The magnetic field inside the Earth's outer core cannot be directly probed. The intensity of the magnetic field at the core–mantle boundary is estimated to be 0.3 mT, and geodynamo models predict a value about ten times larger (3 mT) for the core's interior. Other data, however, indicate an internal field of only around 0.2 mT. This discrepancy has now been resolved: an ensemble inversion of core flow models finds a torsional wave recurring every 6 years, leading to an estimated field strength of around 4 mT inside the core