magnetic field generation
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Author(s):  
О.В. Шереметьева

В работе используется маломодовая модель αΩ-динамо для моделирования режимов генерации магнитного поля при незначительных изменениях поля скорости вязкой жидкости. В рамках этой модели интенсивность α-эффекта регулируется процессом с памятью, который вводится в магнитогидродинамическую систему (МГД-система) как аддитивная поправка в виде функционала Z(t) от энергии поля. В качестве ядра J(t) функционала Z(t) выбрана функция, определяющая затухающие колебания с варьируемым коэффициентом затухания и постоянной частотой затухания, принятой равной единице. Исследование поведения магнитного поля проводится на больших временных масштабах, поэтому для численных расчётов используется перемасштабированная и обезразмеренная МГД-система, где в качестве единицы времени принято время диссипации магнитного поля (104 лет). Управляющими параметрами системы выступают число Рейнольдса и амплитуда α-эффекта, в которых заложена информация о крупномасштабном и турбулентном генераторах. Результаты численного моделирования режимов генерации магнитного поля при различных значениях коэффициента затухания и постоянной частоте затухания отражены на фазовой плоскости управляющих параметров. В работе исследуется вопрос о динамике изменения картины на фазовой плоскости в зависимости от значения коэффициента затухания. Проводится сравнение с результатами, полученными ранее при постоянной интенсивности α-эффекта и при изменении интенсивности α — эффекта, которое определялось функционалом Z(t) с показательным ядром и аналогичными значениями коэффициента затухания. In this paper, we use a low-mode αΩ-dynamo model to simulate the modes of magnetic field generation with insignificant changes in the velocity field of a viscous fluid. Within the framework of this model, an additive correction is introduced into the magnetohydrodynamic system to control the intensity of the α-effect in the form of a function Z(t) from the field energy. As the kernel J(t) of the function Z(t) is chosen the function that determines damped oscillations with the different values of the damping coefficient and a constant damping frequency taken equal to one. The study of the magnetic field behavior is carried out on a large time scales, therefore, for numerical calculations, a rescaled and dimensionless MHD-system is used, where the time of the magnetic field dissipation (104 years) is accepted as the unit of time. The main parameters of the system are the Reynolds number and the amplitude of the α-effect, which contains information about the large-scale and turbulent generators, respectively. According to the results of numerical simulation, an increase in the values of the damping coefficient is characterized an increase in the inhibition effect of the process Z(t) on the α-effect and decrease of the magnetic field divergence region on the plane of the main parameters.


2021 ◽  
Vol 932 ◽  
Author(s):  
Kengo Deguchi

Nonlinear Hall-magnetohydrodynamic dynamos associated with coherent structures in subcritical shear flows are investigated by using unstable invariant solutions. The dynamo solution found has a relatively simple structure, but it captures the features of the typical nonlinear structures seen in simulations, such as current sheets. As is well known, the Hall effect destroys the symmetry of the magnetohydrodynamic equations and thus modifies the structure of the current sheet and mean field of the solution. Depending on the strength of the Hall effect, the generation of the magnetic field changes in a complex manner. However, a too strong Hall effect always acts to suppress the magnetic field generation. The hydrodynamic/magnetic Reynolds number dependence of the critical ion skin depth at which the dynamos start to feel the Hall effect is of interest from an astrophysical point of view. An important consequence of the matched asymptotic expansion analysis of the solution is that the higher the Reynolds number, the smaller the Hall current affects the flow. We also briefly discuss how the above results for a relatively simple shear flow can be extended to more general flows such as infinite homogeneous shear flows and boundary layer flows. The analysis of the latter flows suggests that interestingly a strong induction of the generated magnetic field might occur when there is a background shear layer.


Author(s):  
Doris Breuer ◽  
Tilman Spohn ◽  
Tim Van Hoolst ◽  
Wim van Westrenen ◽  
Sabine Stanley ◽  
...  

