부산대학교 도서관

Turbulent motions of liquid metal in Earth’s  outer core generate the geomagnetic field. Magnetic field observations from low-Earth-orbit satellites, together with advanced numerical simulations, indicate that present-day core motions are dominated by a planetary-scale gyre, a jet in the northern polar region and waves involving the magnetic field. In this Review, we explore the dynamics of core gyres, jets and waves and discuss their impact on the magnetism and rotation of the Earth. The planetary gyre is anticyclonic, offset from the rotation axis towards low latitudes under the Atlantic hemisphere and involves flow speeds of 15–50 km yr−1 that are fastest in a focused westward jet under the Bering Strait. A quasi-geostrophic, Magnetic–Archimedes–Coriolis force balance is thought to control the dynamics of the planetary gyre and high latitude jet. Waves in the core flow with periods ~7 years have been detected at low latitudes, that are consistent with an interplay among magnetic, Coriolis and inertial effects. The arrival of wave energy at the core surface accounts for many of the characteristics of interannual geomagnetic field variations. Fluctuations in outer core flow patterns, including the planetary gyre, account for decadal changes in Earth’s length of day, while interannual changes are well explained by wave processes. Systematic investigations of core–mantle coupling mechanisms in models that include wave dynamics promise new insights on poorly constrained physical properties, including deep mantle conductivity. Long-term satellite monitoring of changes in the Earth’s magnetic field is essential if further progress is to be made in understanding core dynamics, as the high-resolution magnetic record remains short compared with the timescales of waves and convection in the core.
Gyres, jets and waves are thought to have an important role in Earth’s core dynamics. This Review explores these core processes, based on satellite observations and numerical simulations, and discusses the implications for deep-Earth coupling and forecasting geomagnetic field changes.
Key points: Since 1999, satellite observations have provided a reliable global picture of how Earth’s magnetic field is changing on interannual-to-decadal timescales. The most intense changes are found at mid-to-low latitudes under the Atlantic hemisphere and under Alaska and Siberia at high northern latitudes.Global knowledge of geomagnetic field changes, together with an understanding of the motional induction process in the core, enables the general circulation of liquid metal in the outer core to be inferred.Key features of the core flow include a planetary-scale, eccentric, anticyclonic gyre with an intense jet-like concentration under the Bering Strait and waves at low latitudes.Numerical simulations of core dynamics are now approaching conditions relevant to Earth. These demonstrate that a combination of core convection and hydromagnetic waves can account for the observed field variations.Recorded changes in the length of day on interannual and decadal periods over the past century are well explained by changes in the axisymmetric part of the core flow inferred from geomagnetic observations.