of the dynamics of the interstellar medium (ISM), composed of gas, magnetic fields and CR components. We take into account CRs accelerated in randomly distributed supernova (SN) remnants, and assume that SNe deposit small-scale, randomly oriented, dipolar magnetic fields into the ISM. The amplification timescale of the large-scale magnetic field resulting from the CR-driven dynamo is comparable to the galactic rotation period. The process efficiently converts small-scale magnetic fields of SN-remnants into galactic-scale magnetic fields. The resulting magnetic field structure resembles the X-shaped magnetic fields observed in edge-on galaxies.

scholarly journals Cosmic-ray driven dynamo in galaxies

2010 ◽  
Vol 6 (S274) ◽  
pp. 355-360 ◽  
Author(s):  
M. Hanasz ◽  
D. Wóltanski ◽  
K. Kowalik ◽  
H. Kotarba
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Numerical Models ◽  
Rotation Period ◽  
Cosmic Ray ◽  
Small Scale ◽  
Galactic Rotation ◽  
Large Scale Magnetic Field ◽  
Recent Developments

AbstractWe present recent developments of global galactic-scale numerical models of the Cosmic Ray (CR) driven dynamo, which was originally proposed by Parker (1992). We conduct a series of direct CR+MHD numerical simulations of the dynamics of the interstellar medium (ISM), composed of gas, magnetic fields and CR components. We take into account CRs accelerated in randomly distributed supernova (SN) remnants, and assume that SNe deposit small-scale, randomly oriented, dipolar magnetic fields into the ISM. The amplification timescale of the large-scale magnetic field resulting from the CR-driven dynamo is comparable to the galactic rotation period. The process efficiently converts small-scale magnetic fields of SN-remnants into galactic-scale magnetic fields. The resulting magnetic field structure resembles the X-shaped magnetic fields observed in edge-on galaxies.

scholarly journals Numerical models of sunspot formation and fine structure

2012 ◽  
Vol 370 (1970) ◽  
pp. 3114-3128 ◽  
Author(s):  
Matthias Rempel
Keyword(s):  
Magnetic Field ◽  
Fine Structure ◽  
Large Scale ◽  
Numerical Models ◽  
Dynamical Evolution ◽  
Solar Interior ◽  
Small Scale ◽  
Wide Range ◽  
Umbral Dots ◽  
The Magnetic Field

Sunspots are central to our understanding of solar (and stellar) magnetism in many respects. On the large scale, they link the magnetic field observable in the photosphere to the dynamo processes operating in the solar interior. Properly interpreting the constraints that sunspots impose on the dynamo process requires a detailed understanding of the processes involved in their formation, dynamical evolution and decay. On the small scale, they give an insight into how convective energy transport interacts with the magnetic field over a wide range of field strengths and inclination angles, leading to sunspot fine structure observed in the form of umbral dots and penumbral filaments. Over the past decade, substantial progress has been made on both observational and theoretical sides. Advanced ground- and space-based observations have resolved, for the first time, the details of umbral dots and penumbral filaments and discovered similarities in their substructures. Numerical models have advanced to the degree that simulations of entire sunspots with sufficient resolution to resolve sunspot fine structure are feasible. A combination of improved helioseismic inversion techniques with seismic forward modelling provides new views on the subsurface structure of sunspots. In this review, we summarize recent progress, with particular focus on numerical modelling.

scholarly journals Cosmic-ray driven dynamo in galactic disks

2008 ◽  
Vol 4 (S259) ◽  
pp. 479-484 ◽  
Author(s):  
Michał Hanasz ◽  
K. Otmianowska-Mazur ◽  
H. Lesch ◽  
G. Kowal ◽  
M. Soida ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Supernova Remnants ◽  
Large Scale ◽  
Cosmic Ray ◽  
Turbulent Energy ◽  
Field Energy ◽  
Small Scale ◽  
Galactic Rotation ◽  
Dynamo Action ◽  
Magnetic Field Energy