AbstractThe Earth-like planets and moons in our solar system have iron-rich cores, silicate mantles, and a basaltic crust. Differentiated icy moons can have a core and a mantle and an outer water–ice layer. Indirect evidence for several icy moons suggests that this ice is underlain by or includes a water-rich ocean. Similar processes are at work in the interiors of these planets and moons, including heat transport by conduction and convection, melting and volcanism, and magnetic field generation. There are significant differences in detail, though, in both bulk chemical compositions and relative volume of metal, rock and ice reservoirs. For example, the Moon has a small core [~ 0.2 planetary radii (RP)], whereas Mercury’s is large (~ 0.8 RP). Planetary heat engines can operate in somewhat different ways affecting the evolution of the planetary bodies. Mercury and Ganymede have a present-day magnetic field while the core dynamo ceased to operate billions of years ago in the Moon and Mars. Planets and moons differ in tectonic style, from plate-tectonics on Earth to bodies having a stagnant outer lid and possibly solid-state convection underneath, with implications for their magmatic and atmosphere evolution. Knowledge about their deep interiors has improved considerably thanks to a multitude of planetary space missions but, in comparison with Earth, the data base is still limited. We describe methods (including experimental approaches and numerical modeling) and data (e.g., gravity field, rotational state, seismic signals, magnetic field, heat flux, and chemical compositions) used from missions and ground-based observations to explore the deep interiors, their dynamics and evolution and describe as examples Mercury, Venus, Moon, Mars, Ganymede and Enceladus.


2021 ◽  
pp. 124-128
Author(s):  
С.Ю. Маламанов ◽  
В.А. Павловский

Современные вычислительные средства с помощью новейших компьютерных технологий дают возможность производить моделирование и расчёт научных и прикладных задач в самых разных сферах деятельности. Новые возможности, позволяют ставить и решать многие комплексные научные и технические задачи морской гео- и гидрофизики, среди которых особенно актуальны в настоящее время следующие: создание аппаратуры для изучения и измерения электрического и магнитного полей в воде; исследование электрических явлений в море для определения их связи с другими физическими процессами; изучение магнитогидродинамических процессов, возникающих из-за движения морской воды в магнитном поле Земли и многие другие. Некоторые прикладные задачи требуют физически верного описания движения заряженного твёрдого тела, как в проводящей среде, так и на границе раздела сред, например, «газ–жидкость». Кроме того, подобного рода движения могут происходить при наличии изменчивости физических (например, геомагнитного) полей, которые необходимо учитывать. Решение подобных задач стало возможным с помощью современных вычислительных комплексов. Однако при этом следует иметь в виду, что сложный характер взаимодействия гидродинамического и электромагнитного полей обуславливает необходимость рассмотрения достаточно упрощенных моделей, описывающих основные закономерности изучаемых явлений. В настоящей работе представлены результаты численного моделирования генерации индуцированного магнитного поля, вызванной колебательным движением твёрдого шара, с помощь вычислительного комплекса ANSYS.CFX. Заряженный шар совершает колебания в приповерхностном слое границы раздела «морская вода – воздух». Модельная постановка задачи позволяет лучше понять механизм генерации магнитного поля, обусловленный движением твёрдого заряженного тела в проводящей среде. Modern computing facilities with the help of the latest computer technologies make it possible to simulate and calculate scientific and applied problems in a variety of fields of activity. New opportunities make it possible to pose and solve many complex scientific and technical problems of marine geo- and hydrophysics, among which the following are especially relevant at present: the creation of equipment for the study and measurement of electric and magnetic fields in water; study of electrical phenomena at sea to determine their relationship with other physical processes; the study of magnetohydrodynamic processes arising from the movement of sea water in the Earth's magnetic field and many others. Some applied problems require a physically correct description of the motion of a charged solid, both in a conducting medium and at the interface between media, for example, “gas – liquid”. In addition, such movements can occur in the presence of variability of physical (for example, geomagnetic) fields, which must be taken into account. The solution of such problems has become possible with the help of modern computing systems. However, it should be borne in mind that the complex nature of the interaction of hydrodynamic and electromagnetic fields necessitates the consideration of rather simplified models that describe the basic laws of the studied phenomena. This paper presents the results of numerical simulation of the generation of an induced magnetic field caused by the oscillatory motion of a solid ball using the ANSYS.CFX computer complex. The charged ball vibrates in the near-surface layer of the "sea water - air" interface. The model formulation of the problem makes it possible to better understand the mechanism of magnetic field generation caused by the motion of a solid charged body in a conducting medium.


