Flux Patches Research Articles

On the persistence of geomagnetic flux lobes in global Holocene field models

To investigate the longevity and robustness of high-latitude flux patches in the geomagnetic field at the core–mantle boundary, we present time-dependent models of the geomagnetic field for the past 7000 years. Our models use the same data set as previously used for time-dependent archaeomagnetic field modelling, but constrained with additional priori models from time averages of field models covering the last 150, 400 and 3000 years. We find that the data are consistent with flux patches existing in both north and south hemispheres for the past 7000 years, and that the northern hemisphere patches at least have highly dynamic behaviour. Simple averaging of the historical field may not be appropriate to obtain a characteristic time-averaged model of the field for comparison with other geophysical observables. Our results should inform geodynamo studies of thermal core–mantle coupling, and of possible long-term structure in the geomagnetic field.

Polarity Reversals from Paleomagnetic Observations and Numerical Dynamo Simulations

Recent advances in the study of geomagnetic field reversals are reviewed. These include studies of the transitional field during the last geomagnetic reversal and the last geomagnetic excursion based on paleomagnetic observations, and analysis of reversals in self-consistent 3D numerical dynamo simulations. Field models inferred from observations estimate reversal duration in the range of 1–10 kyr (depending on site location). The transitional fields during both the Matuyama/Brunhes reversal and the Laschamp excursion are characterized by low-latitude reversed flux formation and subsequent poleward migration. During both events the dipole as well as the non-dipole field energies decrease. However, while the non-dipole energy dominates the dipole energy for a period of 2 kyr in the reversal, the non-dipole energy merely exceeds the dipole energy for a very brief period during the excursion. Numerical dynamo simulations show that stronger convection, slower rotation, and lower electrical conductivity provide more favorable conditions for reversals. A non-dimensional number that depends on the typical length scale of the flow and represents the relative importance of inertial effects, termed the local Rossby number, seems to determine whether a dynamo will reverse or not. Stable polarity periods in numerical dynamos may last about 1 Myr, whereas reversals may last about 10 kyr. Numerical dynamo reversals often involve prolonged dipole collapse followed by shorter directional instability of the dipole axis, with advective processes governing the field variation. Magnetic upwellings from the equatorial inner-core boundary that produce reversed flux patches at low-latitudes of the core-mantle boundary could be significant in triggering reversals. Inferences from the observational and modeling sides are compared. We summarize with an outlook on some open questions and future prospects.

Stationary, oscillating or drifting mantle‐driven geomagnetic flux patches?

The impact of the heterogeneous lower mantle on the geomagnetic field is under debate, especially the question of whether high‐latitude intense geomagnetic flux patches currently observed at the core surface are stationary, oscillating, or drifting on longer time scales. While the correlation between the location of these patches with that of similar patches found in the time‐averaged paleomagnetic field may suggest stationary behavior, their variability over archaeomagnetic time scales together with their weaker signature in the average paleomagnetic field relative to the present geomagnetic field precludes such a scenario. Here we use numerical dynamos with an imposed heat flux boundary condition based on lower mantle tomography to study the behavior of such intense magnetic flux patches. We design an algorithm to detect centers of intense flux patches and track their time evolution. We find that the time‐dependent nature of those patches comprises oscillatory motion about statistically preferred locations imposed by mantle control, with episodic drift from one preferred location to the other corresponding to an azimuthal migration of fluid downwelling structures that concentrate surface magnetic flux. This statistical behavior provides a possible explanation for both the observed variability of high‐latitude patches on the archaeomagnetic time scale and the similar locations of the current patches and the weaker patches seen in the paleomagnetic field. Our simulations also show that the patches exhibit more time dependence and less coherency in the southern hemisphere, leading to a weaker time‐averaged patch signature in that hemisphere.

