Estimating error associated with a magnetic field model

A common requirement for users of magnetic field models is to have information about the accuracy of the model. Any signal not captured by the model is part of the error, for example the unmodelled crustal and external fields. Some models are used long after they have been produced and errors associated with prediction of the core field inevitably arise. Estimates of ground-based errors were derived as part of the recent World Magnetic Model production. These estimates were derived from vector data from observatories and repeat stations around the world combined with scalar data from marine and airborne surveys. The errors arising from the crustal and external fields have distinct spatial patterns, with local maxima in the auroral and polar regions for the external field. Declination, the element of the magnetic field of greatest interest to many users, is not linear in spherical harmonic model coefficients but can be propagated from the orthogonal components which are linear. This results in further spatial variations (inclination and horizontal and total intensities are also affected). Some of these propagation-related spatial variations are difficult to validate in ground-based measurements because of the poor spatial coverage. We investigate whether satellite data such as those from the Swarm mission can provide such validation. To derive all-inclusive error estimates for a particular magnetic field model, the errors from the crustal and external fields and core field prediction can be combined with the propagated error estimates

Click Here Observation of magnetic diffusion in the Earth's outer core from Magsat, Ørsted, and CHAMP data

International audienceThe frozen flux assumption consists in neglecting magnetic diffusion in the core. It has been widely used to compute core flows from geomagnetic observations. Here we investigate the validity of this assumption over the time interval 1980–2005, using high‐ precision magnetic data from the Magsat, Ørsted, and CHAMP satellites. A detectable change of magnetic fluxes through patches delimited by curves of zero radial magnetic field at the core‐mantle boundary is associated with a failure of the frozen flux assumption. For each epoch (1980 and 2005), we calculate spatially regularized models of the core field which we use to investigate the change of reversed magnetic flux at the core surface. The largest and most robust change of reversed flux is observed for two patches: one located under St. Helena Island (near 20°S, 15°E); the other, much larger, is located under the South Atlantic Ocean. We next calculate frozen‐flux‐constrained field models (i.e., pairs of models for epoch 1980 and 2005 having the same flux through patches delimited by curves of zero radial magnetic field), using a penalty method. We find that the frozen flux constraint does not lead to any significant increase of the global misfit. However, applying the constraint leads to a detectable increase of the scalar residuals at satellite altitude in the region of St. Helena, strongly suggesting a local failure of the frozen flux assumption. The observed flux expulsion within the St. Helena patch could result from the formation of a pair of "core spots," as predicted by numerical simulations of the geodynamo

Short Timescale Core Dynamics: TheoryandObservations

Fluid motions in the Earth's core produce changes in the geomagnetic field (secular variation) and are also an important ingredient in the planet's rotational dynamics. In this article we review current understanding of core dynamics focusing on short timescales of years to centuries. We describe both theoretical models and what may be inferred from geomagnetic and geodetic observations. The kinematic concepts of frozen flux and magnetic diffusion are discussed along with relevant dynamical regimes of magnetostrophic balance, tangential geostrophy, and quasi-geostrophy. An introduction is given to free modes and waves that are expected to be present in Earth's core including axisymmetric torsional oscillations and non-axisymmetric Magnetic-Coriolis waves. We focus on important recent developments and promising directions for future investigation

On the semiannual and annual variations of geomagnetic activity and components

International audienceThe semiannual and annual lines in a long series of magnetic observatories daily values, as well as in the aa-activity index series, are investigated. For both periods, amplitudes and phases of the lines corresponding to the different series present grossly common variations on decadal time scales; relative phases and amplitude ratios between the observatories change with the same time constants. The results are briefly discussed with regards to commonly received theories of the semiannual variation of magnetic activity, and some possible mechanisms for the observed geographical variability are suggested

