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dynamo simulation; hemispherical magnetic field; Martian crustal
field; parameter regime; dynamo wave
Abstract:
The MHD dynamo process, which generates a magnetic field through turbulent motion inside an electrically conducting fluid layer, is of fundamental importance to earth and planetary science. Numerical simulations have been employed to investigate the MHD dynamo process for several decades and can reproduce various observed magnetic field properties, including dipolar and multipolar magnetic fields with different field strengths. In this work, we focus on a particular type of dynamo solution where the magnetic fields are concentrated in either the northern or the southern hemisphere and show wave-like regular reversals and propagations. We surveyed an extensive parameter range and found that these dynamo waves are the predominant solution when the thermal drive is sufficient to maintain a magnetic field but insufficient to drive a chaotic multipolar dynamo. These waves seem to require moderate convection forcing, characterized by a local Rossby number between 0.04 and 0.1. Further investigation shows that the waves likely are αΩ Parker dynamo waves, based on the consistency of the simulated wave frequency with the mean-field dynamo theory. Our results have implications for the observed dichotomy of planetary magnetic fields as well as the possibility of the absence of paleomagnetic records while the dynamos were still operating.