Geomagnetic field hemispheric asymmetry and archeomagnetic jerks

We investigate the origin of the so-called archeomagnetic jerks detected in the French archeomagnetic record over the past three millennia. Although only very large-scale global archeomagnetic field models are currently available, we show that the occurrence of archeomagnetic jerks is intimately linked to what we define as “most eccentric” events, i.e., periods of time when a simple description of the geomagnetic field in terms of an eccentric dipole reveals the center of this eccentric dipole to strongly move away from the Earth's center. From the behavior of the much better known historical field, we interpret the evolution of the center of the eccentric dipole as reflecting the production and gathering of flux patches at the core–mantle boundary within preferential hemispheres. Archeomagnetic jerks would thus correspond to episodes of maximum geomagnetic field hemispheric asymmetry. Such “most eccentric” events could also provide an explanation for some of the properties previously reported in the long-term paleomagnetic field.

Geomagnetic field evolution during the Laschamp excursion

Since the last geomagnetic reversal, 780,000 years ago, the Earth's magnetic field repeatedly dropped dramatically in intensity. This has often been associated with large variations in local field direction, but without a persistent global polarity flip. The structure and dynamics of geomagnetic excursions, and especially the difference between excursions and polarity reversals, have remained elusive so far. For the best documented excursion, the Laschamp event at 41,000 years BP, we have reconstructed the evolution of the global field morphology by using a Bayesian inversion of several high-resolution palaeomagnetic records. We have obtained an excursion scenario in which inverse magnetic flux patches at the core-mantle boundary emerge near the equator and then move poleward. Contrary to the situation during the last reversal (Leonhardt, R., Fabian, K., 2007. Paleomagnetic reconstruction of the global geomagnetic field evolution during the Matuyama/Brunhes transition: Iterative Bayesian inversion and independent verification. Earth Planet. Sci. Lett. 253, 172–195), these flux patches do not cross the hydrodynamic boundary of the inner-core tangent cylinder. While the last geomagnetic reversal began with a substantial increase in the strength of the non-dipolar field components, prior to the Laschamp excursion, both dipolar and non-dipolar field decay at the same rate. This result suggests that the nature of an upcoming geomagnetic field instability can be predicted several hundred years in advance. Even though during the Laschamp excursion the dipolar field at the Earth's surface was dominant, the reconstructed dynamic non-dipolar components lead to considerable deviations among predicted records at different locations. The inverse model also explains why at some locations no directional change during the Laschamp excursion is observed.

The Magnetic Landscape of the Sun's Polar Region

We present observations of the magnetic landscape of the polar region of the Sun that are unprecedented in terms of spatial resolution, field of view, and polarimetric precision. They were carried out with the Solar Optical Telescope aboard Hinode. Using a Milne-Eddington inversion, we find many vertically oriented magnetic flux tubes with field strengths as strong as 1 kG scattered in latitude between 70° and 90°. They all have the same polarity, consistent with the global polarity of the polar region. The field vectors are observed to diverge from the centers of the flux elements, consistent with a view of magnetic fields that are expanding and fanning out with height. The polar region is also found to have ubiquitous horizontal fields. The polar regions are the source of the fast solar wind, which is channeled along unipolar coronal magnetic fields whose photospheric source is evidently rooted in the strong-field, vertical patches of flux. We conjecture that vertical flux tubes with large expansion around the photospheric-coronal boundary serve as efficient chimneys for Alfven waves that accelerate the solar wind.

Accounting for magnetic diffusion in core flow inversions from geomagnetic secular variation

SUMMARY We use numerical dynamos to investigate the possible role of magnetic diffusion at the top of the core. We find that the contribution of radial magnetic diffusion to the secular variation is correlated with that of tangential magnetic diffusion for a wide range of control parameters. The correlation between the two diffusive terms is interpreted in terms of the variation in the strength of poloidal flow along a columnar flow tube. The amplitude ratio of the two diffusive terms is used to estimate the probable contribution of radial magnetic diffusion to the secular variation at Earth-like conditions. We then apply a model where radial magnetic diffusion is proportional to tangential diffusion to core flow inversions of geomagnetic secular variation data. We find that including magnetic diffusion does not change dramatically the global flow but some significant local variations appear. In the non frozen-flux core flow models (termed ‘diffusive’), the hemispherical dichotomy between the active Atlantic and quiet Pacific is weaker, a cyclonic vortex below North America emerges and the vortex below Asia is stronger. Our results have several important geophysical implications. First, our diffusive flow models contain some flow activity at low latitudes in the Pacific, suggesting a local balance between magnetic field advection and diffusion in that region. Second, the cyclone below North America in our diffusive flows reconciles the difference between mantle-driven thermal wind predictions and frozen-flux core flow models, and is consistent with the prominent intense magnetic flux patch below North America in geomagnetic field models. Finally, we hypothesize that magnetic diffusion near the core surface plays a larger role in the geomagnetic secular variation than usually assumed.

