Richard Holme

Ørsted Initial Field Model

Magnetic measurements taken by the Orsted satellite during geomagnetic quiet conditions around Jan-uary 1, 2000 have been used to derive a spherical harmonic model of the Earths magnetic field for epoch 2000.0. The maximum degree and order of the model is 19 for internal, and 2 for external, source fields; however, coefficients above degree 14 may not be robust. Such a detailed model exists for only one previous epoch, 1980. Achieved rms misfit is < 2 nT for the scalar intensity and < 3 nT for one of the vector components perpendicular to the magnetic field. For scientific purposes related to the Orsted mission, this model supercedes IGRF 2000.

Large-Scale Flow in the Core

We discuss the observational constraint of large-scale flow in the core. The time variation of the internal magnetic field has been attributed to internal motions in the Earths fluid core, associated with the geodynamo. From initially simplistic models of magnetic westward drift, more detailed time-dependent models of the field have motivated the calculation of maps of core-surface flow. Advection is assumed to dominate over diffusion (the ‘Frozen-flux’ approximation), suggesting that the magnetic field provides a passive tracer for core flow. Only one equation is available to constrain two components of flow along the core surface; this leads to an unavoidable nonuniqueness in flow determination. Dynamic assumptions have been adopted to provide additional constraints, but the flows vary in morphology depending on which assumption is used. New assumptions are under development, including improved treatments of magnetic diffusion, which may help us to better understand the nature of the flow, or at least the shortcomings in the models we produce. Modern geomagnetic satellite data allow the creation of models of the geomagnetic secular variation with enhanced detail, but as yet, such models have opened up greater modeling problems rather than increased insight as to the flow. A primary difficulty is the shielding of the core field at spherical harmonic degree above 13 by the magnetic field from the Earths lithosphere.

Terrestrial impact of abrupt changes in the North Atlantic thermohaline circulation: Early Holocene, UK

Abrupt cooling events are features of Holocene climate and may recur in the future. We use lake records from Hawes Water, NW England, to quantify the impact of two prominent early Holocene climatic events. Subdecadal oxygen isotope records from sedimentary carbonate (18δOc), dated using thermal ionization mass spectrometry (TIMS) U-series analyses, provide evidence for abrupt cold events, lasting ∼50 and ∼150 yr at 9350 and 8380 yr ago, which correlate with the 9.3 ka and 8.2 ka events recognized in Greenland ice cores. At Hawes Water, mean July air temperatures, inferred from chirono-mid assemblages, decreased by ∼1.6 °C during each event. Calculations show that the isotopic excursions were dominantly caused by decreases in the isotopic composition of meteoric precipitation (18δOp) by ∼1.3‰; this is interpreted as a direct downstream response to cooling and freshening of northeast Atlantic surface water by melting ice sheets. Intermediate in magnitude between events observed in Greenland and central Europe, the effects are consistent with a partial shutdown of the North Atlantic thermohaline circulation.

Characterization and implications of intradecadal variations in length of day

Variations in Earths rotation (defined in terms of length of day) arise from external tidal torques, or from an exchange of angular momentum between the solid Earth and its fluid components. On short timescales (annual or shorter) the non-tidal component is dominated by the atmosphere, with small contributions from the ocean and hydrological system. On decadal timescales, the dominant contribution is from angular momentum exchange between the solid mantle and fluid outer core. Intradecadal periods have been less clear and have been characterized by signals with a wide range of periods and varying amplitudes, including a peak at about 6 years (refs 2, 3, 4). Here, by working in the time domain rather than the frequency domain, we show a clear partition of the non-atmospheric component into only three components: a decadally varying trend, a 5.9-year period oscillation, and jumps at times contemporaneous with geomagnetic jerks. The nature of the jumps in length of day leads to a fundamental change in what class of phenomena may give rise to the jerks, and provides a strong constraint on electrical conductivity of the lower mantle, which can in turn constrain its structure and composition.

Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation

The Earth’s inner core grows by the freezing of liquid iron at its surface. The point in history at which this process initiated marks a step-change in the thermal evolution of the planet. Recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the Earth’s core, resulting in estimates of the timing of inner-core nucleation ranging from less than half a billion to nearly two billion years ago. Recent inner-core nucleation (high thermal conductivity) requires high outer-core temperatures in the early Earth that complicate models of thermal evolution. The nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner-core nucleation to be estimated directly. Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations. Here we examine results from an expanded Precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria. Our analysis provides intensity-based support for the dominant dipolarity of the time-averaged Precambrian field, a crucial requirement for palaeomagnetic reconstructions of continents. We also present firm evidence for the existence of very long-term variations in geomagnetic strength. The most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1.5 billion years ago. This observation is most readily explained by the nucleation of the inner core occurring during this interval; the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the Earth.