2021 ◽  
Vol 92 (12) ◽  
pp. 123506
Author(s):  
A. G. Luchinin ◽  
V. A. Malyshev ◽  
E. A. Kopelovich ◽  
K. F. Burdonov ◽  
M. E. Gushchin ◽  
...  

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Yan-Jun Gu ◽  
Masakatsu Murakami

AbstractSpontaneous magnetic field generation plays important role in laser-plasma interactions. Strong quasi-static magnetic fields affect the thermal conductivity and the plasma dynamics, particularly in the case of ultra intense laser where the magnetic part of Lorentz force becomes as significant as the electric part. Kinetic simulations of giga-gauss magnetic field amplification via a laser irradiated microtube structure reveal the dynamics of charged particle implosions and the mechanism of magnetic field growth. A giga-gauss magnetic field is generated and amplified with the opposite polarity to the seed magnetic field. The spot size of the field is comparable to the laser wavelength, and the lifetime is hundreds of femtoseconds. An analytical model is presented to explain the underlying physics. This study should aid in designing future experiments.


2021 ◽  
Author(s):  
Zhonghai Zhao ◽  
Shu-Kai He ◽  
H. H. An ◽  
Z. Lei ◽  
Y. Xie ◽  
...  

Abstract Understanding the generation and evolution of magnetic fields in high-energy-density plasmas is a major scientific challenge in broad research areas including astrophysics, cosmology, and laser fusion energy. However, the fully three-dimensional (3D) topologies of such dynamic magnetic fields are still unknown yet. Here we report experiments of the first 3D synchronous proton radiography for self-generated magnetic fields in respectively laser-produced low-Z CH and high-Z Cu plasmas. The radiography images show that abundant 3D filamentary structures of magnetic fields grow up in coronal region of CH plasmas, while for Cu, the fields are majorly compressed along the dense surface region whose internal structures are pretty vague. These results are reproduced and explained by a combination of radiation-magnetohydrodynamic, particle-in-cell and Vlasov-Fokker-Planck simulations, where the cross-scale effects of Biermann battery, Nernst advection, resistive diffusion, Righi-Leduc and particularly kinetic Weibel instability are all taken into account. Our findings provide much enlightenment to the role of magnetic field generation in implosion and hohlraum dynamics of laser fusion.


2021 ◽  
Vol 2103 (1) ◽  
pp. 012016
Author(s):  
A M Bykov ◽  
Y A Uvarov

Abstract Supernova remnants (SNRs) are well known sources of the non-thermal radiation, particle acceleration and magnetic field generation and amplification. Synchrotron radiation of the accelerated electrons in the magnetic field is an important emission mechanism in SNRs that can dominate in radio and X-ray energy bands. Turbulent magnetic field yields to formation of the special inhomogeneous (clumpy) structure in the SNR synchrotron X-ray images. This structure could differ significantly on the SNR polarization maps for different types of the magnetic turbulence. A new family of the gas pixel detector X-ray polarimeters that are supposed to have good sensitivity and angular resolution should be well suited for SNR polarimetry. IXPE (NASA) will be the first polarimeter of this kind. Lately a model IXPE synchrotron polarization images of Tycho SNR were simulated in the 3 — 8 keV energy band. It was shown that IXPE observation time of ~ 1 Ms should be enough to distinguish characteristic features that are specific for some types of the magnetic turbulence. We perform simulations of Tycho SNR polarization maps for a wider set of energy bands in order to determine the most suitable energy range for study of the SNR turbulent magnetic field using IXPE. The dependence of the polarization degree on the photon energy is accurately considered in the simulations. IXPE background influence on the observations of Tycho SNR is also discussed here together with possible ways of data processing and interpretation reducing this effect.


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