The Earth's magnetic field is quite complex, and includes global reversals where the north and south magnetic poles flip-flop. While scientists have studied it for centuries, the complexity still holds mysteries about the magnetic field.
The first thing to note is all magnetism we know of comes from moving electric charges and electric currents. For something like a planet, the question then becomes where are the currents?
The Earth has a solid inner core, but then a molten iron outer core - that is the key. As my students are well aware, metals are special because they have free electrons moving throughout (the 'electron sea'). Normally these free electrons are moving randomly through the metal. But then, we need to remember that the earth rotates. The rotation causes the molten iron to flow in one direction, and suddenly we have a circular current of electrons, and this contributes to the magnetic field.
Fluid dynamics is complex and very challenging mathematically. Turbulence happens, and because Earth is spherical, that geometry introduces different rotational velocities in different regions of the molten iron. The complexities of flow and the electric currents that result causes the Earth's magnetic field to be dynamic - it is constantly changing, and on occasion when turbulent regions develop, and sometimes the flow locally can be opposite the main flow, irregularities exist within the global magnetic field. This is likely what is happening in the outer core to cause the South Atlantic Magnetic Anomaly. It has grown over the past few decades, and is a region where the magnetic field has already 'flipped' polarity. At some point, perhaps in the coming centuries, the global field may undergo another reversal, where our compass needles will point opposite where it points today. We'll see how this plays out!
In science, nature always is harder and more complex than what our textbooks tell us, so there are always magnificent mysteries waiting to be solved! Have some fun and take on one of those mysteries that fascinates you!
