How Magnetism Shapes The World? Definition & lot more

Definition:

 Magnetism is defined as the force which magnets apply on each other. It is very similar to gravitational force in the sense that it is also non contact force as though it doesn't require contacting for the push or pull something. Any magnet has two poles that is North pole and South pole. Exactly what happens that unlike poles attract each other however, like poles repel each other.

Opposite poles of magnet attracting is attracting each other.

Same poles of a magnet repelling each other.

                                                            How far can you follow a magnetic compass needle? As far as the North magnetic pole, where the needle starts spinning wildly? Compass needle alliance with the magnetic field line, and on the precise spot of magnetic north, those field lines are vertical. So just your magnetic compass 90 degrees and you can continue your journey either down to the molten iron dynamo surrounding the earth's core, or up.

      But up to where? The answer is - to everywhere - and today, that's where  we are going to go.

                    Magnetism is one-half of electromagnetism, one of the four fundamental forces alongside gravity. Magnetism shares a property with gravity that none of other forces have - it manifests on enormous scales. The nuclear forces are short range and electromagnetic forces are never adding up to much because its positive and negative charges tends to cancel it out. However, the magnetism is generated when electrically charged particles move. Even if the substance is electrical neutral you will still get magnetic field as long as the charges are  moving in opposite directions. That means Magnetic field can add up - and magnetism adds up to having enormous influence on the development of structure in our universe. Understanding magnetic field is of fundamental importance to astrophysicists. 

And so we should probably understand them too. Magnetic field lines form in concentric circles around moving charges. If that charge moves in a loop, that results in a dipole field - sort of torus around the loop with the field threading the loop and shooting off at the poles. Well, a moving charge particle will feel
a force perpendicular to both its direction of motion and to the field lines - and the 0net result of that is that charged particles tends to a spiral around around magnetic field lines. And it that charge already has a circular motion - for example the electric current in an electromagnet, or the aligned electrons spin in a ferromagnet, then that current will want to loop around the magnetic field. But the circular current it produces its own dipole field, and the result is that dipole fields always try to align themselves with other dipole fields. And that is how compass works. 

 

 Speaking of compasses - a minute ago we were standing at North pole with a compass needle pointing straight up. Where that field lines exactly go? As with the gravitational field, in a sense there is only one universal magnetic field. 

 

                          So while most of the earth's dipole field loops back - but some of the field lines connect to this greater magnetic field of the solar system - and even of the galaxy. So lets hitch a ride on field lines and let's see how far it takes us. Now I don't want to spend too much time in the solar system - greater magnetic wonders lie beyond. But while we are here, it's worth following one of Earth's field lines that connects directly to the surface of sun. This is a violent place, speaking magnetically. Here the field generated electric currents flowing in the searing plasma near the sun's surface. The earth's solid inner core and mental regulate the flow in its liquid outer core, resulting in a clean dipole field. But the sun is entirely fluid, and its rate of rotation speeds up near the equator. 

             

              

Sun in fluid State.

That means the dipole field gets twisted over the time. Magnetic  field lines cross each other, and the enormous magnetic energy densities pile-up. We can see those tangled field lines in UV light as charged particles spiral along them, up and down from the sun's surface. When the pressure gets too high, this field lines snap and then reconnect, and in the process spray death magnetic field into the solar system, carrying high energy particles with them. These coronal mass ejection join the solar wind. Follow one of these magnetic blasts, and you will spiral through the solar system on a giangatic magnetic tornado. This is still the sun's magnetic field, which connects here and there to the piddling fields of the planets. 

             About 4x the distance of Pluto, the sun's magnetic field connects to the the field of galaxy itself. Or smashes into it, depending on how you look at it. 