AbstractWe present new developments on the Cosmic–Ray driven, galactic dynamo, modeled by means of direct, resistive CR–MHD simulations, performed with ZEUS and PIERNIK codes. The dynamo action, leading to the amplification of large–scale galactic magnetic fields on galactic rotation timescales, appears as a result of galactic differential rotation, buoyancy of the cosmic ray component and resistive dissipation of small–scale turbulent magnetic fields. Our new results include demonstration of the global–galactic dynamo action driven by Cosmic Rays supplied in supernova remnants. An essential outcome of the new series of global galactic dynamo models is the equipartition of the gas turbulent energy with magnetic field energy and cosmic ray energy, in saturated states of the dynamo on large galactic scales.

scholarly journals Large Scale Solar Magnetic Fields and Their Consequences

1971 ◽  
Vol 43 ◽  
pp. 547-568 ◽  
Author(s):  
Gordon Newkirk
Keyword(s):  
Magnetic Field ◽  
Active Region ◽  
Magnetic Fields ◽  
Differential Rotation ◽  
Large Scale ◽  
Mechanical Energy ◽  
Rotation Period ◽  
Small Scale ◽  
Solar Magnetic Fields ◽  
Radio Bursts

The general properties of large scale solar magnetic fields are reviewed. In order of size these are: (1) Active region, generally bipolar fields with a lifetime of about two solar rotations. These are characterized by fields of several hundred G and display differential rotation similar to that found for the photosphere. (2) UM regions which appear to be the remnants of active region fields dispersed by the action of supergranulation convection and distorted by differential rotation. These are characterized by fields of a few tens of gauss and have lifetimes of several solar rotations. (3) The polar fields which are built up over the solar cycle by the preferential migration of a given polarity towards the poles. The poloidal fields are of a few gauss in magnitude and reverse sign in about 22 yr. (4) The large scale sector fields. These appear closely related to the interplanetary sector structure, cover tens of degrees in longitude, and stretch across the equator with the same polarity. This pattern endures for periods of up to a year or more, is not distorted by differential rotation, and has a rotation period of about 27 days. The presence of these long enduring sector fields may be related to the phenomenon of active solar longitudes. The consequences of large scale fields are examined with particular emphasis on the effects displayed by the corona. Calculated magnetic field patterns in the corona are compared with the density structure of the corona with the conclusion that: (1) Small scale structures in the corona, such as rays, arches, and loops, reflect the shape of the field and appear as magnetic tubes of force preferentially filled with more coronal plasma than the background. (2) Coronal density enhancements appear over plages where the field strength and presumably the mechanical energy transport into the corona are higher than normal. (3) Coronal streamers form above the ‘neutral line’ between extended UM regions of opposite polarity. The role played by coronal magnetic fields in transient events is also discussed. Some examples are: (1) The location of Proton Flares in open, diverging configurations of the field. (2) The expulsion of ‘magnetic bottles’ into the interplanetary medium by solar flares. (3) The relation of Type IV radio bursts to the ambient field configuration. (4) The guiding of Type II burst exciters by the ambient magnetic field. (5) The magnetic connection between widely separated active regions which display correlated radio bursts.

scholarly journals Exploring the magnetic topologies of cool stars

2010 ◽  
Vol 6 (S273) ◽  
pp. 181-187
Author(s):  
J. Morin ◽  
J.-F. Donati ◽  
P. Petit ◽  
L. Albert ◽  
M. Auriére ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Rotation Period ◽  
Spectral Lines ◽  
Doppler Imaging ◽  
Magnetic Field Generation ◽  
Wide Range ◽  
Cool Stars ◽  
The Impact

AbstractMagnetic fields of cool stars can be directly investigated through the study of the Zeeman effect on photospheric spectral lines using several approaches. With spectroscopic measurement in unpolarised light, the total magnetic flux averaged over the stellar disc can be derived but very little information on the field geometry is available. Spectropolarimetry provides a complementary information on the large-scale magnetic topology. With Zeeman-Doppler Imaging (ZDI), this information can be retrieved to produce a map of the vector magnetic field at the surface of the star, and in particular to assess the relative importance of the poloidal and toroidal components as well as the degree of axisymmetry of the field distribution.The development of high-performance spectropolarimeters associated with multi-lines techniques and ZDI allows us to explore magnetic topologies throughout the Hertzsprung-Russel diagram, on stars spanning a wide range of mass, age and rotation period. These observations bring novel constraints on magnetic field generation by dynamo effect in cool stars. In particular, the study of solar twins brings new insight on the impact of rotation on the solar dynamo, whereas the detection of strong and stable dipolar magnetic fields on fully convective stars questions the precise role of the tachocline in this process.