Jerks abound: An analysis of geomagnetic observatory data from 1957 to 2008

We present a two-step method for the removal of external field signals and the identification of geomagnetic jerks in magnetic observatory monthly mean data, providing quantitative uncertainty estimates on jerk occurrence times and amplitudes with minimal a priori information. We apply the method to the complete time series of X-, Y- and Z-components at up to 103 observatory locations in the period of 1957–2008. We find features fitting the definition of jerks in individual components to be frequent and not globally contemporaneous. Identified regional jerks have no consistent occurrence pattern and the most widespread in any given year is identified at <30% of observatories worldwide. Whilst we identify jerks throughout the period of study, relative peaks in the global number of jerk occurrences are found in 1968–71, 1973–74, 1977–79, 1983–85, 1989–93, 1995–98 and 2002–03 with the suggestion of further poorly sampled events in the early 1960s and late 2000s. The mean uncertainties on individual jerk occurrence times and amplitudes are found to be ±0.3 yrs and ±2.1 nT/yr2, respectively, for all field components. Jerk amplitudes suggest possible periodic trends across Europe and North America, which may be related to the 6-yr periods detected independently in the secular variation and length-of-day

Assessing the importance and expression of the 6-year geomagnetic oscillation

The first time derivative of residual length-of-day observations is known to contain a distinctive 6 year periodic oscillation. Here we theorize that through the flow accelerations at the top of the core the same periodicity should arise in the geomagnetic secular acceleration. We use the secular acceleration of the CHAOS-3 and CM4 geomagnetic field models to recover frequency spectra through both a traditional Fourier analysis and an empirical mode decomposition. We identify the 6 year periodic signal in the geomagnetic secular acceleration and characterize its spatial behavior. This signal seems to be closely related to recent geomagnetic jerks. We also identify a 2.5 year periodic signal in CHAOS-3 with unknown origin. This signal is strictly axially dipolar and is absent from other magnetic or geodetic time series

The US/UK World Magnetic Model for 2020-2025

This report contains a complete description of the World Magnetic Model (WMM) 2020. Section 1 contains information that users of WMM2020 require in order to implement the model and software in navigation and heading systems, and to understand magnetic charts, poles and geomagnetic coordinate systems. Section 2 contains a detailed summary of the data used and the modeling techniques employed. Section 3 contains an assessment of the model uncertainties and a description of the error model provided with the WMM2020. Section 4 contains charts of all the magnetic elements at 2020.0 and their expected annual rates of change between 2020.0 and 2025.0. These predicted changes are based upon the best knowledge of the geomagnetic main field evolution at the time the WMM was released. Sponsored by the U.S. National Geospatial-Intelligence Agency (NGA) and the U.K. Defence Geographic Centre (DGC), the World Magnetic Model (WMM) is produced by the U.S. National Oceanic and Atmospheric Administration’s National Centers for Environmental Information (NOAA/NCEI) and the British Geological Survey (BGS). It is the standard model used by the U.S. Department of Defense (DoD), the U.K. Ministry of Defence, the North Atlantic Treaty Organization (NATO) and the International Hydrographic Organization (IHO), for navigation, attitude and heading referencing systems using the geomagnetic field. It is also used widely in civilian navigation and heading systems

The US/UK World Magnetic Model for 2015-2020

This report contains a complete description of the World Magnetic Model (WMM) 2015. Section 1 contains information that users of WMM2015 require in order to implement the model and software in navigation and heading systems, and to understand magnetic charts, poles and geomagnetic coordinate systems. Section 2 contains a detailed summary of the data used and the modeling techniques employed. Section 3 contains an assessment of the model uncertainties and a description of the error model provided with the WMM2015. Section 4 contains charts of all the magnetic elements at 2015.0 and their expected annual rates of change between 2015.0 and 2020.0. These predicted changes are based upon the best knowledge of the geomagnetic main field evolution at the time the WMM was released. Sponsored by the U.S. National Geospatial-Intelligence Agency (NGA) and the U.K. Defence Geographic Centre (DGC), the World Magnetic Model (WMM) is produced by the U.S. National Oceanic and Atmospheric Administration’s National Geophysical Data Center (NOAA/NGDC) and the British Geological Survey (BGS). It is the standard model used by the U.S. Department of Defense (DoD), the U.K. Ministry of Defence, the North Atlantic Treaty Organization (NATO) and the International Hydrographic Organization (IHO), for navigation, attitude and heading referencing systems using the geomagnetic field. It is also used widely in civilian navigation and heading systems