Why do the earth have a magnetic field?

Answer 1

An excerpt from a paper by D.R. Fearn:

The problem of magnetic field stability is complementary to the (much more widely studied) kinematic dynamo problem The dynamo mechanism converts the mechanical energy of convection into magnetic energy. If the field generated is unstable then the instability extracts energy from the field and might be expected to limit the strength of field attainable. This is an important consideration since the toroidal part of the field in the Earth's core is shielded from us by the insulating mantle and we do not know its strength. [Measurements of the DC electrical potential near the top of the mantle have been extrapolated downward to estimate the toroidal field at the core-mantle boundary (CMB), ...). This gives a toroidal field strength of a few gauss ( at the CMB), but with a weakly conducting mantle, a low value is to be expected and this result is not inconsistent with a strong toroidal field in the interior of the core (...)] There are good reasons for believing the toroidal field may be significantly stronger than the observed (poloidal) part of the field. If so, theoretical considerations may be the only way we have of determining the strength of the major component of the field. Linear theory (...) has shown that a broad class of fields, characteristic of the Earth's field, are unstable when their maximum strength exceeds a value of the order of 60 gauss which is a factor 10 greater than the observed poloidal field but is in line with many estimates of the hidden toroidal field. (...) To explain the observed features of the geomagnetic field, it is clearly important to understand how a magnetic instability evolves.

Link below.

Answer 2

because salt from the ocean is electrically charged