GP53B-01
Benefits of Electric Field Measurements in Combination with Precision Magnetic Field Observations on Swarm
Swarm will be the first precision magnetic field satellite mission to measure electric fields as well. The foremost benefit to the magnetic measurements will be to help identify and separate magnetic fields originating externally to the solid earth, which often have an electric field counterpart. Once identified, external magnetic fields can be combined with electric fields to infer ionospheric conductivity, and also to estimate electromagnetic energy exchange between the magnetosphere and upper atmosphere through quasi-static Poynting flux. This latter quantity can reach values of 100 kW per square kilometer of sky and can contribute significantly to the energy budget and dynamical behavior of the upper atmosphere. This talk will review these and other uses of combined electric and magnetic field measurements on satellites, and on Swarm in particular.
GP53B-02 INVITED
Coordinated use of ground-based auroral and high-precision LEO magnetic and electric field measurements to investigate auroral electrodynamics
There are now dozens of sensitive All-Sky Imagers (ASIs) deployed in networks spanning latitudes from the subauroral zone into the polar cap and many hours of magnetic local time. These new networks are collecting data with unprecedented spatial coverage and temporal resolution and in numerous scientifically interesting wavelength ranges. As well, direct satellite overflights of ground-based images that were once rare occurrences are becoming increasingly commonplace. This talk will focus on the scientific opportunities afforded by the integrated use of ground-based auroral images and magnetic and electric field data from existing and planned LEO missions including CHAMP, Oersted, and Swarm. These opportunities include exploring the relationship between field-aligned current and Poynting flux and different types of aurora, as well as reducing spatio-temporal ambiguity in the in situ measurements.
GP53B-03 INVITED
A satellite magnetic perspective of subduction zones, large igneous provinces, rifts, and diffuse plate boundary zones
Large and intermediate-scale tectonic features such as subduction zones, large igneous provinces, rifts, and
diffuse plate boundary zones are often seen to have a magnetic signature visible from the perspective of
near-Earth magnetic field satellites such as CHAMP and Orsted. Why do these tectonic features have a
magnetic signature, while others do not? A new model of the lithospheric field (MF-6, Maus et al., 2008)
extending to spherical harmonic degree 120 (333 km wavelength) has been used to evaluate the magnetic
state of the lithosphere under the assumption that the magnetization is either induced (with a seismic starting
model), or remanent (with a minimum norm approach). Some of the features identified from these images
include the Tethyan and NE Siberian diffuse plate boundary zones, the Red Sea rift, and Cretaceous rift
basins developed on the West African shield. Almost without exception, subduction zones exhibit a magnetic
signature, as do many large igneous provinces. In this talk we discuss some of the new insights this
magnetic perspective provides, and speculate on the controls which determine whether tectonic features will
be expressed magnetically.
http://geodynamics.gsfc.nasa.gov/research/purucker/pspecagu2008.pdf
GP53B-04 INVITED
Circum-Arctic Mapping Project: New Magnetic Anomaly map Linked to the Geology of the Arctic
Recent Circum-Arctic digital compilations of magnetic, gravity and bathymetry data have been analyzed in order to provide a consistent view of the tectonically complex Arctic basins and surrounding continents. The new grids have been mathematically filtered in order to assist in the regional characterization of magnetic and gravity domains and boundaries. In particular, we have analyzed the frequency content, amplitudes, and patterns of regional magnetic anomalies in areas with disputed crustal structure and tectonic evolution (Alpha-Mendeleev Ridge, Canada Basin, Makarov and Podvodnikov basins). The interpretation of the potential field and its derivatives has been compared with a recently released map of Arctic geology. Based on our data analysis, we present a range of kinematic scenarios for the Arctic Ocean domains. Gravity inversion methods were used for discriminating between various plate tectonic scenarios.
GP53B-05 INVITED
Modeling the Earth's Core Magnetic Field Under Core Flow Constraints
Two recent magnetic field models, GRIMM and xCHAOS, describe similar core field acceleration behaviors up to Spherical Harmonic (SH) degree 5, but, at higher degrees, their power spectra differ significantly. These dissimilarities, due to different approaches in smoothing rapid time variations of the core field, have strong implications on the interpretation of the secular variation. Furthermore, the amount of smoothing applied for the highest SH degrees is essentially the modeler's choice. We should therefore look for new, physically meaningful ways of regularizing core magnetic field models. We propose here to constrain field models to be consistent with the frozen flux induction equation. For this a core magnetic field model and a flow model at the top of the outer core, are co-estimated. The flow model is requested to have a smooth spatial and temporal behavior at the core-mantle boundary. The implementation of such constraints and their effects on a magnetic field model built from one year of CHAMP satellite and observatory data, are presented. In particular, it is shown that they efficiently constrain the core magnetic field secular variation and acceleration model components. We also show that for a magnetic field model to be consistent with the frozen flux approximation, the power spectrum of its acceleration must stay above a "minimum" power spectrum curve. Finally, preliminary results are shown for a core field model derived following the same approach and spanning seven year of CHAMP and observatory data.
