[S-EM15] Geomagnetism, paleomagnetism and rock magnetism

[SEM15-05] Temporal variations of the geomagnetic field caused by the length-of-day variations

*Masaki Matsushima¹ (1.Department of Earth and Planetary Sciences, School of Science, Tokyo Institute of Technology)

Keywords:core flow, geomagnetic field, length-of-day, spin-up/spin-down

The geomagnetic field is generated by the dynamo action driven by thermal and/or compositional convective motions in the Earth’s outer core. Temporal variations of the geomagnetic field are also caused by fluid flows in the core. When the rotation rate of the mantle, or the length-of-day (LOD), changes, the core fluid is likely to be influenced by temporal variations of the excess of LOD (ΔLOD). Two processes are to be considered. One is conservation of angular momentum between the core and the mantle, and the other is the interaction between a viscous boundary layer at the core-mantle boundary (CMB) and the mantle. In the former case, the caused flow motion is expected to occur without any delay. In the latter case, however, the so-called spin-up/spin-down should occur, and a phase difference between the ΔLOD and the angular momentum of core flows is expected. Then, the eddy viscosity of core flows can be obtained from comparison between the time variation of ΔLOD and that of the angular momentum of core flows which can be estimated from any geomagnetic field model.

In the present study, numerical simulations of core motions caused by a change of ΔLOD are carried out as a first step. For simplicity, the fluid core is assumed to be composed of spherical shells; that is, the core motions are expressed in terms of rigid rotation of the spherical shells. Then, temporal variations of the geomagnetic field due to changes of ΔLOD are computed and investigated to understand their correlation between the LOD and the geomagnetic field.