Magnetohydrodynamics (M.H.D.) is a branch of physics devoted to the study of the motions of fluids that conduct electricity in the presence of magnetic fields. It applies to liquid metals (mercury, molten alkali metals), weakly ionized gases and plasmas.

When a conductive fluid moves in a magnetic field, it is the seat of an electric field that produces electric currents there; these modify the initial magnetic field; on the other hand, the Laplace forces applied to the matter along the streamlines modify the movement of the fluid. Thus appears an interaction of the electromagnetic and hydrodynamic effects which constitutes the field of study of the M.H.D. The magnitude of the interaction is characterized by a dimensionless number RM called the magnetic Reynolds number; RM is proportional to the electrical conductivity of the fluid, its velocity and the dimensions of the flow. The interaction is generally weak (RM < 1) in liquid metals and weakly ionized gases and strong (RM > 1) in plasmas.

Magnetohydrodynamics is involved in the interpretation of many natural phenomena: magnetic fields and rotational speeds of stars and planets; solar spots, flares and wind; structure of magnetospheres; origin of cosmic rays, radiation of pulsars. The importance of M.H.D. in all these astrophysical events is due to the fact that the dimensions, velocities and, often, the conductivities of the plasmas involved are very high so that matter and the magnetic field are strongly coupled.

Important industrial developments concern liquid metals (electromagnetic pumping of sodium used as a coolant in certain nuclear reactors; pumping, stirring and levitation in certain metallurgical treatments). Applications to ionized gases and plasmas have been the subject of research: magnetic confinement of plasmas in research on controlled fusion, energy conversion (MHD generators to improve the efficiency of thermal power stations, MHD engines for the propulsion of rockets, hypersonic wind tunnels), aerothermal and naval propulsion.

Regards

Magnetohydrodynamics (MHD) helps enormously to understand our closest start, Dr. Md. Fayz-Al-Asad The Sun.

In addition, MHD is crucial for our existence on planet earth since it also relates to the behavior of the Earth's magnetic field.

Best Regards.

In plasma studies on Earth and beyond.

Tokomak..

Thanks a lot for your nice discussion dear professor Dr. Bachir Achour

Thank you so much Dr. Pedro L. Contreras E.

You are most welcome Dr. Md. Fayz-Al-Asad and thank you for replaying.

We recently published a long work on this topic (dedicated only to thin astrophysical plasmas in the sun)

Please, it contains a long literature review as well. It is very instructive for the onset of the non-linear phenomena that involve the kinetic coefficients in such high temperatures and the crossover of the slow and fast magnetosonic waves.

Article An Eigenvalue Analysis of Damping in Optical Thin Plasmas

Best Regards.

Dear Dr. Pedro L. Contreras E.

Thank you for the link to the paper. This paper is very informative for the onset of the non-linear phenomena.

Dear professor Dr. Natalia S Duxbury

Thank you very much for your nice comment.

MHD is a multi pronged field, I have a personal interest in this mainly out of curiosity and my inclination to study energy generation and conversion methods, from which I learnt that MHD was really in vogue in the late 1960s as a potential method for efficient recovery of energy from hot plasmas. Theoretical efficiencies of 60% were expected. However plasma instability was always said to be a problem limiting the efficiency to 40% at best, and the research programs that existed in France were, at least officially, abandoned. However, and curiously, when one searchs for the so called “secrecy of inventions“ act of the USA, one can find a list of research topics that were potentially subject to this act. This list was obtained through FOIA and is from the early 1970‘s, and in the energy generation chapter, it mentions directly MHD energy converters.

There are also some very clever MHD aircraft ideas developed since the 1950s which could be in theory hypersonic (MACH 20), as with MHD you can cancel the shockwave. The energy source for such an aircraft, specially the one needed to create the great ionization in the air around the craft, is still something that can only be applied at the small drone level, but the research has been ongoing and I even recall a patent application from 2004 of an MHD drone that was a plagiarism of work done by a French expert in the late 1970s.

Hannes Alfven won his Nobel for his MHD work, and he is much lesser known by his efforts to explain cosmological phenomena through it, even star formation.

Thanks a lot Camilo Urbina for your nice comment; this is very instructive of the MHD (Magneto-hydrodynamics).

You are most welcome, Prof. Md. Fayz-Al-Asad

The blind peer reviewer, who I guess was a famous specialist in the field since the peer review was very detailed, emphasized that we continued with the onset phenomena, but Venezuelan universities were dismantled so we could not.

So I wrote whatever I could and published it open access so it could be available to all interested.

Thank You, Best Regards.

Prof. Md Fayz-Al-Asad@

What I feel from your question: MHD is useful for understanding the internal structure of the Sun, in particular Coronal Mass Ejection (CME), High speed solar wind, sunspots, Gnevyshev Gap : In case of compact objects i.e. Neutron star, Pulsars, Magnetars --- structure of the Atmosphere of neutron star, pulsar's radio emission ; sun-earth connection i.e. sunspots number and geomagnetic storm, effects on human , sunspots number and covid19, etc.

If you are interested to do research on MHD you first read Review articles on that topic whether it be the sun or neutron star or pulsar or magnetars , then proceed further.

Another interesting topic is : MHD in extreme conditions i.e. plasma behaviour under extreme conditions.

Dear Md. Fayz-Al-Asad,

You are welcome.

Regards

Dear Md. Fayz, in my opinion, MHD can be simply applied wherever there is any kind of fluid flow.

Prof Fayz Md. Fayz-Al-Asad

as an application of MHD in astrophysics:

in Sun ---- formation of sunspots and behaviour of internal magnetic field, Coronal mass ejection (CME)- high speed and low speed solar winds, geomagnetic storm, sun's north & south pole reversal in 22 years ;

In Inter Stellar Material (ISM) --- formation of Local bubble, Supperbubble,

In Neutron Star, Pulsar, Magnetar ---- Atmosphere, Light cylinder

Also see my one day ago reply.

Dear Ramen Parui

Thanks for valuable information.

Dear Ravi Sharma

Thanks a lot for your opinion .

As an engineer I am of course as fascinated with the science as with the applications of it to solve everyday problems. That’s why MHD propulsion has been on my radar for a long time, and there are simple lab models that have been researched giving great insight.

On the energy production side there was a remarkable series experiments involving MHD science in the so called Z machine of Sandia Labs during 2006, which created the highest recorded man made temperatures for several years before CERN topped that in the LHC. However the interesting part of the Z machine results was that the energy balance between the kinetic energy imparted to the Z pinch target compared to the thermal energy output left everyone scratching their heads as it was around 4x. This had all the characteristics that one would want to develop an Inertial Confinement Fusion system, and there was even a segment of scientists that wanted to call attention to this possibility,

but it fell on mostly deaf ears.

MHD is widely applied from the range of solar wind turbulence to medical application. if you have to know more about it's application to study solar activity finds the attachment

Thank you Ram Chandra Pageni

Dear prof. Dr. Camilo Urbina

Thanks a lot for valuable comment on MHD (Magnetohydrodynamic).