09:45 〜 10:00
[SEM15-04] On generation processes of the dipole/quadrupole-family magnetic field in kinematic dynamos
キーワード：惑星磁場、キネマティックダイナモ、赤道対称性
The geo- and planetary magnetic fields are maintained by the dynamo action in their fluid cores. Characteristics of the planetary magnetic fields are closely related to the interior structure and core dynamics, and therefore very diverse among planets. One of the remarkable features is found in the Mercury's magnetic field, compared with the geomagnetic field: the equivalent dipole is offset northward significantly from the center of Mercury, whereas the geomagnetic field is well represented by the geocentric dipole. Such a difference can be explained by a difference in fraction of the quadrupole component. In principle, any scalar/vector fields in the spherical coordinate system are decomposed into the equatorially anti-symmetric and symmetric constituents. Regarding the magnetic field, the euatorially anti-symmetric (symmetric) part is classified as the dipole-family (quadrupole-family). Theoretically, magnetic field generation of diffrent family occurs independently, when the velocity field consists solely of the equatorially symmetric conponent, which is supposed to be dominant in the core flows. At present, it is well knwon that dipole-family magnetic field is generated by equatorially symmetric columnar flows, while the generation mechansims of the quadrupole-family magnetic field is not yet well established. Here we study generation mechanisms of the dipole/quadrupole-family magnetic field in order to enhance our knowledge on dynamo action in the core.
As a first step, we use kinematic dynamo modeling with a velocity field by Kumar and Roberts (1975), which is symmeric with respect to the equator. A stationary solution for the dipole-family field is obtained at the magnetic Reynolds number of about 3880 as is in Kumar and Roberts (1975). On the other hand, stationary solution is not found up to the magnetic Reynolds number of 30,000 for the quadrupole-family field. Then, a velocity field consisting of the differential rotation and the poloidal flow of Y_2^2 mode is applied to the runs of kinematic dynamos. Stationary solutions are found for both the dipole- and quadrupole-family with the velocity field: the critical magnetic Reynolds number is 88 for the dipole-family, and 250 for the quadrupole-family. Magnetic field generation processes in each famaly solution are investigated in terms of energy budget and the Mean-Field Theory. Based on the analyses, we discuss differences between dynamos in the dipole-family and quadrupole-family.
This work was supported by JSPS KAKENHI Grant Number JP21K03725.
As a first step, we use kinematic dynamo modeling with a velocity field by Kumar and Roberts (1975), which is symmeric with respect to the equator. A stationary solution for the dipole-family field is obtained at the magnetic Reynolds number of about 3880 as is in Kumar and Roberts (1975). On the other hand, stationary solution is not found up to the magnetic Reynolds number of 30,000 for the quadrupole-family field. Then, a velocity field consisting of the differential rotation and the poloidal flow of Y_2^2 mode is applied to the runs of kinematic dynamos. Stationary solutions are found for both the dipole- and quadrupole-family with the velocity field: the critical magnetic Reynolds number is 88 for the dipole-family, and 250 for the quadrupole-family. Magnetic field generation processes in each famaly solution are investigated in terms of energy budget and the Mean-Field Theory. Based on the analyses, we discuss differences between dynamos in the dipole-family and quadrupole-family.
This work was supported by JSPS KAKENHI Grant Number JP21K03725.