Numerical Dynamo Research Articles

Constraining a numerical geodynamo model with 100 years of surface observations

SUMMARY To constrain numerical geodynamo models with surface geomagnetic observations, we develop an assimilation scheme for mapping surface geomagnetic data into numerical dynamo solutions. This approach is similar to the optimal interpolation that has been used in atmospheric data assimilation for many years. The scheme is numerically stable and computationally efficient, and has been tested in a series of observing system simulation experiments with either simplified MHD systems or synthetic data from numerical dynamo simulations. In this study, the algorithm is integrated with our MoSST core dynamics model and is tested with 100-yr surface geomagnetic observations, starting in 1900. These experiments use 5 and 20-yr analysis intervals, and forecasts are made every 5 yr starting from 1905. We show that forecast errors gradually reduce over the course of the assimilation. Shorter assimilation runs (starting in 1960 and 1980) result in much larger forecast errors. We investigate the causes of the error reduction by examining how the assimilation impacts the unobserved variables. We discuss the implications for the ability of much longer assimilation runs to constrain the physical fields within the Earth's outer core.

Modelling the palaeo-evolution of the geodynamo

Although it is known that the geodynamo has been operating for at least 3.2 Ga, it remains difficult to infer the intensity, dipolarity and stability (occurrence of reversals) of the Precam-brian magnetic field of the Earth. In order to assist the interpretation of palaeomagnetic data, we produce models for the long-term evolution of the geodynamo by combining core ther-modynamics with a systematic scaling analysis of numerical dynamo simulations. We update earlier dynamo scaling results by exploring a parameter space, which has been extended in order to account for core aspect ratios and buoyancy source distributions relevant to Earth in the Precambrian. Our analysis highlights the central role of the convective power, which is an output of core thermodynamics and the main input of our updated scalings. As the thermal evolution of the Earth's core is not well known, two end-member models of heat flow evolution at the core–mantle boundary (CMB) are used, respectively, terminating at present heat flows of 11 TW (high-power scenario) and 3 TW (low power scenario). The resulting models predict that until the appearance of the inner core, a thermal dynamo driven only by secular cooling, and without any need for radioactive heating, can produce a dipole moment of strength comparable to that of the present field, thus precluding an interpretation of the oldest palaeomagnetic records as evidence of the inner core presence. The observed lack of strong long-term trends in palaeointensity data throughout the Earth's history can be rationalized by the weakness of palaeointensity variations predicted by our models relatively to the data scatter. Specifically, the most significant internal magnetic field increase which we predict is associated to the sudden power increase resulting from inner core nucleation, but the dynamo becomes deeper-seated in the core, thus largely cancelling the increase at the core and Earth surface, and diminishing the prospect of observing this event in palaeointensity data. Our models additionally suggest that the geodynamo has lied close to the transition to polarity reversals throughout its history. In the Precambrian, we predict a dynamo with similar dipolarity and less frequent reversals than at present times, due to conditions of generally lower convective forcing. Quantifying the typical CMB heat flow variation needed for the geodynamo to cross the transition from a reversing to a non-reversing state, we find that it is unlikely that such a variation may have caused superchrons in the last 0.5 Ga without shutting down dynamo action altogether.

Effects of buoyancy and rotation on the polarity reversal frequency of gravitationally driven numerical dynamos

We present the results of 50 simulations of the geodynamo using a gravitational dynamo model driven by compositional convection in an electrically conducting 3-D fluid shell. By varying the relative strengths of buoyancy and rotation these simulations span a range of dynamo behaviour from strongly dipolar, non-reversing to multipolar and frequently reversing. The polarity reversal frequency is increased with increasing Rayleigh number and Ekman number. Model behaviour also varies in terms of dipolarity, variability of the dipole, and core-mantle boundary field spectra. A transition region is found where the models have dipolar fields and moderate (Earth-like) reversal frequencies of approximately 4 per Myr. Dynamo scaling laws are used to compare model dipole intensities to the geodynamo. If the geodynamo lies in a similar transition region then secular evolution of the buoyancy flux in the outer core due to the growth of the inner core and the decrease in the Earth's rotation rate due to tidal dissipation can account for some of the secular changes in the reversal frequency and other geomagnetic field properties over the past 100 Myr.