Magnetic poles and dipole tilt variation over the past decades to millennia

Abstract Due to the strong dipolar character of the geomagnetic field the shielding effect against cosmic and solar ray particles is weakest in the polar regions. We here present a comprehensive study of the evolution of magnetic and geomagnetic pole locations and the Earth’s magnetic core field in both polar regions over the past few years to millennia. North and south magnetic poles change independently according to the asymmetric complexity of the field in the two hemispheres, and the changes are not correlated to variations of the dipole axis. The recent, strong acceleration of the north magnetic pole motion appears to be linked to a reverse flux patch at the coremantle boundary, and an increasing deceleration of the pole over the next years seems likely. Over the whole studied period of 7000 years two periods of comparatively high velocity of the north magnetic pole are observed at 4500 BC and 1300 BC, based on the presently available data and models. Geographic latitudes and longitudes of magnetic and geomagnetic poles based on the studied geomagnetic field models are available together with animations of the poles and polar field behaviour from our webpage http://www.gfz-potsdam.de/geomagneticfield/poles.

Frequent Occurrence of High‐Speed Local Mass Downflows on the Solar Surface

We report on new spectropolarimetric measurements with simultaneous filter imaging observation, revealing the frequent appearance of polarization signals indicating high-speed, probably supersonic, downflows that are associated with at least three different configurations of magnetic fields in the solar photosphere. The observations were carried out with the Solar Optical Telescope on board the Hinode satellite. High-speed downflows are excited when a moving magnetic feature is newly formed near the penumbral boundary of sunspots. Also, a new type of downflows is identified at the edge of sunspot umbra that lack accompanying penumbral structures. These may be triggered by the interaction of magnetic fields swept by convection with well-concentrated magnetic flux. Another class of high-speed downflows are observed in quiet Sun and sunspot moat regions. These are closely related to the formation of small concentrated magnetic flux patches. High-speed downflows of all types are transient time-dependent mass motions. These findings suggest that the excitation of supersonic mass flows are one of the key observational features of the dynamical evolution occurring in magnetic-field fine structures on the solar surface.

Behaviour of the geomagnetic field during the Matuyama–Brunhes polarity transition

The ability to derive Gauss coefficients, up to and including degree 3, and their variation through a geomagnetic polarity transition is studied using simulated palaeomagnetic data. It is concluded that for a specified distribution of palaeomagnetic sites reasonable estimates of the behaviour of the coefficients can be derived even when uncertainties in the data, and in the compilation of contemporaneous records, are considered. Published palaeomagnetic records of the Matuyama–Brunhes transition are then used as basis for deriving the variation of the Gauss coefficients over a 32 kyear period encompassing the reversal. Individual records are interpolated to uniform time intervals of 0.5 kyear and put on to a common time scale by correlating between sites the variation in the latitude of VGP's through the reversal. Relative palaeointensity data are scaled by the geocentric axial dipole field intensity for 2000 at each site, and the Gauss coefficients derived by a matrix inversion employing singular value decomposition. The derived variation with time of the Gauss coefficients suggests that, over the time span of the data, the dipole and non-dipole fields have approximately equal intensities. Plots of the variation of the surface vertical magnetic field through the reversal suggest that immediately prior to the reversal a large patch of reverse flux appears in the southern hemisphere. This may subsequently have been responsible for the weakening of the vertical field leading into the reversal. A similar patch of reverse flux is observed some 20–15 kyear prior to the actual reversal and may be associated with an observed excursion in VGPs at several sites.

