http://www.youtube.com/watch?v=A49VML0_lEY

    The Magnetosphere    may seem a little complicated for something invisible, but keep in mind that one really doesn’t need to understand all the little details to see the big picture.

A BAR MAGNET IN SPACE

The Earth is a bar magnet in space.  Humans make use of this earthly attribute when they use a compass.  The needle of the compass aligns itself with the magnetic lines which surround the Earth and in the process it will indicate which way is north.  A bar magnet has two poles called north and south; hence it is called a dipole. 

The lines of the magnetic field from a bar magnet form closed lines meaning each line has one foot on the north pole of the magnet and one foot on its south pole.  These lines of magnetic force are rather stringy in appearance.  

Perhaps you performed this experiment while at school.  For a two dimensional image place a piece of paper over a magnet and sprinkle on it iron filings. These filings align themselves with the lines of magnetic force.  The stringiness of the lines is then apparent.

A compass helps us to find our way, because on the surface of the Earth the needle of the compass will align itself with the Earth’s magnetic lines and point towards the north magnetic pole. The needle is actually a tiny magnet, with the north end painted red. This red end is attracted to a south pole of a magnet, which on the planet we call the 'north magnetic pole'.  It seems backwards, but much in life is backwards especially when it comes to human conceived definitions.

An ancient compass invented by the Chinese

The compass has been useful to humankind since ancient times. The ancient Greeks and Chinese discovered that certain rare stones, called lodestones, were naturally magnetized.  These stones could attract small pieces of iron in a magical way, and were found to always point in the same direction when allowed to swing freely suspended on a piece of string.  These small pieces of iron were aligning themselves with the magnetic line of force of the Earth.

We refer to the area of space that is occupied by the Earth’s magnetic line of force as its magnetosphere.  By definition then, when space bodies like the Sun and other planets generate a magnetic field, they also have a magnetosphere.

PLASMA: THE FOURTH STATE OF MATTER

There are four such states, three states of which three are familiar: solid, liquid, and gas.  Take water for example, the three states would be ice, water, and steam respectively.  As a solid, ice is in a rigid structure with a ratio of 2 hydrogen atoms to 1 oxygen atom.  Something happens when ice is heated, it melts.  When ice turns to water, it changes states from a solid to a liquid. The molecules of H2O in a liquid are freer to slip and slide.  If you add enough heat to the liquid, it changes states again to steam which is a gas of H2O molecules.  Not only can matter can exist in a state of solid, liquid or gas, it can exist in a fourth state as well.  This state is less familiar to humans because it is rare on Earth, and it occurs when so much heat is added to the gas state that some of  the individual atoms break apart.  With enough energy, atoms are pulverized.

Atoms consist of protons, neutrons and electrons.  In a model of an atom, protons and neutrons are lumped into the center of the atom called the nucleus and electrons orbit the nucleus like the planets orbit the Sun.  What if a planet got knock out of orbit around the sun?  It could happen, but infrequently.  Atoms on the other hand under certain conditions give up an electron and take it back again.  The positively charged protons in the nucleus attract the electrons which have negative charge.  In nature opposite charges attract, and like charges repel.  When the protons and electrons are in balance there is no net charge.  If an atom should lose an electron it then is known as an ion with a positive charge. 

Going back to our example of the third state of water matter, steam, if enough additional heat is added, some of the atoms within this gas lose electrons so that now the gas has some neutral atoms, some ions and some free electrons, but the overall soup is neutral with the negative charges balancing with the positive charges.  This new gas is now in a different state of matter which is called plasma. This plasma has some very interesting properties.

One interesting property of plasma is that it has a density of charge which depends on the number of positive ions and negative electrons in its mix.  A low density plasma may have only a few of each; whereas a high density plasma could possibly have every particle as being either an ion or an electron.  These point charges within the soup of plasma can be controlled by electrical and magnetic fields.  Recall that like charges repel each other and unlike charges attract each other.  In a soup of plasma containing all those point charges there is a lot of repelling and a lot of attracting; kind of like a Friday night at a New York bar. 

Individual charged particles in the plasma may like to mind their own business and not interact with other charges; however turbulence within the plasma soup will not let those individuals be.  Moving charges are electrical currents.  Hence electrical and magnetic fields can have a definite influence on the behavior of plasma if there are sufficient numbers of charged particles in the soup.  Also the motions of the particles in the plasma generate magnetic fields and electric currents from within. This complex set of interactions makes plasma a captivating and multifaceted subject.

Only recently have humans acquired an interest in understanding the nature of plasma.  The big boost came when Van Allen discovered dangerous plasma areas in space known as the radiation belts in the early 1950s. While plasma is common in the universe, examples of plasma are rare on Earth, but lightning is a form of plasma as is neon lighting. 

