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GEOSPACE consortium presentations, to be given at GEOSPACE Consortium Meeting, Coseners House, Abingdon, 3rd - 4th July 2008
(please send all final papers for upload to Chris Place (firstname.lastname@example.org) )
Deriving a core magnetic field model under flow constraints
Two recent magnetic field models, GRIMM and xCHAOS, present similar acceleration energy up to Spherical Harmonic (SH) degree 5. However, the power spectra of their accelerations differ significantly at higher degrees, which in turn has strong implication on the interpretation of the secular variation of these models. These differences are due to different approaches in smoothing rapid time variations of the internal field, the amount of smoothing being left to the choice of the modeler. 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. We further request that the liquid outer core flows to have a smooth space and time behavior at the coremantel boundary. We describe the implementation of such constraints. In particular, for this first approach, we minimize the norm of the first derivative in time of the flow model vector. Preliminary results show that such a constrain has strong effect on the time variation of the core magnetic field models. This constraint effectively regularizes the magnetic field acceleration model for SH degree above 5. Models for the 2004.5 2005.5 epoch will be presented
An extended comparison of Cluster magnetic data with the Tsyganenko 2001 model
E. E. Woodfield1,3, M.W. Dunlop2, R. Holme3, J.A. Davies2, M.A. Hapgood2
1Lancaster University, 2Rutherford Appleton Laboratory, 3University of Liverpool
We have extended our investigation of the magnetic effects of external currents in the magnetosphere using Cluster data and 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. In our previous study covering data from 2003 and 2004 we found that the model performs very well in a global sense; nevertheless, absolute residuals between the data and the model can reach approximately 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 centred on perigee, of approximately 20 nT. To investigate the source of these longer term trends we have extended the data coverage to include more recent data.
A study of the open-closed field line boundary during substorms using incoherent scatter radars.
E. E. Woodfield1, J. A. Wild1, A. J. Kavanagh1
The size of the polar cap and hence the amount of open magnetic flux contained within it is a very important quantity when it comes to understanding the substorm process as well as reconnection rates in general. Ground-based proxies of the open-closed field line boundary (OCFLB) have therefore been of great interest in recent years; signatures such as the spectral width boundary observed by the SuperDARN radars can be used to track changes in the size of the polar cap. Incoherent scatter radar data have also been used in individual studies to identify the footprint of the OCFLB in the ionosphere. In this work we make use of the Frey list of substorms to study the incoherent scatter radar signatures of the OCFLB during substorms using the EISCAT radars. The EISCAT data is supported by contextual information from a variety of ground and space based instrumentation including the CUTLASS HF radars and the four Cluster spacecraft. Our preliminary findings are presented.
Core Flow Modelling from Satellite-Derived 'Virtual Observatories'
Ciaran Beggan1, Kathy Whaler1, Susan Macmillan2
1University of Edinburgh, School of GeoSciences, 2British Geological Survey, Edinburgh
The last decade has seen a significant improvement in the capability to observe the global field at high spatial resolution. Several satellite missions have been launched, providing a rich new set of scalar and vector magnetic measurements from which to model the global field in detail. These data complement the existing record of ground-based observatories, which have continuous temporal coverage at a single point. We wish to exploit these new data to model the secular variation (SV) globally and improve the core flow models that have been constructed to date. Using the approach developed by Mandea and Olsen (2006) we create a set of evenly distributed `Virtual Observatories' (VO), at 400km above the Earth's surface, encompassing satellite measurements from the CHAMP satellite over five years (2001-2005). We invert the SV calculated at each VO to infer flow along the core-mantle boundary. Direct comparison of the SV generated by the flow model to the SV at individual VO can be made. Thus, the residual differences can be investigated in detail. We show comparisons of flow models generated from a number of VO datasets, which provide evidence that the residual distributions observed are consistent with internal and external field effects in the satellite data. We also show that the binning and processing of the VO data can induce artifacts, including sectorial banding, into the residuals.
