Saturday, January 31, 2009

The Sun's Orbit and the Earth's Magnetic Field

Just as the Moon orbits the Earth, and the Earth orbits the Sun, the Sun also orbits the Milky Way galaxy. The Sun takes approximately 186 million years to orbit the Milky Way. Throughout the orbital period, the Sun intersects the arms of the galaxy. These arms are believed by some to be related to the reversal of the Earth's magnetic field. Yes, the Earth's magnetic field does reverse. It happens in periods of a anywhere from tens of thousands to millions of years. To put it in simple terms, a compass's handle would turn over (the arrow that faces north would face south, and the arrow that faces south would face north). A few thousand years before this happens, the magnetic field of Earth starts to weaken, and "holes" appear in it besides the usual ones at the poles. As this process goes on, Earth becomes more and more vulnerable to radiation and large meteors.

This being said, the activity of the magnetic field is also affected by the orbit of the Sun. When the Sun intersects the arm of our galaxy, the magnetic field is disturbed by the (relatively) close encounters with stars and other objects. This causes the magnetic field to be weak more than usual. During a gap between arms, there are little or no magnetic flips. Therefore, the field can be strong for millions of years in a row. This theory is reflected in Earth's history. For example, in the Cretaceous period, the Sun went through the longest empty gap in the galaxy. The magnetic field was quiet for nearly 40 million years. Then the magnetic field flipped five times in the next 20 million years. During this time, a 180 km meteor wiped out of 80% of life on Earth, including the dinosaurs. Also, a similar event occurred in the Permian period. After a 60 million year quiet zone, ten flips happened in the next ten million years. As the rapid flipping was occurring, a space rock more than 300 km across, one of the largest in history, destroyed over 95% of life on the planet. Note this this theory is not widely accepted at the current time.

Map of the Milky Way.  The two events mentioned above are caused by the gap after the Outer Arm going clockwise.

If this theory is incorrect, another possible explanation is the dynamo theory. This theory is based on the fact that nearly everything (except some subatomic particles) have magnetic fields. This is due to the presence of electrons (up to the present, not much is known about where electrons get their charge, as this is one of the fundamentals of the very early Universe). Since the Earth has an iron core, a magnetic field is formed around that. But this is still not enough to explain the magnetic field around Earth, and this is here the dynamo theory comes in.

A combination of the convection on molten iron in the outer core and the Coriolis effect (the same force that allows low pressure systems to form, causing weather) creates powerful electric currents. Then, since electricity and magnetism are connected (see electromagnetism), another magnetic field is produced on top of the first one. With one field sustaining the other, a dynamo is formed. Other examples of dynamo in celestial bodies are conducting plasma in the Sun and other stars, and in active galactic nuclei (based on supermassive black holes). Disturbances in this magnetic field causes it to flip or reverse when the molten outer core's circulation is disrupted (e.g. a major asteroid impact, which gives a valid contradiction to the theory described earlier). The dynamo theory also explains the flipping of the solar magnetic field, which happens every 7 to 15 years, which is very often in contrast to the Earth's. The dynamo theory is the most widely believed at this point in time.


Anonymous said...

Very nice article. You describe the sun moving through 'arms of the galaxy'. These arms contain stars themselves (I think). Are these stars fixed into the arms, while our sun is orbiting in a freer drift around the galactic center? Do the super-massive blackholes have magnetic fields? If they do, are they sucked up by the Black hole's power? Or do these magnetic fields begin radiating beyond the event horizon? It would seem that Earth's magnetic field acts as a shield and not just against the sun's radiation. Is it possible that all matter adrift in the solar system interact in some kind of magnetic level? I heard there is a larger object in Virgo attracting the universe's galaxies. If this object has in itself a magnetic field, then possibly it could be a source of stellar medium found between galaxies. Maybe this is a clue as to how magnetic fields govern the course of smaller bodies. Or magnetic fields are more rare then I think.

Louis said...

From what I have read, it seems that the Sun has a higher orbital velocity around the Milky Way than that of the spiral arms (which are composed of stars) and thus passes through them over time. In addition, black holes do have magnetic fields, but, not being composed of matter, the gravity of black holes has no effect on these fields. Also, magnetic fields only exert forces on charged particles in motion, while the interstellar medium is primarily composed of neutral hydrogen and helium.

Anonymous said...

It could be then that the "arms" of the galaxy are more bound to conditions perhaps imposed by the super massive black hole. While other stars, like our sun is free of these conformities. Then again perhaps it is about where and how groups of particular stars were formed.
Your blog is amazing btw.
And it is mind boggling that supermassive black holes have magnetic fields. My nerd jaw just dropped. Thank you.