Why is the Earth's Magnetic Field is getting weaker day by day?

To understand that why Earth's Magnetic Field is getting weaker day by day, we first need to understand that what is the reason of Earth's Magnetic Field.

Reason Behind Magnetic Field of a Planet

The magnetic field is produced by swirling motions of liquid conducting material in the planet interiors. Materials that can conduct electricity have some electrical charge that is free to move about. Such materials are called metallic and are not necessarily shiny solids like copper, aluminum, or iron. Jupiter and Saturn have a large amount of hydrogen that is compressed so much it forms a liquid. Some of that liquid hydrogen is in a state where some of the electrons are squeezed out of the atoms and are free to move around. A moving charge will produce a magnetic field. The liquid conducting material in a planet's interior can be made to swirl about if the planet is rotating quickly enough. The faster a planet rotates, the more the material gets stirred up and, therefore, the stronger the generated magnetic field. 

What affects Magnetic Field of a Planet?

If the liquid interior becomes solid or if the rotation slows down, the magnetic field will weaken. So in summary what a planet needs in order to produce a strong magnetic field are 
(1) a liquid conducting (metallic) interior and 
(2) rapid rotation to get the conducting material moving about. 

For the terrestrial planets plate tectonics may also play a role. Plate tectonics cools the planet's mantle creating a large enough temperature difference between the core and mantle to produce convection in the metallic core needed to make a magnetic field. Thus decreasing the magnetic effect.

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