Magnetic field emergence in quiet Sun granules

We describe a new form of small-scale magnetic flux emergence in the quiet Sun. This process seems to take vertical magnetic fields from the solar interior to the photosphere, where they appear above granular convection cells. High-cadence time series of spectropolarimetric measurements obtained by Hinode in a quiet region near disk center are analyzed. We extract line parameters from the observed Stokes profiles and study their evolution with time. The circular polarization maps derived from the observed \ion{Fe}{i} 630 nm lines show clear magnetic signals emerging at the center of granular cells. We do not find any evidence for linear polarization signals associated with these events. The magnetic flux patches grow with time, occupying a significant fraction of the granular area. The signals then fade until they disappear completely. The typical lifetime of these events is of the order of 20 minutes. No significant changes in the chromosphere seem to occur in response to the emergence, as revealed by co-spatial \ion{Ca}{ii} H filtergrams. The Stokes I and V profiles measured in the emerging flux concentrations show strong asymmetries and Doppler shifts. The origin of these events is unclear at present, but we suggest that they may represent the emergence of vertical fields lines from the bottom of the photosphere, possibly dragged by the convective upflows of granules. Preliminary inversions of the Stokes spectra indicate that this scenario is compatible with the observations. The emergence of vertical field lines is not free from conceptual problems, though.

Using reversed magnetic flux spots to determine a planet's inner core size

The study of planetary magnetic fields can provide important information on a planet's interior structure and composition. Here we use numerical dynamo models to demonstrate that it is possible to determine information on a fundamental core property: inner core size, from observations of intense, small‐scale radial magnetic flux patches. The inner core size is discernable from the boundary between different types of intense magnetic flux patches inside and outside the inner core tangent cylinder. This type of study could help in determining the solid inner core size in Mercury, Ganymede, Jupiter and Saturn. For Mercury and Ganymede, the inner core size would then provide useful constraints on thermal evolution models for these bodies.

Geomagnetic constraints on stratification at the top of Earth’s core

Abstract The geodynamo requires that at least part of the Earth’s liquid core be convecting so vigorously as to mix it thoroughly to uniform composition and adiabatic temperature. It is possible, however, that part of liquid core is stably stratified, either thermally because of a low temperature gradient or compositionally because light material has separated out. The top of the outer core is the most likely site for stability because the adiabat is steepest there and because light material will rise to the surface. Here I show that part of the observed secular variation, that associated with flux expulsion in the southern hemisphere, can only be caused by fluid upwelling in the electromagnetic boundary layer at the top of the core or by very strong poloidal field gradients at the top of the core. Any stratified layer is limited to roughly the uppermost 100 km if flux expulsion is the mechanism; if the layer is any thicker the region in the southern hemisphere where reverse flux patches are growing must also be the site of very strong field gradients caused by very strong near-surface shear flows.

Eastward and westward drift of the Earth's magnetic field for the last three millennia

We analyse the secular variation captured by the archaeomagnetic field model CALS7K.2 in an effort to determine episodes of eastward and westward motions of Earth's magnetic field at the core–mantle boundary (CMB) over the past 3000 yr. The direction, amplitude and geographical distribution of these motions are described. We find that the clearest azimuthal motions are observed at mid- to high latitudes in the Northern hemisphere, where both eastward and westward motions occur. These azimuthal motions correspond to displacements and distortions of the two main, quasi-stationary, high-latitude magnetic flux patches. Similar motions are not observed in the Southern hemisphere, although this may be a consequence of the poorer data coverage there. The globally averaged drift for the past 1000 yr has been westward since 1400 AD, but eastward between 1000 AD and 1400 AD. In the broad region of the CMB under Europe, the times of transition in the direction of the mean azimuthal motion coincide with the times at which “archaeomagnetic jerks” have been reported. Our results suggest that these are caused by a relatively rapid (<100 yr) change in the direction of the underlying azimuthal flow near the core surface. We find indications that equatorial westward motions of field features at the CMB, similar to those observed during historical times, may have been present for much of the past 3000 yr. When observed, these low-latitude motions are most prominent in the Atlantic hemisphere, which we interpret as a signature of core–mantle thermal coupling.