Geomagnetic dipole moment collapse by convective mixing in the core

Convective mixing in the fluid outer core can induce rapid transient decrease of the geomagnetic dipole. Here we determine rates of dipole moment decrease as a function of magnetic Reynolds number following convective instability in a numerical dynamo and in axisymmetric kinematic flows. Our calculations show that mixing flows induce reversed magnetic flux on the core‐mantle boundary through expulsion of mostly poloidal magnetic field by convective upwellings. The dipole field collapse is accelerated by enhanced radial diffusion and meridional advection of magnetic flux below the core‐mantle boundary. Magnetic energy cascades from the dipole to smaller scales during mixing, producing a filamentary magnetic field structure on the core‐mantle boundary. We find that the maximum rate of dipole moment decrease on century time scales is weakly sensitive to the mixing flow pattern but varies with the velocity of the flow approximately as cRmβ, with Rm the magnetic Reynolds number and (c, β) ≈ (0.2 ± 0.07, 0.78 ± 0.05). According to our calculations, a mixing flow in the outer core with Rm in the range of 200–300 can account for the historically‐measured rate of decrease of the geomagnetic dipole moment, although it is unlikely that a single mixing flow event with this intensity would cause a full dipole collapse or polarity reversal.

An investigation of reversing numerical dynamos driven by either differential or volumetric heating

We present a study of reversing Boussinesq dynamos driven thermally in a rotating spherical shell. For differentially heated dynamos, we investigate the Ekman number dependence of the strong/weak field transition. It is demonstrated that several non-asymptotic features emerge at high Ekman number E > 1 × 1 0 − 3 . At lower E < 1 × 1 0 − 3 , it is shown that reversing dynamos are in weak field states with little influence of the Lorentz force on the flow. A non-magnetic investigation of the onset of convection inside the tangent cylinder (TC) highlights this transition to the lower E regime. Based on these results we choose to study the detailed reversal mechanisms of a dynamo at E = 3.16 × 1 0 − 4 as a compromise between low- E features and numerical expense. For this case comparison with known visual interpretation has been made. We have developed a filtering method where the Lorentz force is being damped consistently in selected regions of the shell. It is demonstrated that upwelling plumes at the inner core boundary (ICB) inside the TC destabilise the magnetic dipole to some extent. However, the induction processes in the main part of the TC interior, in particular at the core-mantle boundary (CMB), stabilise the dipole. The net effect of the TC interior is dipole stabilisation. The polar TC-surface and the equatorial CMB region also contribute to dipole stabilisation. Equatorial upwelling plumes at the ICB outside the TC are necessary to the strong/weak field transition at this Ekman number and act as pronounced reversal builders/triggers. The filtering method highlights the ICB plumes as promoters of dipole instability. For comparison we have therefore also investigated volumetrically heated dynamos where buoyancy is stronger at the CMB. In contrast to the above solutions, these dynamos may be in strong field reversing states. The latter require sufficiently high magnetic Prandtl number Pm. For lower Pm we find solutions with a weak, erratically reversing dipole. The dependence on the inner core size has been determined showing that the erratic dipole dynamos extend to higher Pm for smaller inner cores. A non-magnetic investigation of the onset of convection suggests that these results should be close to the case without an inner core. The filtering method shows that the dipole position of the erratic dipole dynamos is destabilised by flow fluctuations anywhere in the shell. Like for differentially heated dynamos, the reversing strong field dynamos are stabilised by the flow inside the TC where the CMB region reinforces the stabilisation. In sharp contrast, however, the ICB region plays no role to reversals. In fact, the equatorial CMB region contains the processes that build/trigger reversals.