scholarly journals No-z Model for Magnetic Fields of Different Astrophysical Objects and Stability of the Solutions

Data ◽  
2021 ◽  
Vol 6 (1) ◽  
pp. 4
Author(s):  
Evgeny Mikhailov ◽  
Daniela Boneva ◽  
Maria Pashentseva
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Point Of View ◽  
Accretion Discs ◽  
Wide Range ◽  
Astrophysical Objects ◽  
Alpha Effect ◽  
The Stability ◽  
The Magnetic Field

A wide range of astrophysical objects, such as the Sun, galaxies, stars, planets, accretion discs etc., have large-scale magnetic fields. Their generation is often based on the dynamo mechanism, which is connected with joint action of the alpha-effect and differential rotation. They compete with the turbulent diffusion. If the dynamo is intensive enough, the magnetic field grows, else it decays. The magnetic field evolution is described by Steenbeck—Krause—Raedler equations, which are quite difficult to be solved. So, for different objects, specific two-dimensional models are used. As for thin discs (this shape corresponds to galaxies and accretion discs), usually, no-z approximation is used. Some of the partial derivatives are changed by the algebraic expressions, and the solenoidality condition is taken into account as well. The field generation is restricted by the equipartition value and saturates if the field becomes comparable with it. From the point of view of mathematical physics, they can be characterized as stable points of the equations. The field can come to these values monotonously or have oscillations. It depends on the type of the stability of these points, whether it is a node or focus. Here, we study the stability of such points and give examples for astrophysical applications.

Coupled Thermo-Hydro-Geochemical Models of Engineered Barrier Systems: The Febex Project

2000 ◽  
Vol 663 ◽  
Author(s):  
J. Samper ◽  
R. Juncosa ◽  
V. Navarro ◽  
J. Delgado ◽  
L. Montenegro ◽  
...  
Keyword(s):  
Large Scale ◽  
Reference Model ◽  
Numerical Models ◽  
Full Scale ◽  
Small Scale ◽  
Model Parameters ◽  
In Situ Test ◽  
Wide Range ◽  
Engineered Barrier

ABSTRACTFEBEX (Full-scale Engineered Barrier EXperiment) is a demonstration and research project dealing with the bentonite engineered barrier designed for sealing and containment of waste in a high level radioactive waste repository (HLWR). It includes two main experiments: an situ full-scale test performed at Grimsel (GTS) and a mock-up test operating since February 1997 at CIEMAT facilities in Madrid (Spain) [1,2,3]. One of the objectives of FEBEX is the development and testing of conceptual and numerical models for the thermal, hydrodynamic, and geochemical (THG) processes expected to take place in engineered clay barriers. A significant improvement in coupled THG modeling of the clay barrier has been achieved both in terms of a better understanding of THG processes and more sophisticated THG computer codes. The ability of these models to reproduce the observed THG patterns in a wide range of THG conditions enhances the confidence in their prediction capabilities. Numerical THG models of heating and hydration experiments performed on small-scale lab cells provide excellent results for temperatures, water inflow and final water content in the cells [3]. Calculated concentrations at the end of the experiments reproduce most of the patterns of measured data. In general, the fit of concentrations of dissolved species is better than that of exchanged cations. These models were later used to simulate the evolution of the large-scale experiments (in situ and mock-up). Some thermo-hydrodynamic hypotheses and bentonite parameters were slightly revised during TH calibration of the mock-up test. The results of the reference model reproduce simultaneously the observed water inflows and bentonite temperatures and relative humidities. Although the model is highly sensitive to one-at-a-time variations in model parameters, the possibility of parameter combinations leading to similar fits cannot be precluded. The TH model of the “in situ” test is based on the same bentonite TH parameters and assumptions as for the “mock-up” test. Granite parameters were slightly modified during the calibration process in order to reproduce the observed thermal and hydrodynamic evolution. The reference model captures properly relative humidities and temperatures in the bentonite [3]. It also reproduces the observed spatial distribution of water pressures and temperatures in the granite. Once calibrated the TH aspects of the model, predictions of the THG evolution of both tests were performed. Data from the dismantling of the in situ test, which is planned for the summer of 2001, will provide a unique opportunity to test and validate current THG models of the EBS.

