GP31A-0821 0800h
Recognizing the longest wavelength lithospheric magnetic signals obscured by overlap with the core field
We recognize and characterize two characteristic patterns evident in new maps of the lithospheric field produced using the CHAMP and Oersted satellites. The boundaries of these patterns define long-wavelength features in the lithospheric field not previously recognized because they were obscured by overlap with the core field. These boundaries correspond to known crustal thickness variations. We characterize the magnetic field patterns in terms of amplitude and shape, and deduce minimum amplitude magnetization models from the satellite data without specifying magnetization direction. The minimum amplitude trades off with misfit to the data, but we interpret the pattern and direction of magnetization, which are robust.
GP31A-0822 0800h
Mapping the low-altitude cusp: intense small-scale field-aligned currents vs. energetic particle precipitation
We have studied more than 50 cases of cusp observations from low-altitude satellites during the February 16-22, 2002, SIRCUS campaign period. About half of them were inferred from DMSP F-13/14/15 particle spectrometer data and the other half from magnetic field measurements in the lower ELF range made onboard the CHAMP and {\O}rsted LEO satellites. The locations of the satellites during the detection of these signatures were converted into AACGM coordinates, and the geomagnetic latitude of these signatures set in relation to the statistical, IMF-Bz dependent particle cusp latitude derived by Newell et al. (1989). The particle cusp latitude which we inferred from DMSP data matches the statistically expected latitude while the small-scale magnetic field regime appears to cover the equatorward section of the cusp and the poleward section of the low-latitude boundary layer (LLBL). We suggest that the perturbations resulting in small-scale magnetic field variations are generated in the LLBL-cusp transition zone, possibly associated with turbulence or instabilities generated in the process of opening geomagnetic field lines and merging them with the interplanetary field.
GP31A-0823 0800h
Estimation Of Field-Aligned Currents With A Multi-Satellite Mission ({\it Swarm})
The multi-satellite mission Swarm is conceived to investigate the dynamics of the Earth's magnetic field and its interaction with the Earth system in unprecedented detail. The instrumentation on board will provide high precision vector data of the electric and magnetic fields. For a further improvement of the main field and lithospheric field models it is vital to understand and reduce the influence of ionospheric currents. With the planned constellation of two satellite pairs at different heights, one at 550 km and the other at initially 450 km, the mission is particularly well suited to study the complex current systems of the polar ionosphere. The lower pair shall fly side-by-side separated by only $\sim$100 km in east/west direction. This will allow for the first time to determine ionospheric field-aligned currents unambiguously by directly employing the curl-{B} relation. As part of an {ESA}-funded science study auroral current systems were generated by one of the global {MHD} codes of the {GGCM} covering different degrees of activity. The new technique is applied to this set of consistent magnetic field and current data. By using realistic satellite trajectories and instrument performances we recovered the field-aligned current distributions from the magnetic field deflections. The resulting currents are tested against the input currents. The agreement between input model and recovered field-aligned currents is satisfying and much improved compared to the single-satellite estimates. These investigations demonstrate one important aspect of the broad capabilities provided by the unique new space mission project.
GP31A-0824 0800h
Resolving power of satellite magnetic data for lithospheric field characterization
The isolation of the magnetic field of the martian and terrestrial lithospheres is subject to several limitations, such as the altitude of the observations, the presence of external fields that overlap the lithospheric field, and the accuracy of the observing system. We compare and contrast the situation at the two planets, Mars and the Earth. The altitude effect at Mars is more severe than that at the Earth, because of the different curvatures of the two planets. We examine five types of constraints that can be used to place some bounds on the resolving power of magnetic data: 1) comparison of radial and along-track fields via the Hilbert transform, 2) comparison of independently determined global spherical harmonic power spectra, 3) auto and cross-spectral estimates from the radial components of nearby passes, 4) forward modeling approaches, and 5) End-to-End simulations. Wavelengths less than the altitude of the observations are strongly suppressed at both planets, more so for Mars. Some improvements can be made with a multi-instrument gradiometer configuration in the future.
GP31A-0825 0800h
Electromagnetic induction transfer functions estimated from Oersted, CHAMP and SAC-C missions
Electromagnetic induction studies using Oersted, CHAMP and SAC-C satellites' magnetic data will ultimately provide important new constraints on the electrical conductivity of Earth's mantle. In contrast to the spatially sparse geomagnetic observatory data, which provide the basis for most of our present knowledge about deep Earth conductivity, data collected from these satellites provide a full coverage of the Earth with 30 degree spacing every day. Electromagnetic induction transfer functions estimated from Oersted, CHAMP and SAC-C magnetic observations, using robust processing and wavelet techniques, will be shown and a comparison of them will be given. Additionally, an important issue in the emergent field of electromagnetic induction studies with magnetic satellite data, namely the assumed morphology of the external source fields will be discussed.