Polarity reversals in geodynamo models with core evolution

Numerical dynamos with time-variable control parameters that simulate the secular evolution of the core are used to interpret long-term trends in geomagnetic field behavior, including polarity reversal frequency. Dynamos with incremental changes in convective forcing and rotation rate show continuous trends in dipole field intensity and fluid velocity, with systematic variations in polarity chrons in some cases. Otherwise similar dynamos with constant forcing and rotation rate have statistically stationary dipole field intensity and fluid velocity, with random polarity chrons. A reversing dynamo with steady rotational deceleration constrained by tidal friction and a decreasing (regular) inner core growth rate constrained by core thermal history evolves over 10 Myr with minor trends in average dipole intensity, fluid velocity, and polarity chron length. In contrast, a dynamo started in a non-reversing state and subject to an increasing (anomalous) inner core growth rate and constant rotation evolves over 20 Myr to a reversing state with substantial trends in dipole intensity and polarity chron length. The dispersion of polarity chron lengths in the dynamo model with anomalous evolution is qualitatively similar to the observed dispersion of geomagnetic polarity chrons since the end of the Cretaceous Normal Superchron, and the model dipole field frequency spectra are qualitatively similar to estimates of the geomagnetic spectrum at very low frequencies.

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.

Dipole collapse and reversal precursors in a numerical dynamo

Precursors to extreme geomagnetic field changes are examined in a numerical dynamo with a reversing dipolar magnetic field. A dynamo model with compositional convection in a rotating spherical shell produces a strongly dipolar external magnetic field over 6Myr of simulated paleomagnetic time, with stable polarity epochs and occasional dipole collapses, some of which result in polarity reversals or dipole axis excursions. We analyze the model behavior during two dipole collapses, one that leads to a polarity reversal and one that does not, focusing on observable precursors. Reversed magnetic field induced in the dynamo interior by intermittent convective variability is the primary cause of dipole collapse. Spots of reversed magnetic flux emerge on the outer boundary at an early stage, then re-emerge with greater intensity at the height of the collapse. The energy in the external field cascades to higher harmonics as these reversed patches appear. Butterfly diagrams showing the reversed and normal flux contributions to the axial dipole reveal poleward migration of the patches during dipole collapse. Axial dipole reduction by precursory reversed flux is several times larger in the reversing case, compared to the non-reversing case. A butterfly diagram of the geomagnetic field since 1840 shows high latitude reversed flux emerging on the core–mantle boundary. Although the reversed geomagnetic flux is presently too weak to be labeled a reversal precursor, it is consistent with early stage dipole collapse in the dynamo model.

A simple model for mantle-driven flow at the top of Earth’s core

Abstract We derive a model for the steady fluid flow at the top of Earth’s core driven by thermal coupling with the heterogeneous lower mantle. The model uses a thermal wind balance for the core flow, and assumes a proportionality between the horizontal density gradients at the top of the core and horizontal gradients in seismic shear velocity in the lowermost mantle. It also assumes a proportionality between the core fluid velocity and its radial shear. This last assumption is validated by comparison with numerical models of mantle-driven core flow, including self-sustaining dynamo (supercritical) models and non-magnetic convection (subcritical) models. The numerical dynamo models show that thermal winds with correlated velocity and radial shear dominate the boundary-driven large-scale flow at the top of the core. We then compare the thermal wind flow predicted by mantle heterogeneity with the 150 year time-average flow obtained from inverting the historical geomagnetic secular variation, focusing on the non-zonal components of the flows because of their sensitivity to the boundary heterogeneity. Comparing magnitudes provides an estimate of the ratio of lower mantle seismic anomalies to core density anomalies. Comparing patterns shows that the thermal wind model and the time-average geomagnetic flow have comparable length scales and exhibit some important similarities, including an anticlockwise vortex below the southern Indian and Atlantic Oceans, and another anticlockwise vortex below Asia, suggesting these parts of the non-zonal core flow could be thermally controlled by the mantle. In other regions, however, the two flows do not match well, and some possible reasons for the dissimilarity between the predicted and observed core flow are identified. We propose that better agreement could be obtained using core flows derived from geomagnetic secular variation over longer time periods.