Lightning is a massive electrical discharge in the atmosphere that creates a jagged column of plasma. Lightning is the means by which electrons are transported back to the Earth to "recharge" the Earth to its negative polarity.

Another example of plasma of which you may be aware occurs when an electric current is passed through neon gas.  It produces both plasma and light. So your neon lighting is an example of putting plasma to work. Whereas examples on plasma are rare on earth, plasma is most common in the universe, and Earth has plasma clinging to magnetic line of force like pearls on a string in the magnetosphere.

The Sun is a massive luminous ball of gas that is so hot it minces atoms.  That is, it strips all of the electrons off of the atoms so that the ions generally consist of only of a nucleus.  Even large atoms like oxygen and carbon get this treatment.  The corona of the sun is a highly structured region of plasma. This structure is imposed by the solar magnetic field which extends from the solar surface outwards. Since the corona is plasma, it is an excellent electrical conductor.  Recall the example of lightning.  Some of that solar plasma extends into space.  This solar wind as it is called travels at tremendous speeds.  This solar wind plasma flees the sun proper and fills interplanetary space.  The Earth is in the direct path of this speeding hot plasma.

The Sun is responsible for the characteristic shape of the magnetosphere.  Everyday the Sun blows hot plasma, the solar wind, into the interplanetary space.  The hot solar wind pushes against the magnetosphere and distorts it from the nice round shape one would expect of magnetic lines around a bar magnet.  The side of the magnetosphere facing the sun is compressed, and the opposite side is stretched out creating what is called a magnetotail. 

This hot plasma solar wind also carries with it solar magnetic lines of force.  This solar wind is generally diverted around the Earth's magnetosphere. Hence the magnetosphere serves the Earth as its protector from the solar onslaught.  However, under certain conditions some of the Sun’s plasma leaks into the terrestrial magnetosphere at its weak points - the poles and tail. At the poles these particles collide with oxygen and nitrogen particles to form the aurora.

In addition to the ever present solar wind, very potent sun flares or Coronal Mass Ejections (CME) can intensify these leaks into the terrestrial domain.  During what is known as magnetic storms, the Earth has experienced power station and radio transmission disruptions and even satellites have lost functionality.

The magnetic field of the Earth is an invisible shield. In order for life to thrive on this planet, it needs to be defended from the continual supersonic onslaught of the solar wind.  The magnetosphere is our protection; even if it is not a perfect guard. Some of the other planets also have magnetospheres; however Mars only has a very weak one, and consequently the solar wind blows away most of its atmosphere.

The magnetosphere is in the shape of a cavity. On the night side it has a long tail extending hundreds of Earth radii away from the Sun; while on the day side the Earth’s magnetic field is "squished." The entire magnetosphere  is dynamic and changes its shape in response to different conditions of the solar wind.

Plasma when it experiences turbulence will have electrical currents within it because it is composed of a multitude of positive and negative point charges.  These currents are good in that they help to shield the Earth from the destructive solar plasma.

The solar wind is constantly changing the direction by which it passes by the Earth.  Depending on the direction of the solar wind the magnetic cavity could act as a shield deflecting the incident energy or it could act as an accelerator creating charged particle beams that hit the neutral upper atmosphere, causing the polar aurora or ejecting solar plasma into the distant magnetotail.

Plasmas and magnetic fields have a fluid nature as they interact. As a result of this close relationship, magnetic fields are transported by flowing plasmas; the field lines are bent and twisted as the plasma flow bends and twists. In particular the solar plasma blowing out from the Sun brings its own twisted magnetic field lines.

Usually plasma particles are strongly constrained by its magnetic field. They rotate and slide around magnetic field lines, giving them a spiral trajectory.  One could think of them as pearls on a necklace in which the pearls are free to rotate around the chord, but cannot leave the chord.

Therefore since the magnetic fields of the Sun and Earth are separate, even as the solar plasma bombards the Earth, it generally stays separate from the terrestrial plasma simply because solar plasma clings tightly to solar lines of force and because earthly plasma clings to earthly lines of force. Think of plasma as being stuck to or frozen to its magnetic lines.  Sun plasma is Sun plasma and Earth plasma is Earth plasma, and the two plasmas rarely merge.

When these plasmas smash together, they usually form distinct regions separated by a thin boundary called the bow shock. Plasmas are highly conductive of electrical currents which are particularly intense in this area between the bow shock line and the magnetosphere proper.  This turbulent area is called the magnetosheath.  The magnetopause is the name of the boundary of the terrestrial magnetic field.  All the earthly magnetic lines lie within this boundary.  It is shaped somewhat like a bullet with its nose facing the Sun. However as the speed and direction of the solar wind changes, so does the shape of the magnetopause. A strong solar wind can compress the bullet nose even more. The changing direction of the solar wind causes the magnetopause to vary in shape.

