Earth has a magnetic field and it is believed that the cause is
the circular flow of molten iron within the earth - it works
like an electric current, so the earth is one giant electric
magnet,
Earth's field lines is like this:
Earth's magnetic field forms a shield against electrically charged
particals - most of them coming for the sun:
According to scientists, the strength of the Earth's magnetic field
has descreased 10% over the past 150 years....
it could disappear over the next 1000 years....
Some scientist speculate it signals an imminent pole reversal:
click here
Others believe that this is a reason to believe that
the earth is quite "young" (if Earth's magnetic field is too strong,
it causes radition which is harmful to life).