Questions tagged [magnetohydrodynamics]
Magnetohydrodynamics or MHD is a magnetized fluid approximation to the kinetic theory describing a gas composed of charged particles.
107
questions
0
votes
1
answer
14
views
Magnehydrotostatic equilibrium
in plasma physics the magnetostatic equilibrium is defined as
$$
\vec{j}\times\vec{B}=\nabla p
$$
where $\vec{j}$ is the current density $\vec{B}$ is the magnetic field and $p$ is the pressure. But ...
- 167
0
votes
1
answer
30
views
Introduction to magnetohydrodynamics
does anybody have any reference books for introduction to magnetohydrodynamics? I want to dive into this topic and I don´t know about any good reference.
3
votes
1
answer
38
views
How is the solar magnetic field produced?
I have been trying to understand the magnetic field creation mechanism. I know that its origin is not perfectly understood, but what is currently believed (as I know) is that the magnetic fields ...
1
vote
1
answer
28
views
Is it generally accepted that Field Aligned Currents are caused by Force-Free Fields?
I am currently an undergraduate working on a project about FAC (Birkeland Currents) and it seems that most of the sources on the subject are very technical and hard for me to read (particularily ...
- 11
0
votes
2
answers
45
views
Does magnetic force on a current-carrying thin slab depend on current distribution in the slab?
In Purcell and Morin’s Electricity and Magnetism, the magnetic force on a current-carrying sheet is found by using the fact that the force on a segment of wire with length $l$ would be $IB_{avg}l$, ...
- 183
2
votes
1
answer
60
views
Taylor's hypothesis in turbulence
Reading about Taylor's hypothesis for converting spatial to temporal scales and vice-versa, I understand that if $V$ is the bulk speed of the fluid then we could use:
$$\ell = V \cdot \tau$$
To ...
- 418
0
votes
0
answers
24
views
Does a Magnetohydrodynamic Generator remove ionisation?
After ionised helium passes through a magenetohydrodynamic generator, does it become normal helium again?
0
votes
0
answers
40
views
Conservative form of the vector diffusion equation
For some reason I am unable to find a source on the internet about this. I think I have an answer, but I want to be doubly sure about this. All I could find (here), is that for an incompressible fluid,...
0
votes
1
answer
66
views
What are the assumptions made in ideal MHD?
So my understanding is that in ideal magnetohydrodynamics (MHD), we assume a conductivity of infinity, and that makes the electric field in a comoving frame equal to zero. As I was watching one ...
- 138
1
vote
2
answers
68
views
Drift of neutral particles in plasmas
I'm learning about $\mathbf{E}\times \mathbf{B}$ drifts in plasmas. The drift speed is given by $\mathbf{v}=\mathbf{E}\times \mathbf{B}/B^2$. Chen (in Introduction to Plasma Physics and Controlled ...
- 11
4
votes
2
answers
115
views
Plage regions data
Plages are phenomena of stellar activity.
Where can I find openly-accessible data of plage regions of the Sun with other parameters such as faculae region, magnetic field, etc?
- 93
4
votes
1
answer
63
views
Do drifting particles violate Alfvén's theorem?
According to this Wikipedia article, Alfvén's theorem states that "in a fluid with infinite electric conductivity, the magnetic field is frozen into the fluid and has to move along with it". ...
- 555
0
votes
0
answers
37
views
External magnetic field in the magnetic induction equation
In the computation of the induced magnetic field inside a moving liquid with an imposed external field, do we have to consider the effect of the magnetization of the medium?
Let's suppose we have a ...
- 1
1
vote
1
answer
59
views
How to yield the relativistic equations of motion for a particle in a plasma non-dimensionless
The relativistic equations equations of motion for the ions are:
$$
\frac{d\vec{p}}{dt} \ = \ q (\vec{E}+ \vec{u} \times \vec{B}), \ \vec{p} \ = \ m \gamma\vec{v}
$$
In many papers discussing test ...
- 418
1
vote
1
answer
31
views
Transition from inertial to kinetic range in Solar wind turbulence
Most papers studying the turbulence in the solar wind present figures showing the power spectral density (PSD) of the magnetic field. The inertial range, with a power law index of ~ -5/3, is followed ...
- 418
1
vote
1
answer
61
views
Do numerical MHD codes contain particles?
Does an MHD code contain charged particles, or does one just numerically solve the MHD equations? Also, can you suggest a good introductory book/paper to learn the basics of MHD codes?
- 418
1
vote
1
answer
40
views
Why does the transverse Alfven mode not undergo wave steepening and hence resulting in shock waves?
Initial Question:
I am currently working through the second edition of Introduction to Plasma Physics by Donald A. Gurnett & A. Bhattacharjee
In section 8.3.8 on page 304, they state that:
.. on ...
