Stefan Maus

International Geomagnetic Reference Field: the 12th generation

The 12th generation of the International Geomagnetic Reference Field (IGRF) was adopted in December 2014 by the Working Group V-MOD appointed by the International Association of Geomagnetism and Aeronomy (IAGA). It updates the previous IGRF generation with a definitive main field model for epoch 2010.0, a main field model for epoch 2015.0, and a linear annual predictive secular variation model for 2015.0-2020.0. Here, we present the equations defining the IGRF model, provide the spherical harmonic coefficients, and provide maps of the magnetic declination, inclination, and total intensity for epoch 2015.0 and their predicted rates of change for 2015.0-2020.0. We also update the magnetic pole positions and discuss briefly the latest changes and possible future trends of the Earth’s magnetic field.

International Geomagnetic Reference Field—the tenth generation

The International Geomagnetic Reference Field (IGRF) 10th Generation was adopted in 2004 by the International Association of Geomagnetism and Aeronomy (IAGA) Working Group V-MOD. It is the latest version of a standard mathematical description of the Earth’s main magnetic field and is used widely in studies of the Earth’s deep interior, its crust and its ionosphere and magnetosphere. This generation differs from the previous generation with the replacement of the secular-variation model for 2000.0–2005.0 with a main-field model at 2005.0 and a secular-variation model for 2005.0–2010.0. The IGRF is the product of a huge collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and from observatories and surveys around the world. This paper lists the new coefficients and includes contour maps and pole positions.

The influence of nonmigrating tides on the longitudinal variation of the equatorial electrojet

[1] The climatological model of the equatorial electrojet, EEJM-1, derived from Orsted, CHAMP and SAC-C satellite measurements provides the opportunity to investigate the longitudinal variation of the current strength in detail. Special emphasis is put in this study on the effect of nonmigrating tides. We have found that the influence of the diurnal eastward-propagating mode with wavenumber-3, DE3, is particularly strong. In polar orbiting satellite observations the DE3 tidal signal appears as a four-peaked longitudinal structure. We have put special emphasis in our analysis to isolate the DE3 contribution from other sources contributing to the wavenumber-4 structure in satellite data. The amplitude of the DE3 signature in the EEJ not only peaks during equinox seasons, but is also strong around the June solstice. When looking at the modulation of the EEJ intensity the DE3 accounts for about 25% during the months of April through September. It is thus the dominant cause for longitudinal variations. During December solstice months the influence of DE3 is negligible. A secondary three-peaked longitudinal pattern emerges during solstice seasons when the DE3 influence is removed. From the data available it is, however, not clear whether this pattern is related to any tidal drivers.

Third generation of the Potsdam Magnetic Model of the Earth (POMME)

The Potsdam Magnetic Model of the Earth (POMME) is a geomagnetic field model providing an estimate of the Earths core, crustal, magnetospheric, and induced magnetic fields. The internal field is represented to spherical harmonic (SH) degree 90, while the secular variation and acceleration are given to SH degree 16. Static and time-varying magnetospheric fields are parameterized in Geocentric Solar-Magnetospheric (GSM) and Solar-Magnetic (SM) coordinates and include Disturbance Storm-Time (Dst index) and Interplanetary Magnetic Field (IMF-By) dependent contributions. The model was estimated from five years of CHAMP satellite magnetic data. All measurements were corrected for ocean tidal induction and night-side ionospheric F-region currents. The model is validated using an independent model from a combined data set of Orsted and SAC-C satellite measurements. For the core field to SH degree 13, the root mean square (RMS) vector difference between the two models at the center of the model period is smaller than 4 nT at the Earths surface. The RMS uncertainty increases to about 100 nT for the predicted field in 2010, as inferred from the difference between the two models.

Resolution of direction of oceanic magnetic lineations by the sixth‐generation lithospheric magnetic field model from CHAMP satellite magnetic measurements

The CHAMP satellite continues to provide highly accurate magnetic field measurements from decreasing orbital altitudes (<350 km) at solar minimum conditions. Using the latest 4 years (2004–2007) of readings from the CHAMP fluxgate magnetometer, including an improved scalar data product, we have estimated the lithospheric magnetic field to spherical harmonic degree 120, corresponding to 333 km wavelength resolution. The data were found to be sensitive to crustal field variations up to degree 150 (down to 266 km wavelength), but a clean separation of the lithospheric signal from ionospheric and magnetospheric noise sources was achieved only to degree 120. This new MF6 model is the first satellite-based magnetic model to resolve the direction of oceanic magnetic lineations, revealing the age structure of oceanic crust.

