magnetohydrodynamics

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Earth is not the only planet in our solar system with auroras. As the solar wind–a stream of rarefied plasma from our sun–blows through the solar system, it interacts with the magnetic fields of other planets as well as our own. Saturn’s magnetic field second only to Jupiter’s in strength. This strong magnetosphere deflects many of the solar wind’s energetic particles, but, as on Earth, some of the particles get drawn in along Saturn’s magnetic field lines. These lines converge at the poles, where the high-energy particles interact with the gases in the upper reaches of Saturn’s atmosphere. As a result, Saturn, like Earth, has impressive and colorful light displays around its poles. (Image credit: ESA/Hubble, M. Kornmesser & L. Calçada, source video; via spaceplasma)

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Solar magnetohydrodynamics

The sun is a magnetohydrodynamics (MHD) system that is not well understood. It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating:

The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events—nanoflares. Currently, it is unclear whether waves are an efficient heating mechanism.

The field of MHD was initiated by Hannes Alfvén, for which he received the Nobel Prize in Physics in 1970. He described the class of MHD waves now known as Alfvén waves. Observations show that all waves except Alfvén waves have been found to dissipate or refract before reaching the corona. Current research focus has therefore shifted towards flare heating mechanisms.

The magnetic filament above erupted on April 19, 2010. The black “hair-like object” is a speck of dust on the CCD camera.

Credit: SDO/AIA

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FIGURE 1: A 3D snapshot of the Earth’s magetic field; lines in blue are directed inward and lines in yellow are directed outward

FIGURE 2: 500 years after a field reversal

At the heart of our planet lies a solid iron ball, about as hot as the surface of the sun. Researchers call it “the inner core.” It’s really a world within a world. The inner core is 70% as wide as the moon. It spins at its own rate, as much as 0.2o of longitude per year faster than the Earth above it, and it has its own ocean: a very deep layer of liquid iron known as “the outer core.”

Earth’s magnetic field comes from this ocean of iron, which is an electrically conducting fluid in constant motion. Sitting atop the hot inner core, the liquid outer core seethes and roils like water in a pan on a hot stove. The outer core also has “hurricanes”–whirlpools powered by the Coriolis forces of Earth’s rotation. These complex motions generate our planet’s magnetism through a process called the dynamo effect.

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Sometimes fluids are slow-moving enough that it takes timelapse techniques to reveal the flow. Fog is one example, and, as seen above, magnetic silly putty is another. The putty is an unusual fluid in a couple of ways. First, having been impregnated with ferromagnetic nanoparticles, it is sensitive to magnetic fields, making it a sort of ferrofluid. And secondly, being silly putty, it’s a non-Newtonian fluid, meaning that it has a nonlinear response to deformation - a fact that will be familiar to anyone who has tried to knead putty versus striking it. With a strong enough magnet, the putty makes for an impressively tenacious creeping flow. (Video credit: I. Parks; via io9; submitted by Chad W.)

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Accretion Disks

Accretion flows are ubiquitous in astrophysics: they occur around protostars, accreting compact objects in binary systems, and supermassive black holes at the cores of galaxies. Much of professor James M. Stone’s work has concerned studies of the local hydrodynamic and magnetohydrodynamics (MHD) processes that can lead to outward angular momentum transport in accretion disks. As computers become more powerful, previous studies of local patches of an accretion flow are being expanded into global studies that encompass the entire disk.

Accretion flows that cannot cool via emission of radiation become vertically thick and nearly spherical. Thus, they are intrinsically multidimensional. To study the structure and evolution of non-radiative accretion flows, 2D (axisymmetric) hydrodynamical simulations were performed using a non-uniform grid that spanned more than two decades in radius.

The most striking property of the flow is the large fluctuations produced by strong convection. Convective eddies transport a lot of mass both inwards and outwards, but the net mass accretion rate is very small and set by the properties of the flow near the inner boundary. A vanishingly small accretion rate may help to explain the deficit of high energy emission observed from accreting compact sources.

While understanding the properties of hydrodynamical accretion flows is important, it is generally agreed that angular momentum transport is in fact mediated by magnetic stresses. Thus, repeating the global simulations of non-radiative accretion flows with MHD calculations is vital.

Credit: James M. Stone

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@notsafefromchara @fallen-determination @digeridoodler @shouganairu @actanon @magnetohydrodynamic @rustnut @dreamy-94 @snajey and last but not least @wow-im-alive

This was a whole lotta fun, I can’t say it enough, but I must sleep now. Thanks guys for all the delight, and sorry for messing with your art styles (Some of which I totally did not do justice!!), I hope you like regardless!!