- 594
2
votes
0
answers
32
views
Has any one seen an application of the virial theorem (say with magnetic fields) for non-self gravitating system?
Here is the version of the theorem I am most familiar with - consider an ideal magneto-fluid with density $\rho$, bulk velocity field $\mathbf v$, with magnetic field $\mathbf B$ and (isotropic gas) ...
- 63
73
votes
3
answers
7k
views
Why don't the Earth's oceans generate a magnetic field?
Many questions have been asked here about why the Earth has a magnetic field, e.g.,
What is the source of Earth's magnetic field?
How does Earth's interior dynamo work?
How can an ...
- 3,898
0
votes
1
answer
24
views
Alfven waves in the ionosphere
I was given the following problem/question as a part of my homework in physics:
It is raining with thunder in Australia, and the lightnings generate transverse Alfven waves in the ionospheric plasma. ...
- 43
0
votes
0
answers
15
views
Behavior of axisymmetric force-free fields with no radial component at infinity
The magnetic field obeys the equation (From Priest's Solar Magnetohydrodynamics)
$$ \frac{1}{2} \frac{d }{d r} \left( B_\phi^2 + B_z^2 \right) + \frac{B_\phi^2}{r} = 0 \ , $$
where $(r,\phi,z)$ are ...
- 63
1
vote
1
answer
54
views
How do I show that an energy minimising magnetic field subject to constraints is force-free?
I want to use the fact that a magnetic field can be written in the form
$$ \textbf{B} = \pmb{\nabla}u \times \pmb{\nabla}v $$
to show that if $\textbf{B}$ minimises the magnetic energy
$$ E = \int_V\...
1
vote
1
answer
70
views
Wave drag in the interstellar medium
In Earth's atmosphere, an object moving at supersonic or near-supersonic speed experiences wave drag due to the formation of shock waves around the object's leading edge. At high speeds, wave drag is ...
- 3,898
1
vote
0
answers
84
views
Crash course in fluid dynamics
I am teaching myself magnetohydrodynamics and have never taken a class on fluid dynamics before. It seems like knowing some fluid dynamics will help me understand MHD faster, so I was looking for a ...
3
votes
2
answers
136
views
Basic questions about Alfven waves in solar corona
I'm a mathematician trying to understand a paper in mathematical physics (Alfven Reflection and Reverberation in the Solar Atmosphere, by Paul Cally), and although I understand most of the pde theory ...
0
votes
1
answer
40
views
Could the Cauchy momentum equation be analogous to ion current?
I'm trying to understand magnetohydrodynamics, in particular the Cauchy momentum equation
${\displaystyle \rho \left({\frac {\partial }{\partial t}}+\mathbf {v} \cdot \nabla \right)\mathbf {v} =\...
- 103
1
vote
0
answers
48
views
In Numerical Relativity (Neutron Star Mergers for example), the Metric is constantly changing. What does the "time" in each frame measure?
Hi there and really sorry for bothering you!
Recently I have begin to studing GR and I have found some simulations regarding Neutron Star Mergers.
Take a look at this video for example: https://www....
- 117
1
vote
1
answer
83
views
How does a solar wind plasmoid split when it touches the magnetosphere?
I want to try to render (computer graphics) auroras in a physical manner as realistic and realizable as possible. I'm aware that the phenomenon hasn't been completly explained yet (as far as my own ...
- 13
1
vote
2
answers
104
views
MHD instability: $\boldsymbol{k}\cdot\boldsymbol{B}=0$?
In this paper, on page 3, it states that to trigger an instability, a disturbance needs to have a wave vector, $\boldsymbol{k}$, which satisfies,
$$\boldsymbol{k}\cdot\boldsymbol{B}=0,$$
where $\...
- 555
0
votes
2
answers
104
views
Motion of electron in magnetic field along two directions
I was trying to calculate the electron cyclotron frequency under earth magnetic field which has a horizontal and a vertical component.
Now when I consider the initial motion of the electron in all ...
- 574
4
votes
1
answer
59
views
Is there any observable change in Earth's magnetism to an observer in geosynchronous orbit?
I am in high school, and today my physics teacher was teaching us about the Earth's magnetism. This is the first time I've read about this topic, so take these questions as coming from a complete ...
- 142
1
vote
0
answers
135
views
Kelvin-Helmholtz theorem in Relativistic regime [closed]
Using the Carter-Lichnerowicz equation of motion $\Omega_{\mu \nu}u^{\nu}=T\nabla_{\mu}s$, we obtain $Tu^{\mu}\nabla_{\mu}s=0$, where s is the specific entropy, $u^{\mu}$ the velocity of the fluid and ...
- 11
3
votes
0
answers
47
views
How efficient can an astrophysical jet be when it's driven by a black hole of maximal spin?
Some astrophysical jets appear to be driven by black holes. As far as I know, a detailed understanding of how black holes produce jets is still under development, but one general possibility is that ...