National Geophysical Data Center candidate for the World Digital Magnetic Anomaly Map

Marine and airborne magnetic anomaly data have been collected for more than half a century, providing global coverage of the Earth. Furthermore, the German CHAMP satellite is providing increasingly accurate information on large-scale magnetic anomalies. The World Digital Magnetic Anomaly Map project is an international effort to integrate all available near-surface and satellite magnetic anomaly data into a global map database. Teams of researchers were invited to produce candidate maps using a common pool of data sets. Here we present the National Geophysical Data Center (NGDC) candidate. To produce a homogeneous map, the near-surface data were first line-leveled and then merged by Least Squares Collocation. Long wavelengths were found to agree surprisingly well with independent satellite information. This validates our final processing step of merging the short-wavelength part of the near-surface data with long-wavelength satellite magnetic anomalies.

The Swarm Satellite Constellation Application and Research Facility (SCARF) and Swarm data products

Swarm, a three-satellite constellation to study the dynamics of the Earth’s magnetic field and its interactions with the Earth system, is expected to be launched in late 2013. The objective of the Swarm mission is to provide the best ever survey of the geomagnetic field and its temporal evolution, in order to gain new insights into the Earth system by improving our understanding of the Earth’s interior and environment. In order to derive advanced models of the geomagnetic field (and other higher-level data products) it is necessary to take explicit advantage of the constellation aspect of Swarm. The Swarm SCARF (SatelliteConstellationApplication andResearchFacility) has been established with the goal of deriving Level-2 products by combination of data from the three satellites, and of the various instruments. The present paper describes the Swarm input data products (Level-1b and auxiliary data) used by SCARF, the various processing chains of SCARF, and the Level-2 output data products determined by SCARF.

Spatio-temporal characterization of the equatorial electrojet from CHAMP, Ørsted, and SAC-C satellite magnetic measurements

Received 4 May 2007; revised 20 June 2007; accepted 12 June 2007; published 13 September 2007. [1] The equatorial electrojet (EEJ) is an eastward electric current on the day-side, flowing in a narrow band along the dip equator in the ionospheric E region. Recent magnetic observations from the CHAMP, Orsted, and SAC-C satellites, comprising more than 95,000 dip equator crossings from 1999 to 2006, have provided an unprecedented longitudinal coverage of the EEJ magnetic signature. We have used these data to construct an empirical model of the EEJ current climatological mean and day to day variability as a function of longitude, local time, season, and solar flux. Our model has been successfully verified against vertical drift data from the JULIA radar at Jicamarca. We have also used the EEJ observations to estimate the self-correlation of the EEJ, confirming short longitudinal correlation lengths of 15 and finding a temporal correlation length of 2.4 h. Our model’s predictions of the eastward electric field and its standard deviation may provide useful input to various kinds of ionospheric simulations. Coefficients and software are available online at http://models.geomag.us/EEJ.html and http://www.earthref.org.

Scaling properties of potential fields due to scaling sources

The theoretical power spectrum of the 3-dimensional potential field caused by an arbitrary 3-dimensional source distribution is derived for gravity and magnetic data. A function with scale-invariant features has a power spectrum, which is proportional to the frequency raised to minus the scaling exponent. For scaling source distributions, the power spectrum of the gravity and magnetic field is anisotropic and a specific scaling exponent exists only for lower-dimensional cross sections of the field. We suggest an approach which allows, under certain conditions, to derive the power spectrum of a lower-dimensional subset from the power spectrum of a 3-dimensional function. For the special case where the 3-dimensional function has an isotropic scaling exponent β3D, we confirm a known property, namely that a (3-k)-dimensional subset of the function has a scaling exponent of approximately k less than β3D. This property is not applicable to the anisotropic 3-dimensional fields, but it can be applied to source distributions with isotropic scaling exponent. Summarizing our results, the scaling exponents of the density distribution and the gravity field are related by whereas the relationship between the scaling exponents of the susceptibility distribution and the magnetic field reduced to the pole can be stated as follows:

Potential field power spectrum inversion for scaling geology

We propose a method for inverting the power spectrum of gravity and magnetic data. The method is demonstrated on aeromagnetic and bore-well data from the German Continental Deep Drilling Project (KTB). Density and susceptibility distributions in the Earths crust exhibit scaling behavior with power spectra proportional to f -β , where f is the wavenumber and β is the scaling exponent of the source distribution. We model the sources of the potential field by a random function with scaling properties, defined on a half-space with its top at a specified depth beneath the observation plane. Comparing the theoretical power spectrum for this model with the power spectrum of the measured data, we obtain the best values for the depth to source and the scaling exponent as a global minimum of the misfit function. Despite the simplicity of the model, it offers a new understanding of the factors influencing the shape of the potential field power spectrum. In particular, the low wavenumber part of the power spectrum can be dominated by the scaling properties of the source distribution and not by the depth to some kind of basement. The scaling exponent of the field varies with the type of surface geology. The question of whether the scaling exponent can actually be used to identify different types of geology gives an interesting new aspect to power spectrum inversion.