Global Geomagnetic Field Modelling; How to Handle Polar and Auroral Zone Magnetic Data?
Alan Thomson1, Brian Hamilton1, Susan Macmillan1, Sarah Reay1
1British Geological Survey, Edinburgh
Using a combination of 20-second sampled night-side Oersted and Champ data and night-side observatory hourly means we have constructed a new global geomagnetic field model for 1999-2008. The data selection principles are the same as in Thomson and Lesur (T&L, GJI, 169, 951-963, 2007) and the model parameterisation is similar. For example, we solve for a maximum internal spherical harmonic degree of 60, with piecewise linear (seven node) secular variation to degree 13. We use a degree one external field, again piecewise linearly time-dependent, and with added annual/semi-annual variation terms, to represent slower secular change in the quiet magnetosphere. Rapid (e.g. ring current) variations in degree one external and induced fields are parameterised by the VMD index, as first introduced by Lesur (T&L, 2007). No damping or constraints are used in the inverse problem and our new model benefits, compared to T&L (2007), from a more substantial data set, a revised Champ satellite vector data calibration and updated/corrected magnetic indices. To minimise (primarily) high-latitude external fields not included in the model parameterisation we down-weight satellite data according to the sample standard deviation. These vector component weights are estimated from each 20-second segment of the original 1-second data and are used as an indicator of rapid variations on short spatial scales, for example due to field-aligned currents. We also use an additional local weighting for each satellite measurement, in the form of a ‘local area vector activity’ (Lava) index. These are derived from neighbouring ground observatory variations, with weights again calculated for each vector component separately. In this paper we will describe these weight factors and demonstrate the impact they have on global field models. For example, the new model shows improved coherence with published models such as GRIMM (Lesur et al, 2008) and CHAOS (Olsen et al, 2006) and much reduced external field ‘noise’ at high-latitudes, compared with these models and with T&L (2007). The new model in particular has the lowest power spectrum in spherical harmonic degrees 30-50 (approximately) of all these models. Other features of the model, particularly the derived lithospheric field, will be described. Future work, for example on more sophisticated secular change models, will be outlined.
Modelling the quiet-time geomagnetic daily variations using observatory data
Brian Hamilton1, Susan Macmillan1
1British Geological Survey, Edinburgh
We present on-going work towards building a global model of the quiet-time geomagnetic daily variation using observatory data. We select hourly mean data during the solstice months of June and December in years during the minimum and maximum of the solar cycle. We fit Fourier series in time, with a fundamental period of 24 hours, to the data at each observatory. We then use global spherical harmonic expansions to separate the daily variation signal, as characterised by the Fourier coefficients in time, into external and induced internal contributions. The models are assessed by comparison with the input data. The robustness of the separation of the field into external and induced internal sources is discussed.
Reprocessing of POGO Satellite Data - Preliminary Results of some Experiments
Nils Olsen1, Freddy Christiansen1, Andy Jackson2
1DTU Space, Copenhagen, 2ETH, Zurich
Positioning errors are believed to be the largest error source of the magnetic intensity data of the POGO satellite series, which flew between 1965 and 1972. A position error of 100 m in radial direction results in a maximum intensity error of 2.8 nT, while a similar positioning error in horizontal direction only gives about 0.5 nT field intensity error. No advanced gravity field models were available at the time when the satellite positions were calculated several decades ago, and therefore a reprocessing of the POGO positions, using state-of-the-art gravity field models, may lead to better positions and thereby reduced magnetic residuals. Unfortunately, the original tracking data are no longer available for the POGO satellites. One therefore has to rely on the processed positions, which we use as input to the BERNESE orbit determination software. This program adjusts the positions by minimizing the difference between the actual "observed positions" (which in our case are the preprocessed orbits) and predicted positions by introducing small corrections. Application to one month of OGO-4 data results in position changes of a few hundred meters. Fitting a magnetic field model to the magnetic field observations using the reprocessed orbits gives slightly reduced magnetic field residuals.
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