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  • 1
    Publication Date: 2019
    Description: Abstract Mercury is characterized by a very peculiar magnetic field, as it was revealed by the MESSENGER mission. Its internal component is highly axisymmetric, dominated by the dipole, and very weak. This in turns leads to a very dynamic magnetosphere. It is known that there exist relationships between the internally generated field and the external field, although their dynamics are complex. In this study we derive steady and time‐varying spherical harmonic models of Mercury's magnetic field using MESSENGER measurements and interpret these models both in terms of correlated features and of the internal structure of Mercury. The influence of the hemispheric data distribution of MESSENGER is evaluated to grant the robustness of our models. We find a quadrupole‐to‐dipole ratio of 0.27 for the steady magnetic field. The time‐varying models reveal periodic and highly correlated temporal variations of internal and external origins. This argues for externally inducing and internally induced sources. The main period is 88 days, the orbital period of Mercury around the Sun. There is no measurable time lag between variations of external and internal magnetic fields, which place an upper limit of 1 S/m for the mantle conductivity. Finally, the compared amplitudes of external and internal time‐varying field lead to an independent (from gravity studies) estimate of the conductive core radius, at 2,060 ± 22 km. These analyses will be further completed with the upcoming BepiColombo mission and its magnetic field experiment, but the presented results already lift the veil on some of the magnetic oddities at Mercury.
    Print ISSN: 2169-9097
    Electronic ISSN: 2169-9100
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
    Publication Date: 2015-09-29
    Description: We study systematically the estimation of Earth's core angular momentum (CAM) variation between 1962.0 and 2008.0 by using core surface flow models derived from the recent geomagnetic field model C 3 FM2. Various flow models are derived by changing four parameters that control the least-squares flow inversion. The parameters include the spherical harmonic (SH) truncation degree of the flow models, and two Lagrange multipliers that control the weights of two additional constraints. The first constraint forces the energy spectrum of the flow solution to follow a power-law , where l is the SH degree and p is the fourth parameter. The second allows to modulate the solution continuously between the dynamical states of tangential geostrophy (TG) and tangential magnetostrophy (TM). The calculated CAM variations are examined in reference to two features of the observed length-of-day (LOD) variation, namely, its secular trend and 6-year oscillation. We find flow models in either TG or TM state for which the estimated CAM trends agree with the LOD trend. It is necessary for TM models to have their flows dominate at planetary scales, whereas TG models should not be of this scale, otherwise their CAM trends are too steep. These two distinct types of flow model appear to correspond to the separate regimes of previous numerical dynamos that are thought to be applicable to the Earth's core. The phase of the subdecadal CAM variation is coherently determined from flow models obtained with extensively varying inversion settings. Multiple sources of model ambiguity need to be allowed for in discussing whether these phase estimates properly represent that of Earth's CAM as an origin of the observed 6-year LOD oscillation.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
    Publication Date: 2012-05-16
    Description: SUMMARY Magnetic satellite data from the last decade allow to model geomagnetic secular acceleration, the second time derivative of the field, in a highly precise manner. Robust estimates of the secular acceleration (SA) are obtained by using order six B-Splines as representation of the field variability, which in turn allows us to estimate the characteristic SA timescale, τ SA . We confirm a recent finding that τ SA is of order 10 years and fairly independent of the spherical harmonic degree n . This contrasts with the characteristic timescale of geomagnetic secular variation τ SV , which is a decreasing function of n and is 100 yr for n ≤ 5. Conceivably the SA timescale might be related to short-term processes in the core, distinct from convective overturn whose timescale is reflected by τ SV . Previously it had been shown that dynamo simulations reproduce the shape of the secular variation (SV) spectrum and, provided their magnetic Reynolds number Rm has an Earth-like value of order 1000, also the absolute values of τ SV . The question arises if dynamo simulations can capture the observed timescales of geomagnetic SA. We determined τ SA ( n ) for a set of dynamo models, covering a range of values of the relevant control parameters. The selection of models was based on the morphological similarity of their magnetic fields to the geomagnetic field and not on criteria related to the time dependence of the field, or on any aspect of the spectra associated with their field variation. We find that τ SA depends only weakly on n up to degree 10, but for larger n it asymptotically approaches the 1/ n -dependence that is also found for τ SV ( n ). The acceleration timescale at low n varies with magnetic Reynolds number more strongly than τ SV and may also depend on magnetic field strength. For an Earth-like Rm ≈ 1000, τ SA is of order 10 yr for n ≃ 2–10, as found in the field models from satellite data. A simple scaling analysis based on the frozen flux assumption for magnetic variations suggests two contributions to the SA, an advective part that scales with velocity U and has a length scale dependence corresponding to n −1 , and a part that depends on the acceleration of the flow without explicit dependence on the length scale. Their combination can explain the spectral shape of τ SA ( n ) in numerical models, with the latter term dominating at n 〈 10. The characteristic timescale of acceleration of the near surface flow correlates with τ SA in the different numerical models and is of the same order as τ SA . This suggests that the observed 10 yr timescale of geomagnetic SA reflects the characteristic time of core flow acceleration. To explain the geomagnetic SV and SA timescales, we find that the rms velocity near the core surface must be 18 km yr −1 and the rms flow acceleration approximately 2 km yr −2 , although a statistical analysis of the induction equation suggests that most of the latter may occur at flow scales corresponding to harmonic degrees n 〉 12. The ability of dynamo models to match simultaneously SV and SA timescales suggests that dynamic processes in the core at the decadal timescale are not fundamentally different from those at the centennial timescale.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 4
    Publication Date: 2011-03-17
    Description: SUMMARY We analyse the residuals between a continuous geomagnetic field model of the internal field, C 3 FM, and observed secular variation. A large part of the residual variations correlate closely with the D ST -index, suggesting an origin from unmodelled external field variations. Removal of this signal enhances the resolution of fine-scale detail in secular variation; this is useful in considering the phenomenology of geomagnetic jerks. The residual variations between different observatories show even better correlation, suggesting the possibility of the construction of a proxy for the D ST -index. Notable cross-correlation is also seen between the residuals and D ST -index with a lag of about 55 months.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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