This is the heliopause, the boundary of the heliosphere, which defines the limit of the sun's influence in the galaxy. Although it's less of a sphere and more of a teardrop dragged into that shape by the Sun's orbital motion through the galaxy. How do we know that?  Well we have sent compasses that far - or rather magnetometers - on board the Voyager 1 & 2 spacecraft which crossed the boundary into interstellar space a few years ago. but those  don't reveal the shape of heliosphere. That was measured for the first time last year. NASA's IBEX mission, which used a short of solar wind sonar - it might how bursts of solar wind material were reflected back towards the earth from the edge of heliosphere. Beyond the heliosphere, seeing magnetic fields gets trickier. Fortunately,  astronomers are also tricky, and so have tricks. It turns out that Milky Way is full of natural compasses. The interstellar medium - the space between the stars - is scattered with tiny specks of dust produced in last supernova explosion. These specks tend to align with the local magnetic field lines of galaxy in exactly the same way as our iron fillings align around the bar magnet. When the light passes through the dusty interstellar medium it gets scattered by these grains - it bounces off them. But the pattern of alignment of these grains imprints a pattern of the scattered light. The light gets polarized - which means the direction of its electric and magnetic fields pick up a  preferred direction rather than being in  random directions. 

By measuring this polarization we can map the direction of these tiny compass needles, and so map the magnetic field of Milky Way. This has been now done in incredible detail by the Planck mission, which mapped polarization of the cosmic microwave background - the ubiquitous radiation left over from the big bang. The resulting map reveals the whirlwind of magnetism tangled through the galaxy. 

Actually, there is a more traditional way to map the magnetic fields of galaxies. These fields drive the motion of lone electrons throughout the interstellar medium. When radio waves will interact with these electrons, there polarization are also affected. This is a bit different though. The presence of these electrons tend to slow light down just as light is slowed down in air or glass - but to much smaller degree. But that slowing depends on polarization of light. In this case it depends on circular polarization of light. If the electric and magnetic fields of of a collection of photons all tend to point in same direction, we say the light is linearly polarized. But if those fields are not fixed but rather rotate in some detection, we say the light is circularly polarized - and it can be clockwise or anticlockwise rotation. the electrons in their magnetic field tend to slow 1:00 circular polarization direction more than the other. The net result of this is that the linear polarization - which is  sort of the sum of the circular polarization - gets rotated in an effect called Faraday rotation. So by measuring the Faraday rotation of different radio sources we can also map magnetic field. This has been done for the Milky Way using the light from pulsars. We even have clear views of magnetic field in many distant spiral galaxies. We see that the field tend to be threaded along the spiral arms. These are the densest region of those galactic disks - places where magnetic fields have confined the charged particles of the interstellar plasma. And that plasma in turn drags the magnetic field in the orbit around the galaxy. So galaxies have magnetic field. 

But from where those magnetic field come from? Largest magnetic fields can grow and reinforce themselves into particular configurations called Dynamos. 

In the earth's dynamo, swirls of magma are  induced by the coriilis force, and while those are initially turbulent, they rearrange themselves into a series of self-supporting flows. 

These amplify what starts out as a very weak and disordered field into the ordered & powerful field that surrounds the Earth. The exact process for the Milky Way is not well understood, but the ingredients for a dynamo are

all there: we have differential rotation in the disc which can lead to Coriolis induced helical flows, and those flows can also be induced by supernova explosions. Those supernovae may also gives us the seeds of magnetic fields that can be amplified buy the galactic dynamo. However it got there, the Milky Way has developed itself into a substantial magnetic field. And that field helps build the Milky Way in return. Magnetic fields generated by collapsing gas clouds help to slow the rotation of those clouds - expel angular momentum. Without that, those clouds would never be able to collapse all the way into stars. Magnetic field also facilitate star formation after the star dies. Magnetic blasts accompany every supernova explosion, and these help to compress gas in the path of that explosion, triggering busts of new star formation. The supernovae are expected to blast their own guts entirely out of the galaxy, which should result in all those newly-formed elements being lost to us. But the galactic magnetic field constrains, that flow, funneling some of it into vast galactic fountains erupting from poles. These galactic fountains are incredibly important for building galaxies.

Thank  you. 

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