|MadSci Network: Astronomy|
Most of these questions are very hard to answer - the answers are a matter of current research! The Sun seems to have a 22-year cycle where the polarity of the field changes through one cycle, giving two approximately 11-year long cycles of activity. The cycle itself is not understood in any detail, but it probably has something to do with a magnetohydrodynamic coupling (that is, a magnetic field and plasma - ionized gas - interact so that the flow of the plasma-gas is modified by the field and the field is modified by the flow) in the areas below the surface where there are field lines, and motion of plasma, called convection. The Sun's differential rotation probably also has something to do with it. The poles spin at a different rate than the equator and this can cause the magnetic fields to be 'wound up'. On Earth the interior is not a gas as in the Sun but a dense liquid with a solid core. There too motion of conducting matter inside a magnetic field undoubtedly causes some sort of magneto-hydrodynamic coupling to occur, but the field reversals are much less periodic than on the Sun. I think no-one knows why. On Earth the reversal of the field may have some effects on ecology, but not many, I think. For instance, near the magnetic poles today there is very little shielding from cosmic rays because the field lines are vertical and charged particles from space can come down to ground level. The radiation is measurably higher at high latitudes, but not by much. If there was a field reversal and the field at equator became small for some time things would not be any worse than near the poles today - in terms of the cosmic radiation, that is. Some animals use the fields to navigate by and they would be unable to use this mechanism for a while - but as it has happened before they appear to adapt and are not wiped out by the change. Some human endeavours are dependent on the magnetic fields, but not many. Navigation by magnetic compass is one, I guess. But we have better means of navigation than by magnetic compass nowadays. All in all I think the impacts on Earth would be small or unnoticeable if the field disappeared and then came back with opposite sign. The change-over probably takes much longer than a person's lifetime so there is no rush to adapt and people would probably not even notice.
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