Paleomagnetic reconstruction of the global geomagnetic field evolution during the Matuyama/Brunhes transition: Iterative Bayesian inversion and independent verification

The Earth's magnetic field changed its polarity from the last reversed into today's normal state approximately 780 000 years ago. While before and after this so called Matuyama/Brunhes reversal, the Earth magnetic field was essentially an axial dipole, the details of its transitional structure are still largely unknown. Here, a Bayesian inversion method is developed to reconstruct the spherical harmonic expansion of this transitional field from paleomagnetic data. This is achieved by minimizing the total variational power at the core–mantle boundary during the transition under paleomagnetic constraints. The validity of the inversion technique is proved in two ways. First by inverting synthetic data sets from a modeled reversal. Here it is possible to reliably reconstruct the Gauss coefficients even from noisy records. Second by iteratively combining four geographically distributed high quality paleomagnetic records of the Matuyama/Brunhes reversal into a single geometric reversal scenario without assuming an a priori common age model. The obtained spatio-temporal reversal scenario successfully predicts most independent Matuyama/Brunhes transitional records. Therefore, the obtained global reconstruction based on paleomagnetic data invites to compare the inferred transitional field structure with results from numerical geodynamo models regarding the morphology of the transitional field. It is found that radial magnetic flux patches form at the equator and move polewards during the transition. Our model indicates an increase of non-dipolar energy prior to the last reversal and a non-dipolar dominance during the transition. Thus, the character and information of surface geomagnetic field records is strongly site dependent. The reconstruction also offers new answers to the question of existence of preferred longitudinal bands during the transition and to the problem of reversal duration. Different types of directional variations of the surface geomagnetic field, continuous or abrupt, are found during the transition. Two preferred longitudinal bands along the Americas and East Asia are not predicted for uniformly distributed sampling locations on the globe. Similar to geodynamo models with CMB heatflux derived from present day lower mantle heterogeneities, a preference of transitional VGPs for the Pacific hemisphere is found. The paleomagnetic duration of reversals shows not only a latitudinal, but also a longitudinal variation. Even the paleomagnetically determined age of the reversal varies significantly between different sites on the globe. The described Bayesian inversion technique can easily be applied to other high quality full vector reversal records. Also its extension to inversion of secular variation and excursion data is straightforward.

High-resolution data of the Iceland Basin geomagnetic excursion from ODP sites 1063 and 983: Existence of intense flux patches during the excursion?

Based on discrete samples, we report new high-resolution records of the ∼ 185 kyr Iceland Basin (IB) geomagnetic excursion from Ocean Drilling Project (ODP) Site 1063 on the Bermuda Rise (sedimentation rate 32 cm kyr − 1 ) and from ODP Site 983 in the far North Atlantic (sedimentation rate 18 cm kyr − 1 ). Two records from Holes 1063A and 1063B are very consistent, and provide the highest resolution of the detailed field behaviour during the IB excursion obtained so far. Inclination records from Holes 983B and 983C in the far North Atlantic are also very consistent, whereas declination anomalies deviate more notably. The pseudo-Thellier (PT) technique was applied along with more conventional palaeointensity proxies (NRM/ARM and NRM/ κ) to recover relative palaeointensity (RPI) estimates from Hole 1063A and Hole 983B. As expected, these proxies indicate that the field intensity generally dropped at both sites during the IB excursion, but also that the history of RPI from the two sites is different. VGPs from Site 1063 indicate that the field at this location experienced some stop-and-go behaviour between patches of intense vertical flux over North America and the tip of South America, areas which coincide fairly well with patches of preferred transitional VGP clustering from reversals and zones of high seismic velocity in the lower mantle. Changes in RPI at this location were generally gradual, possibly due to the proximity of these flux patches, and the first period of VGP-clustering over North America was accompanied by a conspicuous increase in RPI. VGPs from Site 983 track along a different path, and the associated RPI changes are very abrupt and completely synchronous with the onset and termination of the excursion. The differing VGP paths from Sites 1063 and 983 indicate that the global field structure during the IB excursion was not dominated by a single dipole.