One of the key features of turbulent flows is that they contain many different length scales. Look at the plume from an erupting volcano, and you’ll see eddies that are hundreds of meters across as well as tiny ones on the order of millimeters. This enormous difference in scale is one of the major challenges in simulating turbulent flows. Since energy enters at the large scale and is passed to smaller and smaller scales before being dissipated at the tiniest scales of the flow, properly simulating a turbulent flow requires resolving all of these length scales. This is especially challenging for applications like the solar wind – the  stream of charged particles that flows from the sun and gets diverted around the Earth by our magnetic field. The image above shows some of the turbulence in our solar wind. The structures seen in the flow range from the size of the Earth all the way to the scale of electrons! (Image credit: B. Loring, Berkeley Lab)

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Solar ‘Tadpoles’ a.k.a. Supra-Arcade Downflows

These dark, elongated plasma structures, known as supra-arcade downflows (SADs) – also called “tadpoles”(because of their sinuous shape), are sunward/downward-moving features observed in low-density region above post-eruption flare arcades. They were originally thought of as flux tubes contracting under tension after reconnection, but later it was argued that SADs are not flux tubes, rather wakes behind shrinking loops or jet-flows caused by the Rayleigh-Taylor instability. The exact mechanism of the formation of SADs is still not fully understood, despite various MHD models and observational studies.

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Credit & source: NASA/GSFC

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For those of us who are Earthbound, it’s easy to think of liquids and gases as being the most common fluids. But plasma–the fourth state of matter–is a fluid as well. Plasmas are essentially ionized gases, which, thanks to their freely flowing electrons, are electrically conductive and sensitive to magnetic fields. Their motions are described by a combination of the Navier-Stokes equations–the usual equations of motion for a fluid–and Maxwell’s equations–the equations governing electricity and magnetism. Studies of plasma motion often fall under the subject of magnetohydrodynamics and can include topics like planetary auroras, sunspots, and solar flares. (Video credit: SciShow)

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Solar Coronal Waves

One of the most longstanding question in solar physics is why the Sun’s corona is hotter than its surface. This is known as the ’coronal heating problem’, and there’s no definite answer to that question. The Sun is a very complex magnetohydrodynamic system; there are different kinds of flows and instabilities that can play an important role in the energy-transport processes in the corona. The waves – pictured above, are initiated by a Kelvin-Helmholtz instability. The Kelvin–Helmholtz instability occurs not only in clouds and in the ocean, but also in various astrophysical environments. This instability occurs when two flows of different velocities move past one another in opposite directions with a strong enough shear to overcome the tension force. In the solar atmosphere, which is made of hot ionized plasma, the interplay of two hot-plasma jets and the embedded magnetic field might trigger turbulent flows which could help add heating energy to the corona.

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Credit: NASA/JPL/GSFC

Soviet Victor III, known to the USSR as Project 671RTM Shchuka (Pike), showing off it’s distinctive teardrop hull design. This allowed it to maintain faster speeds than other subs of it’s vintage and a much quieter sonar signature. The pod on the tailfin was originally suspected to be a type of magnetohydrodynamic drive, but was revealed to be the housing of a reelable towed passive sonar array.

Solar surges are cool jets of plasma ejected in the solar atmosphere from chromospheric into coronal heights. This particular surge has been captured in a loopy structure and streamed sunwards along the magnetic field lines.

Surges are associated with active regions and they are most likely triggered by magnetic reconnection and magnetohydrodynamic (MHD) wave activity. According to their morphological features, surge prominences can be classified into three types: jet-like, diffuse, and closed loop (above). Jet-like and diffuse surges are associated with coronal mass ejections (CMEs), but the closed-loop surges are not because the initial acceleration of the eruption is slowed down and finally stopped by the overlying coronal loops.

Credit: SDO/ LMSAL

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How do you define the edge of our sun? There’s a distinct surface to it, but our star is also surrounded by the corona, an even hotter region of plasma twisted by magnetic fields. The corona is sort of like the sun’s atmosphere. Farther out in the solar system, we receive a constant barrage of charged particles, known as the solar wind, that streams out from the sun. So where does the corona end and the solar wind begin?

Scientists have been studying the flow structure of the solar wind in search of an answer to this question, and they’ve found that there’s a clear transition point about 32 million kilometers from the sun. At this distance, the sun’s magnetic field weakens to the point where it no longer exerts the same hold on the solar particles and they begin to move turbulently, behaving more like a gas than a plasma. With special measurements and image processing, scientists were able to actually see this flow change in the solar wind! (Video/image credit: NASA; research credit: C. DeForest et al.; via FlowViz)