Search

Cassini observations imply a global ocean underneath Enceladus' ice shell, with hydrothermal seafloor activity. Previous numerical simulations showed that convection in Enceladus' unconsolidated core produces heterogeneous seafloor heat flux explaining the South Pole ice thinning and plume activity. While there is evidence for efficient hydrothermal transport from the seafloor to the plumes, the ocean dynamics is strongly debated. Here, we perform 3-D numerical simulations of the ocean with a very heterogeneous bottom boundary condition from 3-D hydrothermal core simulations. We show that a strong zonal flow inhibits low-latitudes heat transfer while it remains efficient in polar regions, which explains the ice shell variations derived from gravity and topography observations. Using passive tracers, we predict rising times of hours to weeks, compatible with previous predictions. Our simulations confirm that a strong heterogeneous seafloor heat flux concentrates upwellings at the South Pole, efficiently transporting potential bio-signatures from hydrothermal vents to erupting plumes.

It has been proposed that magnetic flux expulsion due to outer core fluid upwellings may affect the geomagnetic secular variation on the core-mantle boundary (Bloxham, 1986). In this process intense horizontal field lines are concentrated below the outer boundary, introducing small radial length scales and consequently strong radial diffusion. We explore such magnetic boundary layers in numerical dynamo simulations with heterogeneous outer boundary heat flux inferred from a tomographic model of lower mantle seismic shear waves velocity anomalies. Our scheme associates magnetic boundary layers to peak horizontal magnetic fields at the top of the shell. In our models mean magnetic boundary layer thickness ranges ≈200-400 km and decreases with increasing magnetic Reynolds number. Extrapolation or interpolation to Earth's core conditions based on total core flow amplitude or its poloidal part gives mean magnetic boundary layer thickness of ≈220 and ≈260-330 km, respectively. We find magnetic boundary layers associated with the azimuthal field at the equatorial region, whereas magnetic boundary layers associated with the meridional field are found at mid latitudes. Negative outer boundary heat flux anomalies yield preferred locations of expulsion of azimuthal field below Africa and the Pacific (at low latitudes of the Northern Hemisphere.), while positive outer boundary heat flux anomalies yield preferred locations of expulsion of meridional field below the Americas and East Asia. Our results suggest that the local diffusion time is on the order of several kyr and the local magnetic Reynolds number is on the order of ≈10, both much smaller than classical estimates., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Observations from the Cassini spacecraft imply the presence of a global salty ocean underneath Enceladus’ ice shell, with ongoing hydrothermal activity at its seafloor. Previous numerical simulations showed that convection in Enceladus’ unconsolidated core generates enough dissipation to sustain the ocean and produces a strongly heterogeneous heat flow pattern at the seafloor that looks promising for explaining the surface ice topography. Yet, how the ocean in-between connects the deeper core to the surface ice shell is unknown. The dynamics of the ocean is thus the missing piece of the Enceladus puzzle towards a fully self-consistent model of the heat transfer in this moon. We fill this gap by performing 3D numerical simulations of the buried ocean of Enceladus. We show that, for the strongly heterogeneous heat flow pattern expected at the seafloor from the unconsolidated core model, azimuthal deflection of strongly localised plumes by the Coriolis force inhibits the heat transfer at low latitudes. The resulting heat flow pattern at the ocean/ice interface is qualitatively consistent with the surface ice topography and in decent quantitative agreement with an ice shell model derived from observations. Using passively advected tracers particles, we predict rising times for hydrothermal products of the order of 1-2 months, in line with previous observations-based predictions. We are thus now able to draw a complete self-consistent model of the heat transfer in Enceladus, explaining the global characteristics of this moon: global ocean, strong dissipation, reduced ice-shell thickness at the south pole and seafloor activity.

Speleothems can provide high-quality continuous records of the direction and relative paleointensity of the geomagnetic field, combining high precision dating (with U-Th method) and rapid lock-in of their detrital magnetic particles during calcite precipitation. Paleomagnetic results for a mid-to-late Holocene stalagmite from Dona Benedita Cave in central Brazil encompass ~1900 years (3410 BP to 5310 BP, constrained by 12 U-Th ages) of paleomagnetic record from 58 samples (resolution of ~33 years). This dataset reveals angular variations of less than 0.06° yr

The South Atlantic Anomaly (SAA) is a region at Earth’s surface where the intensity of the magnetic field is particularly low. Accurate characterization of the SAA is important for both fundamental understanding of core dynamics and the geodynamo as well as societal issues such as the erosion of instruments at surface observatories and onboard spacecrafts. Previous studies applied crude measures of the SAA. Here we propose new measures to better characterize the SAA area and center, accounting for global dipole changes and shape anisotropy. Applying our characterization to a geomagnetic field model covering the historical era, we find that the mean SAA area increase and westward drift rates are twice slower than previously reported. Our results reveal that the SAA area and center are much more time-dependent, including episodes of area decrease, eastward drift and rapid southward drift. We interpret these special events in terms of the secular variation of relevant large-scale geomagnetic flux patches on the core-mantle boundary. Our characterization may be used as a constraint on Earth-like numerical dynamo models.

