Earth's magnetic field cannot be due to permanent magnets, because a. the core is too hot. b. the field changes its strength. c. the field changes its orientation. d. all of the above

Permanent magnets lose their magnetic properties if they are heated above a certain temperature known as the Curie temperature. The Earth's core is significantly hotter than this temperature limit. This extreme heat makes it impossible for any magnetic material to maintain its magnetism in the core. Consequently, the Earth's magnetic field cannot be due to permanent magnets since they would demagnetize under such high temperatures. The core temperature is incredibly high, reaching up to 5,700 degrees Celsius, similar to the surface of the Sun. This immense heat thus eliminates the possibility of permanent magnets being the source of the Earth's geomagnetic field.

The strength of the Earth's magnetic field isn't constant; it varies over time. Observations show fluctuations and changes in magnetic field intensity at different locations and periods. For example, there have been past periods of reduced field strength, known as geomagnetic excursions. Permanent magnets cannot produce such dynamic changes in magnetic field strength. These variations imply that the source of the Earth's magnetic field must be capable of evolving and fluctuating, which is something permanent magnets cannot do. The dynamo theory, which suggests the movement of molten iron and nickel in the Earth's outer core generates the magnetic field, better explains these variations.

The orientation of the Earth's magnetic field is not fixed; it undergoes gradual shifts and sometimes even complete reversals, where the magnetic north and south poles switch places. These magnetic pole reversals are irregular but have occurred multiple times over geologic history. The fact that the Earth's magnetic field changes direction eliminates the possibility of it being generated by permanent magnets. Permanent magnets would maintain a constant orientation without such drastic changes. Instead, the fluid motion in the Earth's outer core, which drives the geodynamo process, accounts for these shifts in magnetic orientation. This ongoing movement of conductive materials creates and sustains a dynamic magnetic field that can change over time.