Just as wind causes the swaying of trees, the solar wind changes the shape of the magnetosphere.  Like a real bullet there is a standing shock front ahead of the magnetopause due to the supersonic speed of the solar wind. As the near-Earth solar wind passes through the bow shock, it abruptly slows down and some of its energy is converted to heat.

BOW SHOCK

The bow shock which is created by the hot solar wind blowing against the magnetosphere.  The supersonic solar wind creates a bow shock on the Sun side or squished side of the magnetosphere.  Visualize a rock in a stream.  As a wave goes around the rock it forms a curved wave just ahead of the rock.  The bow shock in front of a magnetosphere is very similar to that wave.  It is like the wave in front of a boat as the ship passes through the water or like a plane flying faster than the speed of sound: it will produce a bow shock in the air before it.  A sonic boom is heard as the plane passes the speed barrier.  The bow shock forms in front of an obstacle, in this case the Earth, and helps force the solar wind to go around the magnetosphere.

Most of the solar plasma flowing around the magnetosphere is contained in the magnetosheath which is an area just outside the Earth’s magnetosphere.  It contains the Sun's magnetic field lines and plasma which are carried outward by the solar wind. The two magnetic fields interface at the magnetopause.

Comparatively little solar wind plasma gets inside magnetosphere. The magnetosheath contains solar wind plasma that has passed through the bow shock, and that plasma has been slowed down, compressed, heated, and diverted around the Earth.  Particles from the solar wind can enter the magnetosphere either by leaking through the magnetopause or entering the area over the poles known as the cusps. The Solar wind is variable.  The changing direction of the magnetic field plays an important role within the magnetosphere.

In the turbulent plasma of the bow shock, Alfven waves are created. According to a laboratory model, when two of these waves interact, the resulting plasma wave looks like feathers.  In this picture two Alfven waves interact, and they are propagating from left to right.  A discussion relating to this image appears in a research paper by Maggs, et. al. of the University of California, Los Angeles which was published in Plasma Physics and Controlled Fusion, 42, ppB15-B26 (2000) 

THE CUSPS

Between the Sunward magnetic field and the tail ward magnetic field are two funnel-shaped areas, called the polar cusps. These two areas of the magnetosphere are its weak spots in that they are permanently open windows to the solar plasma in the magnetosheath.  It is here in the cusps that shocked solar particles move easily from the bow shock to the Earth’s upper atmosphere, the ionosphere.  The cusps not only look like funnels; they act as funnels as well for solar plasma.  The cusps are anchored at the Earth’s poles, and like sunflowers, they bend their head toward the Sun. At these locations the magnetic field lines are nearly perpendicular to the Earth's surface and there is little magnetic resistance so it is easy for particles to spiral down into the atmosphere where they are responsible for some of the aurora we see. The exact position of these funnels is variable.  Just as the shape of the magnetosphere changes depending on the solar wind pressure and the direction, so too do the cusps alter their position depending on the pressure and direction of the solar wind. 

The cusps are bordered on the sun side with closed terrestrial magnetic lines of force compressed by the pressure of the solar wind, and bordered on the anti sun side by terrestrial lines that are so stretched out by the solar wind that they reach deep into what is called the magnetospheric tail. Shocked solar protons and heavy ions such as He2+ and O(3-8)+ have easy access to one or both of the polar cusps.

THE PLASMA SHEET

On the night side which is the side opposite the Sun, the magnetosphere is stretched out into a long magnetotail.  In the center of this tail is what is called the plasmasheet.  Understanding something about the plasmasheet is necessary to understanding why magnetic reconnection takes place there.

The plasma sheet is a thick layer of very hot plasma centered on the tail's equator. This region is rather dynamic in thickness, density and energy.  Magnetic force is weaker here so that the plasma has more freedom of movement.  In the center of plasmasheet a neutral sheet exists in which an electric current is little constrained by any magnetic field.  Flowing across the tail's equator from flank to flank, from east to west ("dawn to dusk") is a current known as the cross tail current.  As all currents must close, the cross tail current closes along the outer edges of the magnetosphere.

THE TAIL LOBES

The tail lobes are two regions of relatively smooth magnetic field, north and south of the plasma sheet. Field lines of the lobes are parallel until they converge above the poles. The magnetic lines point towards Earth north of the equator and away from Earth south of it. In the neutral sheet these oppositely directed magnetic field line are stretched way out creating the circumstances in which magnetic reconnection often occurs.