We use rotating convection simulations in a thin spherical shell to study fluid dynamics in subsurface oceans of icy moons. We find two types of persistent results, characterized by larger outer boundary heat flux either at polar regions or at the equatorial region. Simulations corresponding to larger Rossby numbers result in polar cooling with moderate lateral heterogeneity in heat flux, whereas lower Rossby numbers give equatorial cooling with more pronounced heat flux heterogeneity. The polar cooling scenario is in agreement with inferences for the heat flux at the top of Titan's ocean, which may provide a dynamical constraint for the vigor of convection in this layer. Our results may help unraveling the internal dynamics and the interactions among the different layers within the hydrosphere of Titan. Possible implications for the deep interiors of other icy moons are envisaged.

Since the establishment of direct estimations of the Earth's magnetic field intensity in the first half of the nineteenth century, a continuous decay of the axial dipole component has been observed and variously speculated to be linked to an imminent reversal of the geomagnetic field. Furthermore, indirect estimations from anthropologically made materials and volcanic derivatives suggest that this decrease began significantly earlier than direct measurements have been available. Here, we carefully reassess the available archaeointensity dataset for the last two millennia, and show a good correspondence between direct (observatory/satellite) and indirect (archaeomagnetic) estimates of the axial dipole moment creating, in effect, a proxy to expand our analysis back in time. Our results suggest a continuous linear decay as the most parsimonious long-term description of the axial dipole variation for the last millennium. We thus suggest that a break in the symmetry of axial dipole moment advective sources occurred approximately 1100 years earlier than previously described. In addition, based on the observed dipole secular variation timescale, we speculate that the weakening of the axial dipole may end soon.

Geomagnetic jerks are the shortest temporal variations of the magnetic field generated in the Earth's core. The physical mechanism producing such abrupt changes as well as their spatio-temporal characteristics are not well understood. In order to explore geomagnetic jerks generation and their characteristics, we use a set of synthetic core flow models to solve the radial magnetic induction equation. We analyze changes of trend in the secular variation time series using a cubic polynomial fit, by invoking a new formalism of jerk amplitude per unit duration time. A new visualization scheme allows interpreting jerk amplitudes and occurrences in space and time. We find that a mild time-dependence of flow amplitude, while keeping a fixed pattern, reproduces geomagnetic jerk amplitudes. The polynomial fits were compared with two line-segments fits at ten sampled magnetic observatories about historical jerk occurrences. The differences between the misfits in the two approaches are small, which may question the definition of geomagnetic jerks as sharp “V-shape”. The local time series in our models exhibit secular acceleration changes of sign that reproduce some main observed characteristics of geomagnetic jerks: (i) a range of amplitudes that encompass those observed in geomagnetic jerks, (ii) non-simultaneous occurrence, (iii) non-global occurrence, (iv) spatial variability of amplitudes and (v) strongest amplitudes in the radial component.

International audience

The diminishing strength of the Earth’s magnetic dipole over recent millennia is accompanied by the increasing prominence of the geomagnetic South Atlantic Anomaly (SAA), which spreads over the South Atlantic Ocean and South America. The longevity of this feature at millennial timescales is elusive because of the scarcity of continuous geomagnetic data for the region. Here, we report a unique geomagnetic record for the last ∼1500 y that combines the data of two well-dated stalagmites from Pau d’Alho cave, located close to the present-day minimum of the anomaly in central South America. Magnetic directions and relative paleointensity data for both stalagmites are generally consistent and agree with historical data from the last 500 y. Before 1500 CE, the data adhere to the geomagnetic model ARCH3K.1, which is derived solely from archeomagnetic data. Our observations indicate rapid directional variations (>0.1°/y) from approximately 860 to 960 CE and approximately 1450 to 1750 CE. A similar pattern of rapid directional variation observed from South Africa precedes the South American record by 224 ± 50 y. These results confirm that fast geomagnetic field variations linked to the SAA are a recurrent feature in the region. We develop synthetic models of reversed magnetic flux patches at the core–mantle boundary and calculate their expression at the Earth’s surface. The models that qualitatively resemble the observational data involve westward (and southward) migration of midlatitude patches, combined with their expansion and intensification.

The South Atlantic Anomaly (SAA) is a region of weak geomagnetic field intensity at the Earth’s surface, which is commonly attributed to reversed flux patches (RFPs) on the core-mantle boundary (CMB). While the SAA is clearly affected by the reversed flux region below the South Atlantic, we show that the relation between the intensity minimum at Earth’s surface and RFPs is not straightforward. We map a field-dependent intensity kernel (Constable, 2007a) to study the relation between the radial geomagnetic field at the CMB and the field intensity at Earth’s surface. Synthetic tests highlight the role of specific patches (reversed and normal) in determining the location of the surface intensity minimum and demonstrate that the SAA can indeed be explained by a few intense patches. We show that the level of axial dipolarity of the field determines the stability of the relation between the SAA minimum and RFPs. The present position of the SAA minimum is determined by the interplay among several robust geomagnetic flux patches at the CMB. The longitude of the SAA minimum appears near the longitude of the Patagonia RFP due to the low-latitude normal flux patches (NFPs) near Africa and mid-Atlantic which diminish the effect of the Africa RFPs. The latitude of the SAA minimum is lower than the Patagonia RFP latitude due to the South Pacific high-latitude NFP and the axial dipole effect. The motion of the SAA minimum is explained by the motions and changes in intensity of these robust geomagnetic flux patches. Simple secular variation (SV) scenarios suggest that while the SAA path can be explained by advection, its intensity decrease requires magnetic diffusion. In addition these SV scenarios provide some speculative predictions for the SAA.

