Krista M. Soderlund

A dynamo explanation for Mercury's anomalous magnetic field

Recent MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) measurements have shown that Mercurys magnetic field is axial-dominant, yet strongly asymmetric with respect to the equator: the field strength in the Northern Hemisphere is approximately 3 times stronger than that in the Southern Hemisphere. Here we show that convective dynamo models driven by volumetric buoyancy with north-south symmetric thermal boundaries are capable of generating quasi-steady north-south asymmetric magnetic fields similar to Mercurys. This symmetry breaking is promoted and stabilized when the core-mantle boundary heat flux is higher at the equator than at high latitudes. The equatorially asymmetric magnetic field generation in our dynamo models corresponds to equatorially asymmetric kinetic helicity, which results from mutual excitation of two different modes of columnar convection. Our dynamo model can be tested by future assessment of Mercurys magnetic field from MESSENGER and BepiColombo as well as through investigations on Mercurys lower mantle temperature heterogeneity and buoyancy forcing in Mercurys core.

The competition between Lorentz and Coriolis forces in planetary dynamos

AbstractFluid motions within planetary cores generate magnetic fields through dynamo action. These core processes are driven by thermo-compositional convection subject to the competing influences of rotation, which tends to organize the flow into axial columns, and the Lorentz force, which tends to inhibit the relative movement of the magnetic field and the fluid. It is often argued that these forces are predominant and approximately equal in planetary cores; we test this hypothesis using a suite of numerical geodynamo models to calculate the Lorentz to Coriolis force ratio directly. Our results show that this ratio can be estimated by Λd∗≃ΛiRm−1/2

Radar detection of the brine extent at McMurdo Ice Shelf, Antarctica, and its control by snow accumulation

\Lambda _{d}^{*} \simeq \Lambda _{i} Rm^{-1/2}

The influence of magnetic fields in planetary dynamo models

(Λi is the traditionally defined Elsasser number for imposed magnetic fields and Rm is the system-scale ratio of magnetic induction to magnetic diffusion). Best estimates of core flow speeds and magnetic field strengths predict the geodynamo to be in magnetostrophic balance where the Lorentz and Coriolis forces are comparable. The Lorentz force may also be significant, i.e., within an order of magnitude of the Coriolis force, in the Jovian interior. In contrast, the Lorentz force is likely to be relatively weak in the cores of Saturn, Uranus, Neptune, Ganymede, and Mercury.

Planetary magnetic fields: Observations and models

We derive the surface density and brine infiltration depth/extent at McMurdo Ice Shelf, Antarctica, from combined analysis of radar profiles and radar statistical reconnaissance of the surface from 2011-2012 austral summer airborne observations. Most of the brine boundaries appear controlled, directly or indirectly, by the snow accumulation pattern. The infiltration is bounded westward by an ablation area and resides just above the pore close-off depth over most of its extent. The eastern brine limit matches a light-snow corridor, suggesting a reversed pressure gradient at depth that might sharply slow down the infiltration. Brine into ice is confirmed at the deepest locations north and east of Williams Field. The ice-ocean interface is undetected west of the infiltrated zone, except in localized patches. We hypothesize this echo-free zone to be due to high scattering below the surface, possibly from a network of accreted ice and/or ice platelets at the ice-ocean interface.