Selected publications   (Ausgewählte Veröffentlichungen)

Ulrich R. Christensen


Christensen, U.R., A deep dynamo generating Mercury’s magnetic field, Nature, 444, 1056-1058, 2006.  -  A new dynamo model is proposed, in which convection is restricted to a deep sublayer of Mercury’s core. The model explains the previously enigmatic low strength of the magnetic field.


Christensen, U.R., Aubert, J., Scaling properties of convection-driven dynamos in rotating spherical shells and applications to planetary magnetic fields, Geophys. J. Int., 166, 97-114, 2006. – An extensive set of numerical dynamo simulations is used to derive rules that relate the characteristic field strength, basic magnetic field geometry, flow velocity and heat transport efficiency in planetary dynamos to parameters such as the power of convection and the rotation rate. The scaling laws can explain the magnetic field strength of Earth and Jupiter.


Christensen, U.R., Tilgner, A., Power requirement of the geodynamo from ohmic losses in numerical numerical and laboratory dynamos, Nature, 429, 169-171, 2004.  -  Numerical dynamo models and the Karlsruhe laboratory dynamo are used to establish that the rate of ohmic dissipation in a dynamo is basically controlled by the magnetic Reynolds number. The power needed by the geodynamo is estimated to be 0.2 – 0.5 TW, less than previously assumed.


Kutzner, C., Christensen, U.R., From stable dipolar to reversing numerical dynamos, Phys. Earth Planet. Inter., 131, 29-35, 2002. It is shown in numerical models that dynamos showing reversals and excursions similar to those observed in the geomagnetic field occur in a transitional range of Rayleigh numbers (Ra) between non-reversing dipole-dominated dynamos at low Ra and dynamos with a multipolar field at high Ra.


Christensen, U., Zonal flow driven by strongly supercritical convection in rotating spherical shells, J. Fluid Mech., 470, 115-133, 2002. - Numerical models are used to show that very strong zonal flow can be excited by convection in rotating spheres. Scaling laws are derived for the zonal and non-zonal flow and for the heat transport. They explain the observed wind velocity at the surface of the gas planets.


Ritter, J.R.R., Jordan, M., Christensen, U., Achauer, U., A mantle plume beneath the Eifel volcanic field, Germany, Earth Planet. Sci. Lett., 186, 7-14, 2001. -  A regional seismic tomography study using teleseismic P-wave arrivals shows the presence of a mantle plume below the weakly active neovolcanic  Eifel region. The plume extends to at least 400 km depth.


Christensen, U., Olson, P., Glatzmaier, G.A., Numerical modeling of the geodynamo: A systematic parameter study, Geophys. J. Int., 138, 393-409, 1999. – In the first systematic parameter study the existence of dynamo solutions and some of the dynamo properties are mapped out in the accessible part of the parameter space. It is shown that dynamos at low magnetic Prandtl number exist only in strongly rotating cases (low Ekman number).


Olson, P., Christensen, U.R., Glatzmaier, G.A., Numerical modeling of the geodynamo: Mechanisms of field generation and equilibration, J. Geophys. Res, 104, 10383-10404, 1999.Analysing the flow structure and magnetic field geometry in simple numerical dynamos models, it is shown that both the poloidal and the toroidal field are generated by helical flow in convection columns outside the tangent cylinder (alpha effect).


Ribe, N., Christensen, U., The dynamical origin of Hawaiian volcanism, Earth Planet. Sci. Lett., 171, 517-531, 1999. – Using three-dimensional mantle convection models of a plume rising below a moving lithospheric plate, the buoyancy flux and excess temperature of the Hawaiian plume are estimated from the observed melt production and the properties of the topographic sea-floor swell. The temporal variations of melt production, including the so-called rejuvenated stage, are explained.


Christensen, U., The influence of trench migration on slab penetration into the lower mantle, Earth Planet. Sci. Lett., 140, 27-39, 1996. - Numerical mantle convection models of a subducting slab interacting with a phase boundary and/or viscosity boundary at 660 km depth identify the rate of trench migration as a major controlling factor and show how various styles of slab behaviour in the transition zone, as are observed by seismic tomography, can arise.


Harder, H., Christensen, U., A one-plume model of martian mantle convection, Nature, 380, 507-509, 1996. – A three-dimensional convection model is presented in which the phase transition to the perovskite structure occurs slightly above the core-mantle boundary of Mars. It is shown that this would lead to a mantle convection pattern with one single strong plume, which explains the topographic and gravity signal of and the concentration of younger volcanism into the Tharsis region.


Christensen, U., Hofmann, A.W., Segregation of subducted oceanic crust in the convecting mantle, J. Geophys. Res., 99, 19,867-19,884, 1994.The generation of oceanic crust and its gravitational segregation in the deep mantle is simulated in mantle convection models. Crustal segregation is shown to be the possible cause for the anomalous D”-layer above the core-mantle boundary and can explain some of the isotopic pattern observed in mantle-derived rocks.


Ribe, N.M., Christensen, U., Three-dimensional modelling of plume - lithosphere interaction, J. Geophys. Res., 99, 669-682, 1994. The first three-dimensional numerical convection model of a plume rising below a moving plate explains quantitatively the geometry and height of the Hawaiian seafloor swell.


Arndt, N.T., Christensen, U.R., The role of lithospheric mantle in continental flood volcanism: Thermal and geochemical constraints, J. Geophys. Res., 97, 10967-10981, 1992. - Numerical models of lithospheric stretching in the presence or absence of a mantle plume and the analysis of isotopic and trace element data refute the common hypothesis that massive melting of continental sublithospheric mantle contributes significantly to the generation of continental flood basalts.


Christensen, U., Harder, H., Three-dimensional convection with variable viscosity, Geophys. J. Int., 104, 213-226, 1991. – The first three-dimensional numerical mantle convection models with temperature-dependent and/or non-Newtonian viscosity are presented. They show that upwelling is likely to be in the form of columnar plumes and suggest that nonlinear rheology is essential for generating the toroidal component of surface plate motion.


Christensen, U., Yuen, D.A., Layered convection induced by phase transitions, J. Geophys. Res., 90, 10291-10300, 1985.The conditions under which a phase change boundary enforces layered mantle convection are systematically studied. It is found that the thermodynamic properties of the phase change between upper and lower mantle are probably insufficient for layered convection in the present Earth.


Christensen, U., Thermal evolution models for the Earth, J. Geophys. Res., 90, 2995-3008, 1985.Parameterized thermal evolution models with a weak dependence of the heat loss (Nusselt number) on the internal mantle viscosity (Rayleigh number), which is suggested by convection models with strongly temperature-dependent viscosity, are analyzed. Such models predict a rather low ratio of radiogenic heat production to heat loss (Urey ratio), which is in better agreement with geochemical models of the Earth than conventional parameterized convection models.


Christensen, U., Yuen, D.A., The interaction of a subducting lithospheric slab with a chemical or phase boundary, J. Geophys. Res., 89, 4389-4402, 1984.In numerical models the conditions for the penetration of a rheologically strong slab into the lower mantle is studied. It is shown that this can be prevented by a strongly negative Clapeyron slope of an isochemical phase boundary or if a substantial fraction of the density contrasts between upper and lower mantle is of compositional origin.


Christensen, U., Convection with pressure and temperature-dependent non-Newtonian rheology, Geophys. J. R. astr. Soc., 77, 343-384, 1984.The first numerical mantle convection models with a strongly temperature-dependent and non-Newtonian rheology are presented. It is shown that the properties of stationary convection cells are very similar to convection of a Newtonian fluid with a weaker degree of temperature-dependence.