The lobe region is nearly a vacuum, but it contains a relatively strong magnetic field and can store magnetic energy.  Further down the tail the plasma density increases, as ions from the solar wind infiltrate the lobes.The mantle is thin layer that marks a transition from the lobes.  Ions here are a mixture of solar ions and terrestrial ions. 

A cross section of the magnetotail shows the parts of the plasma sheet separating the lobes.  First, in the center is the neutral sheet, the middle layer, which separates the north and south plasma sheets layers.  This trio is again sandwiched by the north and south plasma sheet boundary layers (PSBL).  Also in the equatorial region there is the low latitude boundary layer (LLBL).  The mantle is a boundary layer between the lobes and the magnetosheath.  These different areas play different roles, but they either all work together to provide us protection from the solar wind or under certain circumstances they all work together to accelerate the incursion of solar plasma.

The plasma sheet differs from the lobes in that the magnetic field is weaker and its plasma is denser.  The weak magnetic field allows the plasma more freedom of movement.  That is, the plasma is not so stuck to the magnetic lines of force.  As ions and electrons are free to move about, they create electrical potential or voltage.  Hence there are more electrical currents in this area.

All the magnetic field lines in the area of the plasma sheet cross the equator, whereas the field lines in the lobes are stretched so greatly that they lose their connection to the other pole.  They are known as open magnetic lines in that only one foot is attached to the planet.

THE PLASMASPHERE

The plasmasphere is between the radiation belts.  The inner belt was discover by Van Allen in 1956 as a region that contained a high amount of radiation.  If spacecraft and astronauts were exposed to this radiation for a prolonged time, it would cause them serious damage.  The inner radiation belt is mostly protons and is positively charged.  The outer Van Allen belt is more defused and is negatively charged having more electrons in its plasma population.  Whereas the inner belt is more stable, the outer belt varies in size according to geomagnetic conditions.

In the plasmasphere the geometry is right for radially polarized waves.  Waves of this type are of giant pulsations.  It requires an Alfven wave excited by a resonator.  As it propagates across magnetic field lines, it slowly changes from poloidal to toroidal (donut shaped).  In certain cases a standing wave across magnetic shells is caused by the increasing plasma pressure in the plasmapause. The pulsing poloid plasmasphere concentrates Helium+ and other positive ions in the center of the poloid; while electrons dominate the outside of the poloid.  The fast moving electrons are super heated to relativistic speeds.  This creates a huge cylindrical capacitor in space.

Here we have positive charges concentrated around the donut hole and negative charges just outside the donut.  These charges are held apart by the plasmasphere.  This creates an electrostatic field good for storing electrical energy.

The plasmasphere is an extension of the partially ionized ionosphere.  The ionosphere is just above the Earth’s atmosphere.  Hence the plasmasphere is in the inner magnetosphere close in to planet; so close in fact that for the most part it corotates with the Earth.  This corotation makes for some interesting phenomena.  All magnetic lines in the plasmasphere are closed, and its plasma generally hails from the ionosphere but of course some of the plasma is from the solar wind. The plasma density gradually diminishes with distance from the Earth.  Its moving plasma can be thought of as a river of plasma swirling around with the rotating Earth.  

Generally invisible to us the plasmasphere is loaded with a special ion of He+ which can easily be detected by instruments in the space crafts.  So the plasmasphere can be made visible to us because  He+ vibrates a certain frequency which can be picked by the satellites and imaged.  The plasmasphere draws these He+ ions from the ionosphere.

During periods of low activity, when corotation dominates the near-Earth plasma flow, the plasmasphere becomes "saturated" with upflowing ionospheric plasma and extends to 6 Earth radii or beyond.  Its density decreases steadily with increasing distance from the Earth.

THE RING CURRENT

A model of the ring current shows an egg like region surround by a ring. It is adjacent to the outer boundary of the plasmasphere and the outer radiation belt within which one finds the ring current. This donut shaped area that contains a plasma of electrons, ions and neutral atoms.  The neutral atoms are trapped there by the pull of the Earth’s gravitation.  In the area of the ring current the particles can also drift from magnetic line to an adjacent magnetic line and in so doing drift around the globe. This drifting nature of the particles is what is fascinating about the ring current.   Positively charged ions drift westward around the Earth while the negatively charged electrons drift eastward.  This flow results in a westward-directed ring of current that encircles the Earth at distances, typically, of several Earth radii. The magnetic field produced by the ring current causes a worldwide decrease in the field strength on the ground at low and middle latitudes.  The decrease earthly magnetic field strength is even greater during extreme magnetic storms.  In fact scientist use the fluctuating magnetic field on the ground to help them understand the status of the ring current.

REFERENCE:

http://www.iki.rssi.ru/mirrors/stern/Education/Intro.html