CO2 — A Champ Magnetic Field Model

We describe here a model of the magnetic field created specifically to be used with data from the Champ satellite, valid for the initial period of the Champ mission (July 2000 — December 2001). The model contains initial field, linear secular variation and external field contributions. Although Champ data provide the most important input to the modelling, we also use data from other satellites (Orsted and Orsted-2/SAC-C) and ground-based observatories, to increase the robustness of the model. The model will be improved upon in the future (in particular as we better understand the local time dependencies of the external and induced magnetic fields) but for now is the recommended standard model for use with applications of Champ data.

Lattice-gas and lattice-Boltzmann models of miscible fluids

We introduce new lattice-gas and lattice-Boltzmann models for simulating miscible fluids in two dimensions. The inclusion of a nonlocal interaction produces a lattice gas with lower diffusivity than achieved before. To overcome some observed unphysical properties of this lattice gas, we introduce a lattice-Boltzmann analogue of the model. We first formulate a miscible two-component lattice-Boltzmann model with local interactions only, and show that its diffusivity is determined by an eigenvalue of the linearized collision operator. Diffusivity is then reduced by including nonlocal interactions. The utility of the model is demonstrated by a simulation of two-dimensional viscous fingering.

Ninth generation international geomagnetic reference field released

The coefficients for the new 9th Generation International Geomagnetic Reference Field (IGRF) were finalized at the XXIII General Assembly of the International Union of Geophysics and Geodesy (IUGG), held in Sapporo, Japan, in July 2003. The IGRF is widely used as a mathematical representation for the Earths magnetic field in studies of the Earths deep interior, crust, and ionosphere and magnetosphere. It is the product of a collaborative effort between magnetic field modelers and the institutes involved in collecting and disseminating magnetic field data from observatories and surveys around the world and from satellites.

Modeling the Jovian magnetic field and its secular variation using all available magnetic field observations

We present new models of Jupiters internal magnetic field and secular variation from all available direct measurements from three decades of spacecraft observation. A regularized minimum norm approach allows the creation of smooth, numerically stable models displaying a high degree of structure. External field from the magnetodisk is modeled iteratively for each orbit. Jupiters inner magnetosphere is highly stable with time, with no evidence for variation with solar activity. We compare two spherical harmonic models, one assuming a field constant in time and a second allowing for linear time variation. Including secular variation improves data fit with fewer additional parameters than increasing field complexity. Our favored solution indicates a ∼0.012% yr−1 increase in Jupiters dipole magnetic moment from 1973 to 2003; this value is roughly one quarter of that for Earth. Inaccuracies in determination of the planetary reference frame cannot explain all the observed secular variation. Should more structure be allowed in the solutions, we find the northern hemispherical configuration resembles recent models based on satellite auroral footprint locations, and there is also evidence of a possible patch of reversed polar flux seen at the expected depth of the dynamo region, resembling that found at Earth and with implications for the Jovian interior. Finally, using our preferred model, we infer flow dynamics at the top of Jupiters dynamo source. Though highly speculative, the results produce several gyres with some symmetry about the equator, similar to those seen at Earths core-mantle boundary, suggesting motion on cylinders aligned with the rotation axis.

A comparison of Cluster magnetic data with the Tsyganenko 2001 model

As part of an investigation of the magnetic effects of external currents in the magnetosphere, we have compared two years of perigee Cluster data to the Tsyganenko 2001 (T01) field model. Cluster data are not included in the T01 database and therefore can be used to independently verify the model. The model performs very well in a global sense; nevertheless, absolute residuals between the data and the model can reach ~20 nT near perigee. These deviations take two forms: a sharp, bipolar signature and well-defined trends over a larger spatial region. The bipolar signatures in the residuals are moderately stable, repeating on the phase period of the Cluster orbit. The bipolar nature of the signatures reflects variations in the Cluster data, therefore indicating that the spacecraft may be observing a field-aligned current. Although the size of the magnetic field perturbation in this region is not well determined by T01, the location of the observed field-aligned current system is accurately predicted. The bipolar signatures are observed in close proximity to the edge of the ring current, estimated from Cluster energetic electron spectrograms, indicating that they are associated with region 2 field-aligned currents. Longer-duration trends in the residuals indicate a slight difference between the model predictions and the Cluster data for various locations and seasons. For example, throughout most of 2003 and the first half of 2004, there is a residual in the total magnetic field for an hour centered on perigee, of ~20 nT.