Effects of thermally heterogeneous structure in the lowermost mantle on the geomagnetic field strength

We have conducted a study of numerical dynamos in a rapidly rotating spherical shell with prescribed non-uniform heat-flux patterns at the outer boundary to examine effects of thermal structure at the core–mantle boundary (CMB) on the geodynamo, especially on the magnetic field strength. Large heat-flux heterogeneity with equatorial symmetry enhances strength of the dipolar magnetic field for a sufficiently small Ekman number, whereas that with equatorial anti-symmetry does not change the strength but affects the tilt of the magnetic dipole axis. The thermal wind induced by heat-flux heterogeneity at the CMB has strong influence on the flow structure and on the magnetic field intensity. These results suggest that thermally heterogeneous structure of the lowermost mantle might give rise to an anomalously strong geomagnetic field such as that during the Cretaceous Normal Superchron.

Lateral variations in CMB heat flux and deep mantle seismic velocity caused by a thermal–chemical-phase boundary layer in 3D spherical convection

Numerical simulations of thermo-chemical, multi-phase, compressible mantle convection in a three-dimensional spherical shell are used to investigate the relationship between lateral variations in seismic shear-wave velocity V s above the core–mantle boundary (CMB) and lateral variations in heat flux across the CMB ( q CMB), when compositional variations and the post-perovskite phase transition are included. For simple thermal convection, the V s– q CMB relationship is reasonably but not perfectly linear. The post-perovskite transition introduces a non-linearity that amplifies fast V s anomalies in cold regions, but there is still a unique mapping between δ V s and q CMB. Lateral variations in composition such as piles of dense material introduce another non-linearity that affects hot upwelling regions, and introduces a non-uniqueness in δ V s– q CMB if the dense material (e.g., MORB) is seismically fast compared to the surrounding material. In this case, dense piles are ringed by sharp, low- V s anomalies. If the CMB is covered by a global dense layer than variations in δ V s and q CMB are reduced but so is the mean value of q CMB. In all cases, the peak-to-peak lateral variation in q CMB is similar to or larger than twice the mean value, which might create problems for generating a dynamo according to existing numerical dynamo simulations. Analytical scalings are developed to explain the observed trends.

The magnetic structure of convection-driven numerical dynamos

The generation of a magnetic field in numerical simulations of the geodynamo is an intrinsically 3-D and time-dependent phenomenon. The concept of magnetic field lines and the frozen-flux approximation can provide insight into such systems, but a suitable visualization method is required. This paper presents results obtained using the Dynamical Magnetic Field line Imaging (DMFI) technique, which is a representation of magnetic field lines accounting for their local magnetic energy, together with an algorithm for the time evolution of their anchor points. The DMFI illustrations are consistent with previously published dynamo mechanisms, and allow further investigation of spatially and temporally complex systems. We highlight three types of magnetic structures: (i) magnetic cyclones and (ii) magnetic anticyclones are expelled by, but corotate with axial flow vortices; (iii) magnetic upwellings are amplified by stretching and advection within flow upwellings, and show structural similarity with helical plumes found in rotating hydrodynamic experiments. While magnetic anticyclones are responsible for the regeneration of a stable axial dipole, herewe showthat excursions and reversals of the dipole axis are caused by the emergence of magnetic upwellings, which amplify and transport a generally multipolar magnetic field from the inner to the outer boundary of the models. Geomagnetic observations suggest the presence of magnetic structures similar to those found in our models; thus,we discuss howour results may pertain to Earth's core dynamo processes. In order to make DMFI a standard tool for numerical dynamo studies, a public software package is available upon request to the authors (supplementary material is available at: http://www.ipgp.jussieu. fr/∼aubert/DMFI.html).

Gravitational dynamos and the low-frequency geomagnetic secular variation.

Self-sustaining numerical dynamos are used to infer the sources of low-frequency secular variation of the geomagnetic field. Gravitational dynamo models powered by compositional convection in an electrically conducting, rotating fluid shell exhibit several regimes of magnetic field behavior with an increasing Rayleigh number of the convection, including nearly steady dipoles, chaotic nonreversing dipoles, and chaotic reversing dipoles. The time average dipole strength and dipolarity of the magnetic field decrease, whereas the dipole variability, average dipole tilt angle, and frequency of polarity reversals increase with Rayleigh number. Chaotic gravitational dynamos have large-amplitude dipole secular variation with maximum power at frequencies corresponding to a few cycles per million years on Earth. Their external magnetic field structure, dipole statistics, low-frequency power spectra, and polarity reversal frequency are comparable to the geomagnetic field. The magnetic variability is driven by the Lorentz force and is characterized by an inverse correlation between dynamo magnetic and kinetic energy fluctuations. A constant energy dissipation theory accounts for this inverse energy correlation, which is shown to produce conditions favorable for dipole drift, polarity reversals, and excursions.

