MadSci Network: Astronomy

Re: Why does the sun reverse its magnetic poles?

Date: Mon Feb 19 15:19:25 2001
Posted By: Peter Thejll, Staff, Solar-Terrestrial Physics, Danish Meteorological Institute
Area of science: Astronomy
ID: 982434330.As

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 

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 
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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