1,720,964 research outputs found
Dynamo models for magnetic field generation in Uranus and Neptune
No full textThe magnetic fields of the ice giants are multipolar and non-axisymmetric. Voyager-II- data and HST aurorae-observations suggest magnetic power spectra with similar power in the first three spherical harmonic degrees and a peak in the order m=1. Multipolar, non- axisymmetric fields can be modeled with several different approaches including a high density stratification in the dynamo region, strongly turbulent convection, a dynamo generated by fast zonal jets and a geometrical setup with a deep stably stratified fluid layer below the dynamo region. Earlier studies with this geometry found multipolar fields and in a few cases reproduced the peak in the magnetic power spectra at order m=1 (Stanley and Bloxham, 2006). Here we explore the robustness of the multipolarity (similar power for l=1,2,3) and the m=1-peak for a range of parameters and geometrical setups using 3D numerical dynamo models. We compare our results to internal structure models of the ice giants in order to constrain the parameters and geometrical setups that are in accordance with the magnetic field observations
The exceptional magnetic fields of Uranus and Neptune: Possible generation mechanisms
No full textUnlike other planetary magnetic fields in our solar system the fields of the ice giant planets Uranus and Neptune are neither dipole- dominated nor axisymmetric. Several approaches to explain this ob- servation include turbulent convection in the dynamo region, a high density stratification, low and radially varying electrical conductiv- ity and a dynamo generated by the observed fast zonal jets. Planetary structure models as well as earlier dynamo model results suggest the possible existence of a non-convecting fluid layer below the convecting dynamo region. Such a fluid layer would not stabilize the magnetic field like a solid electrically conducting core would. This might help explain the complex field morphology. Here we present 3D numerical dynamo models in a rotating spherical shell assuming an incompressible fluid with constant electrical conductivity. We investigate the influence of a stably stratified fluid layer on magnetic field morphology by varying its thickness. The magnetic power spectra in harmonic order up to m=3 show the highest power in m=1, similar to observations of the ice giants' spectra. The results and applicability are discussed by considering alternative models leading to power spectra with a peak in m=1 as well as recent research on the ice giants' internal structure and a possible dichotomy based on e.g. their luminosity
Magnetic field morphology of the ice giants linked to their internal structure
No full textThe magnetic fields of the ice giants are multipolar and non-axisymmetric. Voyager-II-data and aurorae-observations suggest magnetic power spectra with similar power in the first three spherical harmonic degrees and a peak in the order m=1. Multipolar, non-axisymmetric fields can be modeled with several different approaches including a high density stratification in the dynamo region, strongly turbulent convection, a dynamo generated by fast zonal jets and a geometrical setup with a deep stably stratified fluid layer below the dynamo region. Earlier studies with this geometry found multipolar fields and in a few cases reproduced the peak in the magnetic power spectra at order m=1 (Stanley and Bloxham, 2006). Here we explore the robustness of the multipolarity (similar power for l=1,2,3) and the m=1-peak for a range of parameters and geometrical setups using 3D numerical dynamo models. We compare our results to internal structure models of the ice giants in order to constrain the parameters and geometrical setups that are in accordance with the magnetic field observations
Dynamo models for magnetic field generation in Uranus and Neptune
No full textThe magnetic fields of the ice giants are multipolar and non-axisymmetric. Voyager-II- data and HST aurorae-observations suggest magnetic power spectra with similar power in the first three spherical harmonic degrees and a peak in the order m=1. Multipolar, non- axisymmetric fields can be modeled with several different approaches including a high density stratification in the dynamo region, strongly turbulent convection, a dynamo generated by fast zonal jets and a geometrical setup with a deep stably stratified fluid layer below the dynamo region. Earlier studies with this geometry found multipolar fields and in a few cases reproduced the peak in the magnetic power spectra at order m=1 (Stanley and Bloxham, 2006). Here we explore the robustness of the multipolarity (similar power for l=1,2,3) and the m=1-peak for a range of parameters and geometrical setups using 3D numerical dynamo models. We compare our results to internal structure models of the ice giants in order to constrain the parameters and geometrical setups that are in accordance with the magnetic field observations
Magnetic field morphology of the ice giants linked to their internal structure