GP31A-0826 0800h
Continuous calibration of ground-based magnetic-observatory fluxgate magnetometers
Continuous data collection at magnetic observatories, such as those operated by the USGS and other Intermagnet programs, is accomplished using fluxgate magnetometers. Since these sensors are subject to baseline drift, periodic absolute measurements are made at each observatory. This is accomplished using a proton magnetometer, which can deliver accurate intensity data, and a theodolite coupled to a single-axis fluxgate magnetometer (DIM), which can deliver accurate magnetic directional data, although the theodolite measurements must currently be made by hand. Through data processing, the absolute data can be used to adjust for the response drift of the fluxgate sensor and thereby place the variational data onto an absolute scale. Drawing lessons from satellite magnetometer calibration methods, we show how to use absolute data to not only correct for fluxgate sensor response drift, but also how to correct for both small rotations in the orientation of the fluxgate and for nonorthogonality between the sensor elements of the fluxgate. This calibration of fluxgate data for response, rotation, and nonorthogonality, can be accomplished continuously in time by inverting a 3x3 matrix and using a set of basis functions consisting of B-splines. The near-real-time calibrated data are of potential interest to the directional-drilling and satellite-magnetometer communities.
http://geomag.usgs.gov
GP31A-0827 0800h
A new proposal for Spherical Cap Harmonic modeling and its application on various geomagnetic data.
We recently proposed a new method for regional modeling of potential fields. Based on the solution of Laplace's equation in a conical region associated with particular boundary conditions, our method utilizes new mathematical functions able to represent faithfully the spatial variations of potential fields. We first refine our previous boundary value problem in order that the solutions satisfy the physical properties of a geomagnetic field and in order to have fully orthogonal basis functions inside the region under study. To outline the relevance of this new formalism, we tackle here two kinds of inverse problems using this novel method. First, by joint inversion, we derive a regional model over a small region such as France, by processing simultaneously aero-magnetic, repeat station and CHAMP satellite data. We also discuss the consistency of the predicted magnetic field values with geological features and existing magnetic maps of this region. Then, using four years of CHAMP vectorial data, we try to compute over the entire sphere a lithospheric model at 400km by stitching together different local models obtained in adjacent regions. These two results allow us firstly to highlight the continuity between this local modeling technique and that of the Spherical Harmonics, and secondly to outline the importance of regional modeling to the knowledge of the geomagnetic field at short and medium spatial wavelengths.
GP31A-0828 0800h
Close encounters of CHAMP, Oerested, and SAC-C satellites
Utilizing observations of the field taken every 20 seconds between January, 2001 and May 2004, we assembled a database of the angular separation of the sub-satellite tracks, and close encounters of the CHAMP, Oersted, and SAC-C satellites. We expect that this data base will be useful for defining the variability in time and space of the geomagnetic field, for calibration and intercalibration of the three satellites, for detailed local studies of the lithospheric field via gradients, and as a test bed for the upcoming Swarm mission. The products produced included 1) daily maps and files of the angular separation of the two satellites, 2) maps and a data base showing the location of close encounters (1,2,5,10,15, and 20 degree separations) of the three satellites, and 3) for the 1 degree separations, a database where the main field as determined by CM4 has been removed from the total field observations. The sub-satellite tracks of CHAMP and Oersted were separated by less than 1 degree some 412 times in the 3.4 years studied. Most of these close encounters were at high latitudes. The comparable figure for Oersted and SAC-C was 328, and for CHAMP and SAC-C it was 173.
