[SIT22-P32] Study of the required Rayleigh number to sustain geodynamo with various inner core radius
Keywords:geodynamo, the past Earth, the inner core size
In previous dynamo simulations, the property of Ra around the onset of dynamo action has been revealed for various radius ratios [Heimpel et al., 2005]. The tendency of the dynamo action has also been investigated by previous studies in the present ratio ri/ro = 0.35 [Christensen and Aubert, 2006], but the behavior of dynamo with Ra has not been fully understood with the smaller inner core. We perform dynamo simulations with a range of 1.9 Racrit < Ra < 9.7 Racrit, where Racrit is the critical Rayleigh number, and with the aspect ratio ri/ro = 0.15, 0.25, and 0.35, to quantitatively understand the tendencies with the smaller inner core. The results with ri/ro = 0.25 show that the sustained dipolar dominant dynamo occurs just above the onset of the dynamo action. In the case with larger Ra than that for the dipolar regime, non-dipolar components of the magnetic field is sustained. Finally, dynamo failed in the case with Ra > 8.1 Racrit. The simulation results reveal that the magnetic energy density is largest at the dynamo onset, and becomes smaller at larger Ra. This implies that dynamo is not likely to be sustained in intense convection under the assumed setting. The results of ri/ro = 0.25 and 0.35 imply that the revealed tendency revealed by the present study can be applied to ri/ro = 0.15. Comparing the simulation results in the same Ra/Racrit(= 3.6), the magnetic dipole moment becomes smaller with the smaller inner core. This suggests a possibility that the convection favorable for the dipolar dominant dynamo is difficult to occur with the smaller inner core. We also find that the Ra range of the dipolar dominant cases becomes narrower with the smaller inner core, implying that Ra was very selective at the past Earth. To interpret the simulation results with the real physical quantities, the average temperature difference between the core-mantle boundary and the inner core boundary, ΔT, is estimated. ΔT in ri/ro = 0.25 is larger than that in ri/ro = 0.35, so required buoyancy to sustain a dipolar dominant dynamo is larger with the smaller inner core.