International audience

Archeomagnetic field models may provide important insights to the geodynamo. Here we investigate the existence and mobility of reversed flux patches (RFPs) in an archeomagnetic field model. We introduce topological algorithms to define, identify, and track RFPs. In addition, we explore the relations between RFPs and dipole changes and apply robustness tests to the RFPs. In contrast to previous definitions, patches that reside on the geographic equator are adequately identified based on our RFPs definition. Most RFPs exhibit a westward drift and migrate toward higher latitudes. Undulations of the magnetic equator and RFPs oppose the axial dipole moment (ADM). Filtered models show a tracking behavior similar to the nonfiltered model, and surprisingly new RFPs occasionally emerge. The advection and diffusion of RFPs have worked in unison to yield the decrease of the ADM at recent times. The absence of RFPs in the period 550-1440 A.D. is related to a low in intermediate degrees of the geomagnetic power spectrum. We thus hypothesize that the RFPs are strongly dependent on intermediate spherical harmonic degrees 4 and above. Filtered models and comparison with the GUFM1 model suggest that RFPs are robust features of the geomagentic field for the last millennia. Modelos do campo arqueomagnético podem fornecer importantes informações sobre o geodínamo. Nesta dissertação, estudou-se a existência e a mobilidade de lóbulos de fluxo reverso (RFPs, em inglês) no campo arqueomagnético. Testes de robustez foram aplicados para diferentes modeles. Além disso, foram exploradas as relações entre RFPs e as variações do momento do dipolo axial (ADM, em inglês). Para tanto, foram desenvolvidos algoritmos topológicos para definir, identificar e mapear os RFPs em modelos do campo arqueomagnético (modelo CALS3k.4b e modelo GUFM1). O equador magnético foi usado para definir a polaridade ao invés do equador geográfico. Uma vez que cada ponto da grade de 1 grau x 1 grau de latitude/longitude é associado com um hemisfério magnético e uma polaridade (normal ou reversa), o passo seguinte é identificar os picos de lóbulos de fluxo reverso. No passo final de identificação um critério de intensidade é imposto para evitar lóbulos que possuem intensidade muito baixa. Para mapear lóbulos de fluxo reverso no tempo, foi codificado um algoritmo que calcula a distancia de cada lobulo para todos os lóbulos do próximo intervalo de tempo. Diferentemente das definições anteriores, lóbulos que residem no equador geográfico são adequadamente identificados com a nova definição de RFPs. A maioria dos RFPs exibe deriva para Oeste e mais de 75% deles migram em direção a altas latitudes. Ondulações do equador magnético e RFPs resultam de campo com momento oposto ao ADM. Modelos filtrados apresentam resultados de mapeamento semelhantes aos de modelos não-filtrados, e em alguns casos, surgem nos modelos filtrados novos RFPs. Resultados dos modelos CALS3k.4b e GUFM1 para o período de 1840 AD até 1990 AD mostram concordância para o mapeamento de lóbulos de fluxo reverso com maiores similaridades para os filtros mais fortes. Os resultados desta dissertação são compatíveis com a ideia que a advecção e a difusão de RFPs tem operado em conjunto causando o decréscimo do ADM nos últimos séculos. A ausência de RFPs no período de 550-1440 AD sendo relacionada a baixos valores nos graus intermediários do espectro de potencia. Dessa forma, sugere-se que os RFPs são fortemente dependentes dos harmônicos esféricos de graus 4 e acima. Modelos filtrados e comparações com o modelo GUFM1 sugerem que lóbulos de fluxo reverso são feições robustas do campo magnético terrestre nos últimos milênios.

Archeomagnetic field models may provide important insights to the geodynamo. Here we investigate the existence and mobility of reversed flux patches (RFPs) in an archeomagnetic field model. We introduce topological algorithms to define, identify, and track RFPs. In addition, we explore the relations between RFPs and dipole changes and apply robustness tests to the RFPs. In contrast to previous definitions, patches that reside on the geographic equator are adequately identified based on our RFPs definition. Most RFPs exhibit a westward drift and migrate toward higher latitudes. Undulations of the magnetic equator and RFPs oppose the axial dipole moment (ADM). Filtered models show a tracking behavior similar to the nonfiltered model, and surprisingly new RFPs occasionally emerge. The advection and diffusion of RFPs have worked in unison to yield the decrease of the ADM at recent times. The absence of RFPs in the period 550–1440 A.D. is related to a low in intermediate degrees of the geomagnetic power spectrum. We thus hypothesize that the RFPs are strongly dependent on intermediate spherical harmonic degrees 4 and above.