- 50.9k
2
votes
1
answer
35
views
Manipulation of the diffusive term in MHD induction equation
I am trying to solve the magnetohydrodynamic (MHD) equations with a spatially varying resistivity, $\eta$. To remove some of the numerical stiffness from my finite volume approach, I am trying to get ...
- 21
3
votes
2
answers
53
views
MHD generators. Are magnets really needed?
I have a question to those who are really proficient in MHD generation. There exist some claims that degree of interaction between flowing molecules (gases) and ions on one hand and flowing molecules (...
- 79
4
votes
1
answer
236
views
In plasma physics, why are the motional electric field and the frozen-in-flux condition represented by the same equation? ($E = -u \times B$)
I'm trying to refine my understanding of space plasmas, and feel like there's an intuitive understanding here that I'm just missing.
We commonly refer to a motional electric field in the solar wind. ...
- 43
1
vote
0
answers
71
views
Nuclear Fusion and Quantum Annealing
This is a pie in the sky idea, and a problem finding sort of question that can probably be dismantled easily, but in light of alternative energy needs with climate change, I think it would be ...
- 1,111
1
vote
1
answer
64
views
Do the MHD equations break down under certain circumstances (and possible connection to fusion experiments)?
The Euler equations produce singularities (in finite time) under certain conditions see here.
When rings of fluid approach each other, in certain simplified scenarios, the Euler equations that ...
0
votes
1
answer
27
views
In MHD, why is the magnetic force aquared?
I'm trying to understand how the following equation:
$-\nabla P + j\ \times\ B + pg = 0 $
Reduces to:
$\frac{P_0}{L_0}=\frac{B_0^2}{\mu_0 \rho_0}$
user244328
0
votes
0
answers
31
views
Newtonian/Relativistic Solutions of Fluid Disks in orbit around a massive body
I have been trying to make an RMHD (Relativistic Magneto-Hydro Dynamics) Set-up of a Disk around a Massive Rotational Body.
The initial paper I have been studing a paper which uses a Fishbone & ...
- 221
2
votes
1
answer
81
views
Multiple Ion Species MHD
If I have a fully ionized plasma consisting of electrons and a mixture of different positive ion species of varying masses and charges...how can I use the MHD model to describe the plasma fluid motion,...
- 554
0
votes
0
answers
149
views
Relation between conduction current density and total current density. Ohm's law
I am reading section 2.6 of Shercliff's book on MHD. He first establishes that (following his notation)
\begin{align}
\sum p &= q \delta \hspace{1em} \text{and} \tag{1} \\
\sum p(\mathbf u + \...
- 49
1
vote
1
answer
38
views
Interstellar medium shock heating
How fast would a hypothetical spacecraft need to get to experience significant heating from interaction with the interstellar medium (ISM)? Significant, in this context, means a steady-state ...
- 223
1
vote
0
answers
86
views
MHD pump liquid metal flow speed
a rather simple question, are there any studies or some information about the flow speed of liquid metal inside a tube?
My setup is basically a 50cm diameter circular tube (could be another diameter) ...
- 71
0
votes
1
answer
28
views
Tuning Drifts in Tokamaks
If we managed to make over 50 % of the drifting of the guiding centers of the gyrating particles inside a tokamak to occur into the inwards direction, away from the tokamak walls, would that allow us ...
- 1,495
1
vote
1
answer
266
views
Question about code units and physical units (hydrodynamics simulations)
I'm working on a code that implements smoothed particle hydrodynamics (SPH) method for solving the equations of magnetohydrodynamics (MHD) with self-gravity.
In research papers regarding existing ...
- 13
5
votes
1
answer
99
views
What exactly sets the scale in Fusion power plants?
In fusion power plants, larger devices are said to achieve fusion more easily, next step tokamaks like DEMO are even larger than ITER. In a quantitative way, what exactly isn't scale invariant in a ...
- 637
0
votes
0
answers
58
views
Magnetic cage for keeping plasma around itself
Let's assume that we have an object in vacuum, venting hydrogen plasma around itself. This object has a powerful internal electrical source, connected to a cage of superconducting wire around it, in ...
- 143
2
votes
1
answer
407
views
Frozen in Magnetic Field Lines
A perfectly conducting fluid undergoes an axisymmetric motion and contains an azimuthal magnetic field $\textbf{B}_θ$ . Show that $ \dfrac{B_θ}{r}$ is conserved
by each fluid element.
Question from ...
- 89
2
votes
1
answer
174
views
Linearization of MHD equations for a cold plasma and low frequencies
I'm trying to convert the next equations for a cold plasma (consider $P=0$ the pressure), for low frequencies and ignoring dissipative effects:
$$\rho \left(\left(\frac{\partial \vec{v} }{\partial t}\...
