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Transcript of Celestial Magnetism
Like the earth, the sun also has a magnetic field. This field extends into space, though unlike earth the sun's field changes shape and strength frequently.
Earth is not the only planet with a magnetic field. In fact, most of the other planets in our system have one, though some are very different to our own.
What causes it?
How strong is it?
If the earth spun faster, it would have a stronger magnetic field. Likewise the field would be stronger if more of the core were liquid.
The field is pretty weak once it reaches the surface of the earth, but is still strong enough to protect us from the solar wind.
The strength of earth's magnetic field varies depending how close to the poles you are, but at the equator it is 31 µT, and at 50° latitude it is 58 µT. this is extremely weak compared even to a simple fridge magnet.
Currently, earth's magnetic field is weakening by about 5% every hundred years. It could potentially reach zero, or it could switch polarity. This may cause multiple poles to occur, and the field will certainly weaken.
How does it protect us?
The Earth's Magnetic Field
What causes it?
The sun's magnetic field occurs because within the sun are areas of plasma, containing charged particles that move following convection currents. This movement of charged particles creates a magnetic field.
The sun's magnetic field is similar in strength to a fridge magnet - 5 millitesla.
The sun's magnetic field changes polarity approximately every 11 years.
The field interacts with other planets and forms a spiral shape because the sun rotates every 27 days. This is called a 'parker spiral' after its discoverer.
How does it affect us?
Magnetic fields in celestial bodies, including the earth, cause many phenomena, such as sunspots or the aurorae.
The Moon and Mars
Both Mars and the Moon have areas on their surface that are permanently magnetised.
This suggests that at some point, they both had fields generated by the dynamo effect.
This correlates with the presence of volcanoes on Mars that hint that the core is hot.
Neptune's magnetic field is created in a very similar way to Uranus's in that oceans of water with ionic impurities become the conductors that induce the field. These oceans are about as conducting as the ones on Earth.
The magnetic pole is 47° away from the geographical pole, but like Uranus undergoes variation due to the angle of solar wind particles hitting the planet.
The field is 25 times stronger than Earth's.
Mercury, surprisingly, does have a magnetic field. It's not particularly strong, so it doesn't trap many particles, hence why Mercury has no atmosphere.
There is a possibility that mercury has a molten core, and so its magnetic field forms in a similar way to that of the Earth - through the dynamo effect.
Venus has no magnetic field at all. The solar wind is stopped from stripping away its atmosphere by the ionosphere.
This gives Venus a magnetosphere that is much more like a comet's tail than that of other planets.
Jupiter has an extremely strong magnetic field - about 20,000 times the strength of Earth's.
Its field is caused by the dynamo effect. Inside the core of Jupiter, the pressure is enough to make hydrogen metallic.
The movement of this metallic hydrogen is what induces the magnetic field.
Jupiter's magnetic poles are similar to Earth's in that they are slightly offset from the geographical poles, but for Jupiter the magnet's polarity is the other way around.
Saturn has a magnetic field thought to be induced in a similar way to Jupiter's - through the movement of metallic hydrogen within the core.
Saturn's magnetic field is about 600 times stronger that Earth's.
Remarkably, Saturn's magnetic axis is almost perfectly lined up with its axis of rotation, unlike other planets.
The magnetic axis of Uranus is about 60° off its rotational axis. However as the angle of the flow of solar wind reaching Uranus changes constantly, the magnetosphere varies greatly with each rotation of the planet.
Uranus's field is around 50 times stronger than Earth's.
The field is created by 'oceans' of methane, ammonia and water - the water contains these impurities as ions, which are the charge carriers that induce the magnetic field.
Aurorae are caused because the Earth's magnetic field traps charged particles with lots of energy from the solar wind.
The particles are attracted to the poles because here the magnetic field of the Earth is strongest. The field lines pull them down into the atmosphere.
Electrons collide with oxygen and nitrogen molecules in the air and excite their electrons in turn to higher energy levels. As the electrons return to the ground state, they emit photons of light and these cause the colours.
The common green shade is caused by oxygen about 95km above the earth. Red comes from higher altitude oxygen, up to 320km high. Blue and purple colours come from nitrogen.
Aurorae also emit light in the UV and X-ray spectrum that Nasa's Polar Satellite has photographed for the first time.
The Earth's Magnetic field:
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Sunspots are areas on the surface of the sun that are cooler. They're about 4200K on the surface and 4600K on the inside. This is about 1200K cooler than the rest of the sun's surface.
They have a magnetic pressure. This is because they have strong magnetic fields - so strong, in fact, that the field lines get so densely crowded they push up out the surface and bring with them loops of plasma. The pressure comes from the density of the field lines all repelling each other.
Sunspots appear as dark patches on the surface of the sun because they are much cooler.
Sunspots often show what can be called "magnetic ropes", as the field lines burst out the surface from one spot and fall back in through another. The two spots will have opposite polarity.
Sunspots are believed to be caused by differential rotation. This occurs because, being gaseous, the photosphere of the sun rotates faster at the equator than at the poles. This warps the sun's magnetic field until the twisted field lines are forced through the surface in the form of sunspots.
The magnetic field strength of a sunspot is about 0.15T
The earth possesses a magnetic field that extends into space all around the planet, sometimes called the magnetosphere. It plays an important role in protecting the planet from the solar wind.
The Earth's magnetic field is thought to occur because of the dynamo effect, caused by the presence of liquid iron and nickel in the outer core that move as the earth rotates.
Inside these liquid metals, electric currents flow and these induce the magnetic field around the earth that extends far into space.
As it turns out, earth's polarity is not constant. It has switched several times in the past, as can be noticed by examining the way molten rock has cooled in the mid-Atlantic ridge. the last switch was 770,000 years ago. currently, the magnetic south pole is in the northern hemisphere.
The poles also have a tendency to wander around by about 10km a year, at the moment getting closer to the geographical poles.
The earth's magnetic field protects us by shielding us from radiation moving through space called the solar wind. The solar wind greatly distorts the shame of our magnetic field, causing it to have a long, comet-like tail extending behind the earth away from the sun.
The solar wind compresses the field on the sunward side to only 6-10 times the radius of the earth. A shape called a 'bow shock' forms on the sunward side and is similar to a sonic boom in shape. This bow shock deflects particles around the earth so they don't actually reach the atmosphere.
On the other side, the solar wind drags the magnetosphere far out into space. The exact length is not known, but it is estimated to be about 1000 times the radius of the earth, and is called the magnetotail.
Some particles are drawn in at the poles, and these cause the aurorae.
The magnetosphere prevents charged particles from entering the atmosphere and stripping away the ozone layer. This means the magnetic field of the earth also helps prevent damaging radiation like UV and x-rays reaching the troposphere.
The sun's magnetic field interacts with those of other planets. If this interplanetary magnetic field has an opposite polarity to that of the planet it is interacting with, the fields link and partially cancel at the point of contact.
the sun also affects us through the solar wind. This escapes through the coronal magnetic field near the poles. The solar wind is the main thing that affects us - the stream of charged particles would strip our atmosphere away if it weren't for the earth's own magnetic field providing protection.
Strong solar winds, caused by solar flares, can sometimes affect communications on earth that rely on electromagnetic signals.
All references accurate on 08/07/2014