5:15 PM - 6:45 PM
[PPS05-P06] Stably Stratified Layer in Mercury’s Outer Core: Insights from Axial Symmetry of its Intrinsic Magnetic Field and the Lowes Radius
Keywords:Mercury, axial symmetry, Lowes radius, dynamo simulation, intrinsic magnetic field
Each magnetized planet is thought to have an intrinsic magnetic field due to "dynamo action", in which the flow of electrically conductive material inside the planet generates a magnetic field. The Gauss coefficients of Mercury's and Saturn's intrinsic magnetic field have been estimated using vector magnetic field data from MErcury, Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER), which was the first spacecraft to enter Mercury's polar orbit, and Cassini, which observed Saturn's magnetic fields (Anderson et al., 2012; Cao et al., 2012). One of the planetary electromagnetic problems arising from these observations is that among the magnetized planets of the solar system, the intrinsic magnetic fields of Mercury and Saturn are very axisymmetric, apparently contradicting Cowling's anti-dynamo theorem (Cowling, 1933) that states there are no axisymmetric stationary dynamos.
When a time-varying magnetic field generated in a convection layer passes through a stably stratified layer occupying the upper part of the outer core, the induced current produces an opposite magnetic field, which weakens the diffusing magnetic field (skin effect). Based on this idea, planetary dynamo calculations have been performed for internal structures with a stably stratified layer in the upper half of the outer core (Takahashi et al., 2019) so far. However, there has been no quantitative discussion of the relationship between the thickness of the stably stratified layer and the axisymmetry of intrinsic magnetic fields.
The energy density spectrum of the magnetic field calculated from the Gauss coefficients (Mauersberger spectrum) consists of two different degree dependences for the Earth, and the slope of the spectrum can be used to estimate the radius of the equivalent current sphere, which is called the Lowes radius (Lowes, 1974). Tsang and Jones (2020) compared the observed magnetic field of Jupiter with the dynamo calculation results in terms of magnetic energy density spectra and showed the Lowes radius corresponds to the lower limit of the dynamo radius. However, Hermean Lowes radius is much smaller than that core radius, and thus Mercury's dynamo radius has not yet been estimated in detail.
We performed dynamo calculations of Mercury with different radii of the convective and stably stratified layer boundary and quantitatively compared the obtained intrinsic magnetic fields. The results show that the thicker the stably stratified layer, the stronger the axisymmetry of the magnetic field on Mercury's surface, and the smaller the radial component of the magnetic field. Focusing on the time variation of the magnetic field, the characteristic timescale at the higher degrees is shorter than that of the lower degrees, indicating that the magnetic field with shorter wavelengths is more intensely time-varying. It is considered that the magnetic field produced by the flow in the convection layer is reduced by the geometrical decay at the core-mantle boundary and that the shorter wavelength magnetic field is selectively weakened by the skin effect.
The observed Lowes radius is smaller than the calculated Lowes radius. Therefore, the observed Lowes radius can be interpreted as the lower limit of the dynamo radius as in the case of Jupiter (Tsang and Jones, 2020). As the dynamo radius is increased, the calculated Lowes radius tends to approach the dynamo radius. This suggests that when the stable stratification is thicker, the Lowes radius, which only considers the contribution of the poloidal magnetic field, is much smaller than the dynamo radius. In conclusion, the strong axisymmetry of the observed Mercury magnetic field, the weak magnetic field intensity, and the small Lowes radius are consistent with a thick stably stratified layer in the Hermean outer core.
When a time-varying magnetic field generated in a convection layer passes through a stably stratified layer occupying the upper part of the outer core, the induced current produces an opposite magnetic field, which weakens the diffusing magnetic field (skin effect). Based on this idea, planetary dynamo calculations have been performed for internal structures with a stably stratified layer in the upper half of the outer core (Takahashi et al., 2019) so far. However, there has been no quantitative discussion of the relationship between the thickness of the stably stratified layer and the axisymmetry of intrinsic magnetic fields.
The energy density spectrum of the magnetic field calculated from the Gauss coefficients (Mauersberger spectrum) consists of two different degree dependences for the Earth, and the slope of the spectrum can be used to estimate the radius of the equivalent current sphere, which is called the Lowes radius (Lowes, 1974). Tsang and Jones (2020) compared the observed magnetic field of Jupiter with the dynamo calculation results in terms of magnetic energy density spectra and showed the Lowes radius corresponds to the lower limit of the dynamo radius. However, Hermean Lowes radius is much smaller than that core radius, and thus Mercury's dynamo radius has not yet been estimated in detail.
We performed dynamo calculations of Mercury with different radii of the convective and stably stratified layer boundary and quantitatively compared the obtained intrinsic magnetic fields. The results show that the thicker the stably stratified layer, the stronger the axisymmetry of the magnetic field on Mercury's surface, and the smaller the radial component of the magnetic field. Focusing on the time variation of the magnetic field, the characteristic timescale at the higher degrees is shorter than that of the lower degrees, indicating that the magnetic field with shorter wavelengths is more intensely time-varying. It is considered that the magnetic field produced by the flow in the convection layer is reduced by the geometrical decay at the core-mantle boundary and that the shorter wavelength magnetic field is selectively weakened by the skin effect.
The observed Lowes radius is smaller than the calculated Lowes radius. Therefore, the observed Lowes radius can be interpreted as the lower limit of the dynamo radius as in the case of Jupiter (Tsang and Jones, 2020). As the dynamo radius is increased, the calculated Lowes radius tends to approach the dynamo radius. This suggests that when the stable stratification is thicker, the Lowes radius, which only considers the contribution of the poloidal magnetic field, is much smaller than the dynamo radius. In conclusion, the strong axisymmetry of the observed Mercury magnetic field, the weak magnetic field intensity, and the small Lowes radius are consistent with a thick stably stratified layer in the Hermean outer core.