Evershed Clouds as Precursors of Moving Magnetic Features around Sunspots

The relation between the Evershed flow and moving magnetic features (MMFs) is studied using high-cadence, simultaneous spectropolarimetric measurements of a sunspot in visible (630.2 nm) and near-infrared (1565 nm) lines. Doppler velocities, magnetograms, and total linear polarization maps are calculated from the observed Stokes profiles. We follow the temporal evolution of two Evershed clouds that move radially outward along the same penumbral filament. Eventually, the clouds cross the visible border of the spot and enter the moat region, where they become MMFs. The flux patch farther from the sunspot has the same polarity of the spot, while the MMF closer to it has opposite polarity and exhibits abnormal circular polarization profiles. Our results provide strong evidence that at least some MMFs are the continuation of the penumbral Evershed flow into the moat. This, in turn, suggests that MMFs are magnetically connected to sunspots.

Azimuthal winds, convection and dynamo action in the polar regions of planetary cores

We investigate azimuthal winds in planetary cores using a thermal convection-driven dynamo. When inertial forces are not negligible in the equation of motion, the inertially driven thermal winds are cyclonic. When the Lorentz forces are strong enough, we find anticyclonic thermal winds as observed in the Earth's polar region from secular variation data. Under strong thermal convection, the azimuthal flow is created by the magnetic mode with one or more coherent, strong upwellings inside the tangent cylinder (TC), offset from the polar axis. We also find that, as the convection in the TC becomes stronger, these vortex plumes shrink in size, consistent with the convection being controlled by the magnetic field. In addition, strong upwellings in the TC could expel magnetic field in its path, creating regions of weak or even reverse flux patches. These patches drift westward, but at a significantly slower angular speed than the rotation about the vortex itself. Calculations with electrically conducting and stress-free boundaries reveal that the mechanism of generation of polar thermal winds is fairly independent of the boundary conditions imposed, provided the Rayleigh number is high enough to excite the magnetic mode.

Fall in Earth's Magnetic Field Is Erratic

Earth's magnetic field has decayed by about 5% per century since measurements began in 1840. Directional measurements predate those of intensity by more than 250 years, and we combined the global model of directions with paleomagnetic intensity measurements to estimate the fall in strength for this earlier period (1590 to 1840 A.D.). We found that magnetic field strength was nearly constant throughout this time, in contrast to the later period. Extrapolating to the core surface showed that the fall in strength originated in patches of reverse magnetic flux in the Southern Hemisphere. These patches were detectable by directional data alone; the pre-1840 model showed little or no evidence of them, supporting the conclusion of a steady dipole up to 1840.

Enhanced programmed cell death of iron‐deficient erythrocytes

Exposure of erythrocytes to osmotic shock, oxidative stress, and energy depletion activates Cl--sensitive and Ca2+-permeable cation channels. Subsequent Ca2+ entry triggers eryptosis, characterized by erythrocyte shrinkage, membrane blebbing, and phosphatidylserine exposure all features typical for apoptotic death of nucleated cells. Erythrocytes exposing phosphatidylserine are recognized, bound, engulfed, and degraded by macrophages. Eryptosis thus fosters clearance of affected erythrocytes from circulating blood. Iron deficiency leads to anemia, in part by decreasing erythrocyte life span. In this study, phosphatidylserine exposure, cell size, and cytosolic Ca2+ were measured by FACS analysis of annexin-V binding, forward scatter, and Fluo-3 fluorescence, respectively. Erythrocytes from mice on control diet were compared with erythrocytes from mice exposed 10 weeks to iron-deficient diet. Iron deficiency significantly (P<0.001) enhanced erythrocyte annexin-V binding (from 2.4 to 3.7%), decreased forward scatter (from 544 to 393), and increased cytosolic Ca2+ concentration. 45Ca2+ flux measurements and patch clamp experiments revealed enhanced Ca2+ uptake (by 2.3-fold) and cation channel activity. The half-life of fluorescence-labeled, iron-deficient, or Ca2+-loaded erythrocytes was significantly reduced compared with control erythrocytes. Thus, the experiments reveal a novel mechanism triggered by iron deficiency, which presumably contributes to accelerated clearance of erythrocytes in iron deficiency anemia.