How turbulent convection currents in Earth’s outer core make its magnetic field
This video is a prequel to one that will appear here: http://ve42.co/Spin
Huge thank you to Prof. Jon Arnou who spent a lot of time explaining how planetary magnetic fields are created. He changed a lot of my preconceptions. His research page is here: http://ve42.co/SpinLab
OK, so what were my preconceptions?
1. That the Earth’s magnetic field is a passive thing – it shouldn’t need a continuous input of energy to maintain itself (that seemed reasonable to me because the magnetic field has been around for a long time and it seems mostly stable). But as it turns out, the Earth is a giant electromagnet, and so of course those currents dissipate their energy as they encounter resistance in the liquid metal through which they flow. So the energy to continuously create these currents comes from the kinetic energy of the liquid metal flows in the Earth’s outer core.
2. If it’s convection, I’m thinking hot things rising, cooler things falling. But apparently the main effect driving convection is the compositional differences at the boundary with the Earth’s inner core. This is because of the differential freezing at the boundary. Things like iron freeze into the inner core, while elements like sulfur do not. Hence the pockets of lighter material which then rise outwards.
3. I didn’t get why the fluid motion was necessary for the generation of the magnetic field. I mean if it’s a conducting liquid, it can conduct currents whether it moves or not. But the key is that the liquid metal can ‘trap’ magnetic fields. I imagine this like how iron channels magnetic fields. Then once these fields are channeled, they can be pulled and stretched, making more magnetic field.
4. Fluids operate very differently in rotating frames of reference. This is something I didn’t intuitively grasp. But, as fluids move from the inner core outwards, those particles are moving much more slowly in the direction of rotation than the matter that has been there for a long time, which means the convection currents get deflected and form helices.
Listening to Prof. Arnou’s explanation made possible my explanation in the other video (it’s really just a summary of what he says here). So thanks Prof. Arnou!