It's not.
The equator
The closer the lines the stronger the magnetic field.
The Earth's magnetic field is probably caused by convective currents in Earth's interior.
The field gets stronger
The outer liquid core produces the strong magnetic field of the Earth, through the dynamo effect.
The magnetic field is stronger at the poles.
It would be stronger as you approach the Equator of the planet.
If by "middle", you mean the center of the sphere (inside the core), then yes the magnetic density will be high, there, just as it is inside of a magnet. If you mean at the equator, then that is not the strongest place. If by "bottom", you mean the South Pole, the magnetic field density will be stronger at the poles (north and south) than any other place on the surface of the Earth.
Earth's magnetic field strength at the equator is about 30 microtesla.
Venus does not have a magnetic field.
Charged particles are attracted to magnetic fields, and therefore are attracted to the magnetic poles of the Earth.
near both magnetic poles
the magnetic field of earth is parallel to the earth surface
The auroras that are experienced near the polar regions occur because of the strength of the earth's magnetic field. The magnetic field is not very strong at the equator.
the molten iron core
Stronger near the poles, thus Antarctica.
Earth has a magnetic field weaker than Jupiter but stronger than Venus.Jupiter 4.2 gauss at the equator to 10--14 gauss at the polesEarth less than 30 microteslas (0.3 gauss) at the equator to over 60 microteslas (0.6 gauss) around the polesVenus does not have a magnetic field.