Numerical models of zonal flow dynamos: an application to the ice giants

The weakly dipolar and strongly tilted magnetic fields of Uranus and Neptune are apparently generated by a dynamo process distinct from that which produces axial dipoles. We study a suite of numerical dynamos driven by convection in a rapidly rotating spherical shell and focus on cases with relatively high magnetic diffusivity. Models are presented with magnetic Prandtl number Pm = 0.1–5.0 and Rayleigh numbers between 10 and 80 times the critical Rayleigh number for convection. In the cases with high magnetic diffusivity, the fluid flow has a dominant effect over the magnetic fields, which are characteristically quadrupolar and octupolar and strongly variable in time. The dipolar component is typically weak, and strongly tilted from the axis of rotation. Most of the cases we present result in low Alfvén and Elsasser numbers, in agreement with previous studies of nondipolar dynamos. Our results suggest that the peculiar magnetic fields of Uranus and Neptune result from dynamo action driven by convectively generated, strong zonal flow in the electrolytic fluid envelope.

Tests of core flow imaging methods with numerical dynamos

SUMMARY We test the quality of a new core flow imaging method that incorporates constraints on flow helicity, using synthetic magnetic secular variation data from 3-D self-consistent numerical dynamo models. Comparison with the dynamo model flows reveals that our imaging method delineates most of the main large-scale flow features, both in pattern and magnitude. The dynamo model flows are characterized by high-latitude vortices, some equatorial symmetry, columnar convection and a significant amount of flow along radial magnetic field contours. Our inversion method correctly images these aspects of the flows. The correlation coefficient between the dynamo velocity and the imaged velocity exceeds 0.5 in cases with large-scale flow and magnetic field pattern, but degrades substantially in more complex cases when the scale of the secular variation is small. The magnitude of the imaged velocity depends on the a priori-assumed ratio of tangential divergence to radial vorticity k, in some resemblance to the damping parameter in spectral methods, although with our method the misfit is insensitive to k-values. Including tangential magnetic diffusion in core flow inversion improves the quality of the imaged velocity pattern. The largest artefacts in the imaged velocities are due to unmodelled radial magnetic diffusion and truncation of the input magnetic field.

Detecting thermal boundary control in surface flows from numerical dynamos

The geomagnetic field and secular variation exhibit asymmetrical spatial features which are possibly originating from an heterogeneous thermal control of the Earth's lower mantle on the core. The identification of this control in magnetic data is subject to several difficulties, some of which can be alleviated by the use of core surface flow models. Using numerical dynamos driven by heterogeneous boundary heat flux, we confirm that within the parameter space accessible to simulations, time average surface flows obey a simple thermal wind equilibrium between the Coriolis and buoyancy forces, the Lorentz, inertial and viscous forces playing only a secondary role, even for Elsasser numbers significantly larger than 1. Furthermore, we average the models over the duration of three vortex turnovers, and correlate them with a longer time average which fully reveals the signature of boundary heterogeneity. This allows us to quantify the possibility of observing mantle control in core surface flows averaged over a short time period. A scaling analysis is performed in order to apply the results to the Earth's core. We find that three vortex turnovers could represent between 100 and 360 years of Earth time, and that the heat flux heterogeneity at the core-mantle boundary could be large enough to yield an observable signature of thermal mantle control in a time average core surface flow within reach of the available geomagnetic data.