No full textThe magnetic fields of the ice giants are multipolar and non-axisymmetric. Voyager-II-data and aurorae-observations suggest magnetic power spectra with similar power in the first three spherical harmonic degrees and a peak in the order m=1. Multipolar, non-axisymmetric fields can be modeled with several different approaches including a high density stratification in the dynamo region, strongly turbulent convection, a dynamo generated by fast zonal jets and a geometrical setup with a deep stably stratified fluid layer below the dynamo region. Earlier studies with this geometry found multipolar fields and in a few cases reproduced the peak in the magnetic power spectra at order m=1 (Stanley and Bloxham, 2006). Here we explore the robustness of the multipolarity (similar power for l=1,2,3) and the m=1-peak for a range of parameters and geometrical setups using 3D numerical dynamo models. We compare our results to internal structure models of the ice giants in order to constrain the parameters and geometrical setups that are in accordance with the magnetic field observations
The exceptional magnetic fields of Uranus and Neptune: Possible generation mechanisms
No full textUnlike other planetary magnetic fields in our solar system the fields of the ice giant planets Uranus and Neptune are neither dipole- dominated nor axisymmetric. Several approaches to explain this ob- servation include turbulent convection in the dynamo region, a high density stratification, low and radially varying electrical conductiv- ity and a dynamo generated by the observed fast zonal jets. Planetary structure models as well as earlier dynamo model results suggest the possible existence of a non-convecting fluid layer below the convecting dynamo region. Such a fluid layer would not stabilize the magnetic field like a solid electrically conducting core would. This might help explain the complex field morphology. Here we present 3D numerical dynamo models in a rotating spherical shell assuming an incompressible fluid with constant electrical conductivity. We investigate the influence of a stably stratified fluid layer on magnetic field morphology by varying its thickness. The magnetic power spectra in harmonic order up to m=3 show the highest power in m=1, similar to observations of the ice giants' spectra. The results and applicability are discussed by considering alternative models leading to power spectra with a peak in m=1 as well as recent research on the ice giants' internal structure and a possible dichotomy based on e.g. their luminosity
Numerical dynamo models for magnetic field generation in the ice giants
No full textThe magnetic fields of Uranus and Neptune are not dipole-dominated and are generally more complex than the other planetary magnetic fields in our solar system. Several hypotheses have been proposed to explain their nature. Among these, the existence of a deep stably stratified fluid layer below the dynamo region or a dynamo operating in a large Rayleigh number turbulent regime are two prominent approaches. Both yield magnetic power spectra similar to those observed at the ice giants. A stable fluid layer in the deeper interior may also explain Uranus' low luminosity and could be the signature of a super-ionic water phase (Stanley and Bloxham, 2004). Dynamo action in a turbulently convecting ice layer, on the other hand, also explains the surface heat flow pattern and zonal flow structure, which shows a retrograde equatorial jet flanked by prograde jets (Soderlund et al., 2013). Here we present 3D numerical dynamo models based on data from recent ice giant structure models for the internal density stratification, electrical conductivity profile and aspect ratio. We aim to compare the proposed hypotheses to constrain the parameters and geometry leading to magnetic fields that are comparable to those of the ice giants in morphology and strength by particularly evaluating magnetic power spectra. Furthermore we examine the transition from prograde to retrograde equatorial jets in the turbulent models
Numerical dynamo models for magnetic field generation in the ice giants
No full textThe magnetic fields of Uranus and Neptune are not dipole-dominated and are generally more complex than the other planetary magnetic fields in our solar system. Several hypotheses have been proposed to explain their nature. Among these, the existence of a deep stably stratified fluid layer below the dynamo region or a dynamo operating in a large Rayleigh number turbulent regime are two prominent approaches. Both yield magnetic power spectra similar to those observed at the ice giants. A stable fluid layer in the deeper interior may also explain Uranus' low luminosity and could be the signature of a super-ionic water phase (Stanley and Bloxham, 2004). Dynamo action in a turbulently convecting ice layer, on the other hand, also explains the surface heat flow pattern and zonal flow structure, which shows a retrograde equatorial jet flanked by prograde jets (Soderlund et al., 2013). Here we present 3D numerical dynamo models based on data from recent ice giant structure models for the internal density stratification, electrical conductivity profile and aspect ratio. We aim to compare the proposed hypotheses to constrain the parameters and geometry leading to magnetic fields that are comparable to those of the ice giants in morphology and strength by particularly evaluating magnetic power spectra. Furthermore we examine the transition from prograde to retrograde equatorial jets in the turbulent models
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