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
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.,
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.
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.