On the use of calibrated relative paleointensity records to improve millennial‐scale geomagnetic field models

Current millennial‐scale time‐varying global geomagnetic field models suffer from a lack of intensity data compared to directional data, because only thermoremanently magnetized material can provide absolute information about the past field strength. The number of archeomagnetic artifacts that can provide such data diminishes rapidly prior to 3000 B.C. Sediment cores provide time series of declination and inclination and of variations of magnetization: the latter can reflect relative geomagnetic field variations if suitably normalized. We propose a calibration technique based on predictions from global models and use the CALS7K.2 model to calibrate relative paleointensity records from 22 globally distributed locations and assess whether they reflect actual field variations. All except a few contain useful information for 0 to 7 ka and could be used to improve the existing models. Using synthetic data from a numerical dynamo simulation, we show that with the existing directional data the distribution of intensity data has an important influence on model quality. Intensity data from a broad range of latitudes seem particularly important. This study opens the possibility of extending global time‐varying geomagnetic field models further back in time than the current 7 kyr interval.

Marginal stability of almost adiabatic planetary convection

Almost adiabatic states are typical for the deep convective interiors of all known planets and their moons, e.g., the deviations from the adiabatic state in the Earth's outer core and in the MHD dynamo region of Jupiter are about or less than 10−5%. We approximated the equations governing convection in the deep interiors of planets and their moons to obtain a system, which is more accurate than the traditional Boussinesq equations. Fortunately, our system, which adopts almost uniform entropy instead of the temperature as the basic thermodynamic states, can still be investigated by standard methods. We considered the marginal stability of well-mixed almost adiabatic states in rapidly rotating thick spherical shells, whose inner to outer radius ratio does not exceed that of the modern Earth. The critical Rayleigh-type numbers, frequencies and solution structures of the marginal states were determined by both analytical and numerical methods. Our new estimates differ from those obtained previously using the Boussinesq equations, suggesting that the earlier Boussinesq results for convection in the deep planetary interiors should be re-assessed. The small molecular Prandtl number limit was adopted to model the marginal stability of thermal planetary convection. It was found that the critical Rayleigh number for convection sharply diminished as the radius of the inner rigid core is increased. We modelled the instability of the combined compositional–thermal turbulent geo-convection for Prandtl number unity. When thermal convection is in opposition to compositional convection, extremely large critical Rayleigh numbers are possible. This might happen for a terrestrial planet during its later stage of evolution. Pure compositional convection has been investigated in the large compositional Prandtl number limit, for which the critical Rayleigh number is rather large and the variations of all critical parameters are small. The large size of the critical Rayleigh number ensures that the actual values used in numerical dynamo experiments are only moderately supercritical.

Quasi-geostrophic kinematic dynamos at low magnetic Prandtl number

Rapidly rotating spherical kinematic dynamos at very low Ekman and Prandtl numbers are computed using the combination of a quasi-geostrophic (QG) model for the velocity field and a classical spectral 3D code for the magnetic field. The QG flow is computed in the equatorial plane of the sphere; it corresponds to Rossby wave instabilities of a geostrophic internal shear layer produced by differential rotation. The induction equation is computed in the whole sphere after the QG flow has been expanded along the rotation axis. Differential rotation and Rossby wave propagation are the key ingredients of this dynamo which can be interpreted in terms of Parker-Ω dynamo. Taking into account the quasi-geostrophy of the velocity field enables us to increase time and space resolution to compute the dynamics. For the first time, we report on numerical dynamos with very low Ekman numbers (10−8). Because the magnetic and velocity fields are computed on different grids, we compute dynamos for very low magnetic Prandtl numbers exhibiting a scale separation between magnetic and velocity field. These dynamos are asymptotically close to rapidly rotating, metallic planetary cores.

Palaeomagnetic interpretation of dynamo simulations

SUMMARY The properties of geomagnetic field excursions and reversals are commonly inferred from one palaeomagnetic site only. However, dynamo simulations suggest that the relative dipole contribution is lower during transitional times than in stable periods. Local field properties thus become more important and cause increased local differences in virtual dipole signatures. These differences in turn account for a larger site dependence of inferred excursion and reversal properties. A self-consistent numerical dynamo simulation is used to test and quantify this effect. Estimates of reversal durations are found to vary by an order of magnitude depending on the site location. The same is true for excursions. Moreover, not all excursions are global phenomena. A larger number of smaller excursions are present at higher latitudes. This can be attributed to a latitudinal increase in secular variation that is also found in palaeomagnetic data. Larger excursions are more likely to be observed globally, but the particular signature may vary significantly from site to site. It is speculated that these excursions have a different dynamical origin from secular variation.