Geomagnetic polarity reversals, Earth's core evolution, and conditions for dynamo action in the cores of terrestrial exoplanets.

摘要

Planetary dynamos are responsible for the generation of large-scale magnetic fields and are ubiquitous in the solar system. Magnetic fields generated by dynamo action in a planetary core offer unique insight into the internal structure, composition, and energetics of the planet. This dissertation consists of two main parts, the first focuses on long period fluctuations in Earth's magnetic field and the second explores conditions for dynamo action in the cores of terrestrial exoplanets. The first part consists of three projects using first-principle numerical magnetohydrodynamic models of the geodynamo to investigate the relationship between two fundamental, but poorly understood, aspects of the geomagnetic field: magnetic polarity reversals and the influence of core evolution. The first project explores the dependence of various dynamo properties on the relative strengths of buoyancy and rotation, and identifies several dynamical regimes whose magnetic field fluctuations over time are consistent with the paleomagnetic field. We find that normal evolution of buoyancy production in the core and planetary rotation rate over 100 Myr produce a negligible change in dynamo polarity reversal rate and field intensity, implying that the observed fluctuations in the geomagnetic reversal rate requires either anomalous core evolution or a rough dynamo regime boundary. The second project models the long time-scale evolution of the Earth's core using time-dependent control parameters, which are constrained by the secular cooling of the core and tidal deceleration. We find that fluctuations in the geodynamo are closely coupled to the evolution of the core, which implies a connection between the long time-scale trends in the seafloor geomagnetic polarity reversal rate and the rate of core evolution over the last 100 Myr. In the third project we investigate the hypothesis that the long period (∼200 Myr) oscillation in paleomagnetic reversal frequency is controlled by the heat flow amplitude at the core-mantle boundary (CMB) by calculating a continuous 200 Myr long geodynamo simulation subject to an oscillation in core heat flow. We demonstrate that an increase in the superadiabatic core heat flow evolves the model from a superchron to a reversing state, and vice-versa, producing a simulated reversal record similar to the seafloor paleomagnetic reversal record. This implies that fluctuations in the thermal evolution of the core are recorded in the paleomagnetic record, with periods of high core heat flow corresponding to frequent polarity reversals, similar to the present-day geomagnetic reversal rate (∼4 Myr--1), and periods of low heat flow corresponding to superchron states with no polarity reversals, similar to the CNS. In the second part we explore the conditions for dynamo action in the core's of terrestrial exoplanets and the possibility of their detection. We construct internal structure models for terrestrial exoplanets with 1-10 Earth masses and an Earth-like composition and structure. In order to maximize the magnetic field intensity at the planet surface, which is maintained by dynamo action in the convecting core, we assume these planets are in an optimal thermal state where the temperature profile in the mantle thermal boundary layers is at the silicate melting point. We find that magnetic field intensity increases slightly with mass and core size, such that the maximum magnetic dipole moment is about 23 times the geomagnetic dipole moment for a 10 Earth-mass planet with a large core. We find that estimates of the electron cyclotron emission spectrum for nearby exoplanet magnetic fields are generally below the current detection thresholds of the largest radio telescopes, but may be detectable in the future.