Lesson 7: Earth's Dynamo

🎯 Objectives

• Explain the three ingredients of a planetary dynamo
• Use the adiabatic flux criterion to decide whether a core convects
• Understand the role of the inner core and of rotation
• Recognise what MHD-INT's thermal model still cannot explain

🧪 Exercise

1. Simulate Earth and Mars with the thermal model enabled. Note the core–mantle flux and the adiabatic flux for each, and explain which one has a dynamo.
2. Using the figures in the document, calculate how many degrees per billion years the edge of Earth's core cools in the model.
3. In Synthetic Mode (PRO), with the thermal model, simulate an Earth with a 5-, 10- and 20-day rotation. From what rotation rate does the dynamo become multipolar?
4. Look into the stable layer at the top of Mercury's core: why would it weaken its field?

⚠️ Model limits

• This lesson uses the thermal model redesigned in October 2026, available from MHD-INT v5.2.3. Earlier versions have a different thermal model (marked experimental) and give different results.
• The thermal model is parametrised: it solves neither the fluid nor the induction equations; it uses scaling laws from dynamo simulations (Christensen and Aubert 2006).
• The only fitted constant is the ratio between the internal field and the dipole (b ≈ 3.7), calibrated so that today's Earth gives 0.31 G. Earth matching is not a prediction.
• The simulation starts from the current state: it reconstructs neither Mars's history nor the age of Earth's inner core.
• Mercury comes out about ten times stronger than measured; the stable layer at the top of the core is not modelled.
• For the Earth with a 10-day rotation, the lunar tide keeps acting and the rotation changes during the simulation; the rotation plot uses the initial state.
• Both experiments (10-day rotation, Mars with 1 TW) are hypothetical.