Because it is both fluid and a good electrical conductor, the outer core can produce and sustain the Earth's magnetic field by working like a gigantic dynamo. Although the magnetic field that reaches the surface of the Earth is weaker than the field of an ordinary refrigerator magnet, it is nevertheless responsible for controlling things like a compass needle, and the Northern Lights. Dr. Rochester is interested in how the presence of this magnetic field changes the rotation of the Earth. As the Earth rotates, its speed may change because of the changes in the magnetic field. These very small changes in the Earth's rotation speed mean changes in the length of the day, amounting to a few thousandths of a second over a time span of a decade.
Nothing stands alone.
You can't look at it and understand it without bringing in knowledge from many other areas. It's a very interdisciplinary type of field. So many things in science are connected with one another. That is why research is endlessly fascinating.