scholarly journals Characterising the surface magnetic fields of T Tauri stars with high-resolution near-infrared spectroscopy

2019 ◽  
Vol 630 ◽  
pp. A99 ◽  
Author(s):  
A. Lavail ◽  
O. Kochukhov ◽  
G. A. J. Hussain
Keyword(s):  
Magnetic Field ◽  
High Resolution ◽  
Magnetic Fields ◽  
Large Scale ◽  
Near Infrared ◽  
Field Measurements ◽  
T Tauri Stars ◽  
Small Scale ◽  
Surface Magnetic Field ◽  
T Tauri

Aims. In this paper, we aim to characterise the surface magnetic fields of a sample of eight T Tauri stars from high-resolution near-infrared spectroscopy. Some stars in our sample are known to be magnetic from previous spectroscopic or spectropolarimetric studies. Our goals are firstly to apply Zeeman broadening modelling to T Tauri stars with high-resolution data, secondly to expand the sample of stars with measured surface magnetic field strengths, thirdly to investigate possible rotational or long-term magnetic variability by comparing spectral time series of given targets, and fourthly to compare the magnetic field modulus ⟨B⟩ tracing small-scale magnetic fields to those of large-scale magnetic fields derived by Stokes V Zeeman Doppler Imaging (ZDI) studies. Methods. We modelled the Zeeman broadening of magnetically sensitive spectral lines in the near-infrared K-band from high-resolution spectra by using magnetic spectrum synthesis based on realistic model atmospheres and by using different descriptions of the surface magnetic field. We developped a Bayesian framework that selects the complexity of the magnetic field prescription based on the information contained in the data. Results. We obtain individual magnetic field measurements for each star in our sample using four different models. We find that the Bayesian Model 4 performs best in the range of magnetic fields measured on the sample (from 1.5 kG to 4.4 kG). We do not detect a strong rotational variation of ⟨B⟩ with a mean peak-to-peak variation of 0.3 kG. Our confidence intervals are of the same order of magnitude, which suggests that the Zeeman broadening is produced by a small-scale magnetic field homogeneously distributed over stellar surfaces. A comparison of our results with mean large-scale magnetic field measurements from Stokes V ZDI show different fractions of mean field strength being recovered, from 25–42% for relatively simple poloidal axisymmetric field topologies to 2–11% for more complex 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.

Methanol masers and magnetic field in IRAS18089-1732

2017 ◽  
Vol 13 (S336) ◽  
pp. 285-286
Author(s):  
Daria Dall’Olio ◽  
W. H. T. Vlemmings ◽  
G. Surcis ◽  
H. Beuther ◽  
B. Lankhaar ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
High Mass ◽  
Small Scale ◽  
Star Forming ◽  
Large Scale Field ◽  
The Impact ◽  
Theoretical Simulations ◽  
The Magnetic Field

AbstractTheoretical simulations have shown that magnetic fields play an important role in massive star formation: they can suppress fragmentation in the star forming cloud, enhance accretion via disc and regulate outflows and jets. However, models require specific magnetic configurations and need more observational constraints to properly test the impact of magnetic fields. We investigate the magnetic field structure of the massive protostar IRAS18089-1732, analysing 6.7 GHz CH3OH maser MERLIN observations. IRAS18089-1732 is a well studied high mass protostar, showing a hot core chemistry, an accretion disc and a bipolar outflow. An ordered magnetic field oriented around its disc has been detected from previous observations of polarised dust. This gives us the chance to investigate how the magnetic field at the small scale probed by masers relates to the large scale field probed by the dust.

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