http://planetary-mag.net/virtual
GP31A-0829 0800h
New Parameterization of External and Induced Fields in Geomagnetic Field Modeling
It is a common practice to consider low-degree external and secondary (induced) field contributions when deriving spherical harmonic models of the Earth's core and crustal fields. Traditionally, the external field is described by the spherical harmonic coefficient $q_1^0$ which consists of a static (or slowly varying) part and a part that varies linearly with the $D_{\rm st}$-index, while induced contributions are considered assuming a constant ratio $Q_1$ of induced to external coefficients. $Q_1=0.27$ was found from Magsat data by {\it Langel et al.}, and this value has been used by several authors when deriving recent field models from {\O}rsted and CHAMP data. An alternative approach of considering induced field contributions is based on the decomposition of $D_{\rm st}=E_{\rm st}+I_{\rm st}$ into external ($E_{\rm st}$) and induced ($I_{\rm st}$) parts by means of a realistic (radially symmetric) model of mantle conductivity. In this new approach the temporal behavior of external and corresponding induced coefficients are parameterized by $E_{\rm st}(t)$ and $I_{\rm st}(t)$, respectively. Deriving field models from {\O}rsted and CHAMP data using the old and new parameterization demonstrates the superiority of the new approach. However, the model residuals are correlated between the two satellites in both cases, indicating the existence of an unmodeled magnetospheric signal. This is probably due to baseline-instabilities of $D_{\rm st}$. We account for this by allowing the ``static'' external coefficient $q_1^0$ to change with time, which results in a significant reduction of the large-scale residuals.
http://www.dsri.dk/Oersted/Field_models/
GP31A-0830 0800h
Wavelet Frames: an Alternative to Spherical Harmonic Representation of Potential Fields?
Potential fields are classically represented on the sphere using spherical harmonics. However, this decomposition leads to strong numerical difficulties when dealing with regional features. To overcome this drawback, we developed a new representation of the magnetic and the gravity fields based on wavelet frames. We describe how to build a wavelet frame on the sphere. The chosen frames are based on the Poisson multipoles wavelets, which are of special interest for geophysical modelling, since their scaling parameter is linked to the multipole depth (Holschneider et al., 2003). Our first results in representing the Earth's core magnetic and gravity fields from data evenly or unevenly distributed prove the interest of such a representation. The comparison of the obtained wavelet models with spherical harmonic models underlines the superiority of wavelets when dealing with globally, unevenly distributed datasets or regional ones.
GP31A-0831 0800h
Geomagnetic Data Assimilation: A Method for Determining Error Covariances
We present initial testing of a technique for assimilating geomagnetic data into numerical geodynamo modeling. The technique is analyzed using a one-dimensional system consisting of coupled non-linear equations for momentum, induction and energy variations in a periodic domain. We construct a synthetic assimilation test by making magnetic field observations on a "nature" (or true solution) which differs from the model by some random errors with known statistics. We wish to understand how the magnetic observations can be used to constrain all of the system variables. The coupling in the model between the different variables can be used to obtain correlations between their errors. The ensemble method involves carrying out a number of model runs, each with a different perturbed initial state. The resulting ensemble is then used to determine error correlations between each of the state variables. In this way, the difference between the numerical and observed values of surface magnetic flux indicates corrections to velocity and temperature within the core. This technique leads to a "balanced" forcing on the numerical solution which should lead to a more consistent solution. Results of assimilation in this system can then be used to understand how to carry out geomagnetic assimilation in our MoSST core dynamics model.
http://mosst.gsfc.nasa.gov
GP31A-0832 0800h
Estimating Antarctic near-surface magnetic anomalies from Oersted and CHAMP satellite magnetometer observations
Significant improvement in predicting near-surface magnetic anomalies can result from the highly accurate magnetic observations of the CHAMP satellite that is orbiting at about 400 km altitude. In general, regional magnetic signals of the crust are strongly masked by the core field and its secular variations due to wavelength coupling in the spherical harmonic representation and thus are difficult to isolate in the satellite measurements. However, efforts to isolate the regional lithospheric from core field components can exploit the correlations between the CHAMP magnetic anomalies and the pseudo magnetic effects inferred from gravity-derived crustal thickness variations. In addition, we can use spectral correlation theory to filter the static lithospheric field components from the dynamic external field effects. Employing these procedures, we processed the CHAMP magnetic observations for an improved magnetic anomaly map of the Antarctic crust. Relative to the much higher altitude Oersted and noisier Magsat observations, CHAMP magnetic anomalies at 400 km altitude reveal new details on the effects of intra-crustal magnetic features and crustal thickness variations of the Antarctic. Moreover, these results greatly facilitate predicting magnetic anomalies in the regional coverage gaps of the ADMAP compilation of Antarctic magnetic anomalies from shipborne, airborne and ground surveys. Our analysis suggests that considerable new insights on the magnetic properties of the lithosphere may be revealed by a further order-of-magnitude improvement in the accuracy of the magnetometer measurements at minimum orbital altitude.