GP53B-06
Forward and Adjoint Quasi-Geostrophic Models of the Geomagnetic Secular Variation
We introduce a quasi-geostrophic model of core dynamics, which aims at describing core processes on geomagnetic secular variation timescales. It extends the formalism of Alfvén torsional oscillations by incorporating non-zonal motions. Within this framework, the magnetohydrodynamics takes place in the equatorial plane; it involves quadratic magnetic quantities, which are averaged along the direction of rotation of the Earth. In addition, the equatorial flow is projected on the core-mantle boundary. It interacts with the magnetic field at the core surface, through the radial component of the magnetic induction equation, which we write in the frozen flux approximation. That part of the model connects the dynamics and the observed secular variation, with the radial component of the magnetic field acting as a passive tracer. We resort to variational data assimilation to construct formally the relationship between model predictions and observations. Variational data assimilation seeks to minimize an objective function (which contains the distance between model predictions and observations), by computing its sensitivity to its control variables (initial state, model parameters, boundary conditions). The sensitivity is efficiently calculated after integration of the adjoint model. Here we illustrate that framework with twin experiments, performed first in the case of kinematic core flow problem, and then in the case of Alfvén torsional oscillations. In both cases, the use of the adjoint model allows us to retrieve core state variables which, while taking part in the dynamics, are not directly sampled at the core surface. We study the effect of several factors on the solution (regularizations, width of the assimilation time window, data quality); and we discuss the potential of the model to deal with real geomagnetic observations in the future.
GP53B-07
Core Flow Modelling From Satellite-Derived 'Virtual Observatories'
Large satellite vector datasets of the Earth's magnetic field have become available in recent years. Standard magnetic field models of the internal field are generated by parameterising a small subset of these data through a least-squares spherical harmonic representation. An alternative approach is to create a set of 'virtual observatories' (VO) in space, mimicking the operation of fixed ground-based observatories. We derive VO datasets from both CHAMP and Ø rsted satellite measurements using two different binning methods: (a) a grid of 648 VO cylinders of 400km radius, equally spaced at 10° in colatitude and longitude and (b) a global grid of 648 equal-area tesserae. We use two different data selection techniques: (a) data from all local satellite times and (b) data from the local night-side only (20.00 - 06.00hrs). Selection of night-side only vector data were made and corrections to the input data, using external and toroidal fields calculated by Comprehensive Model 4 (CM4), were also applied. We test the effect of a number of selection and correction criteria. We calculate and directly invert the secular variation (SV) from these VO datasets, to infer flow along the core-mantle boundary. By examining the residuals from the flow models, we find temporally and spatially varying biases and patterns in the vector components. We investigate potential causes for these patterns and find evidence for influence from fields both internal and external to the satellite, orbital configuration and effects from the method of binning data. We conclude that the best fit of the flows to the data employs a grid of equal area tesserae using satellite night-side only data. The use of CM4 to correct the satellite data before calculating the VO SV grid removes a strong bias from external sources but, on average, does not greatly improve the fit of the flow to the data. We suggest that, despite best efforts, external fields effects are not completely removed from SV data and hence create unrealistic secular acceleration.
GP53B-08
Ensemble Forecasting of Geomagnetic Fields
Ensemble forecasting is a method first introduced in numerical weather prediction (NWP) in which an ensemble of model runs with perturbed initial states are produced. The initial states come from an assimilation of observations into the model (called the analysis) and the perturbations generally reflect the known error statistics of the initial state. At the end of the forecast runs the variability (or spread) of the different ensemble members is used as an indication of the error growth or uncertainty in the prediction. We use a modification of this approach with the geomagnetic data assimilation system developed at the University of Maryland - Baltimore County and Goddard Space Flight Center. An ensemble of 400 year assimilation runs with 20 year forecasts between the years 1600 and 2000 are carried out in order to assess the applicability of ensemble forecasting to assessing forecast errors in predicting the geomagnetic field. The ensemble generated forecast errors show the error growth between assimilation times, and therefore have some potential to be used in geomagnetic data assimilation. The results are also used to examine whether forecast can be made to regional geomagnetic variations, e.g. South Atlantic anomaly and geomagnetic jerks.