Seismology and Geomagnetism at BGS
BGS GEOSPACE related activities in 2006
Chris Finlay, Andy Jackson and Nicolas Gillet
Progress towards a new historical geomagnetic field model 1590-2005
Ciaran Beggan, Kathy Whaler and Susan MacMillan
Applying Different normalisation models to improve core flow inversion
Applying Satellite Geomagnetism To Probe Ocean Flow
Kumar Hemant, Mark Pilkington, David B. Snyder
Weakly Magnetic Crust in the Canadian Cordillera
Emma E. Woodfield, M. W. Dunlop, R.Holme, J. A. Davies, M. A. Hapgood, R Ayliffe
An Update on the Statistical Comparison of Cluster data with Tsyganenko 2001
BGS Geospace related activities in 2006
Alan Thomson, British Geological Survey,
West Mains Road, Edinburgh EH9 3LA.
A new degree 60 global field model is presented and discussed (BGS/G/L/0706). We emphasise the relatively simple external field model adopted, the use of a wide variety of geomagnetic activity data for satellite quiet data selection and a new ‘virtual magnetic disturbance’ index, used to compare with Dst. The lithospheric model (degrees 15-60) displays good coherence with other recent published models for this epoch, although the coherence degrades above about degree 40, most obviously in relation to poorer active period rejection in the southern polar cap.
Future work is also outlined. We plan to improve the external field model, taking into account Earth’s time varying dipole tilt and the time varying solar wind control of magnetopause, tail and ring currents.
Progress towards a new historical geomagnetic field model 1590-2005
Chris Finlay1, Andy Jackson1 and Nicolas Gillet2.
1Institut fur Geophysik, ETH
, 2Institute of Geophysics and Tectonics, Zurich, Switzerland University of Leeds, U.K.
A new historical geomagnetic field model spanning the interval from 1590 to 2005 is currently under construction by workers in
Zurichand Leedsin collaboration with the GEOSPACE consortium. This talk will report progress towards the inclusion in this new model of Oersted, CHAMP and SAC-C satellite observations, new survey and observatory data, a small number of additional historical data sources and archaeointensity data prior to 1840 that were absent from the previous historical field model gufm1 of Jackson et al (2000).
Applying different normalisation models to improve core flow inversion
Ciaran Beggan1, Kathy Whaler1, Susan Macmillan2.
1School of Geosciences,
2British Geological Survey, Universityof Edinburgh, Edinburgh
Modelling of the flow at the top of the outer core generating the observed secular variation (SV) on the surface of the Earth is open to considerable ambiguity. Methods adopting different physical assumptions do, however, lead to similar flow velocity maps. The question of which physical assumptions apply in the core is still under debate. For example, the assumption of a steady flow remains under scrutiny. Equally, the geophysical evidence for poloidal flow in the upper part of the outer core is ambiguous.
We aim firstly to use core flows to predict the SV more precisely than previous methods (such as non-linear extrapolation). We investigate the use of different
‘best-fit’ or ‘minimisation’ methods in core flow modelling based on past records of SV. We use a method for directly inverting the observed secular variation from observatories and repeat stations, rather than through the use of spherical harmonic models, in order to produce a better prediction of the time evolution of the field over the short term (i.e. 5-10 years). We have tested iterative one-norm minimisation models and find that they better describe the secular variation than the current two-norm models using the steady flow assumption. We show, as examples, flows generated using the dataset modelled by Wardinski (2005) in conjunction with both GUFM and IGRF main field models.
Secondly, we aim to re-examine the hypothesis of whether flow at the top of the outer core can be assumed purely toriodal. We investigate whether models using toroidal and poloidal flows improve the fit to the observations of the secular variation.
Applying Satellite Geomagnetism to
Flow Probe Ocean
J. Hawe, R. Holme.
Earth and Ocean Sciences,
(email@example.com) University of Liverpool, United Kingdom
While many sources which contribute to the geomagnetic field have been extensively studied, one source to receive little attention is the magnetic field generated via motional induction in the oceans. As part of the GEOSPACE consortium, I am attempting to gain a better understanding of the fields generated by this source and the flows that generate them. Recent work has highlighted the potential to investigate such signals across a range of magnitudes and spatial scales. The result of this potential is the subject of this presentation. Here I will show results from investigations looking at both global ocean tides and also the isolation of smaller scale, localised signals in the
Argentine Basin, situated off the Eastern coast of South America, which have previously only identified in TOPEX altimetry and bottom pressure data.
Weakly magnetic crust in the Canadian Cordillera
Kumar Hemant1, Mark Pilkington2, David B. Snyder2.
1School of Earth & Environment,
Universityof Leeds, . Leeds, UK
2Geological Survey of
, Canada 615 Booth Street, Ottawa, ON, Canada K1A 0E9.
Current models of continental crust favour an increase in magnetization with depth. Here we report a counter example from the Canadian Cordillera where almost a full thickness of non-magnetic continental crust is suggested by joint interpretation of magnetic and seismic data.
The magnetic field over the Cordillera is characterized by complex, short-wavelength (<100 km) anomalies associated with intrusive, metamorphic and volcanic rocks that occur at shallow depths (<5 km) within accreted terranes. The long wavelength (>100 km) portion of the Cordilleran field is subdued and mainly featureless, and suggests a lack of magnetic sources at greater depths.
Seismic reflection and refraction data from three major transects in the
Yukonand British Columbia, support this interpretation and indicate that sedimentary-like formations make up the majority of the crust. The dominance of shallow, upper crustal magnetization in the Canadian Cordillera contrasts with the generally-held view that the lower continental crust is the primary source for long-wavelength magnetic anomalies. Sources for these anomalies are often assumed to be located in the lower crust when surface magnetizations are insufficient to produce such anomalies or no correlation exists between the magnetic field and the mapped surface geology. The Canadian Cordillera appears to be an example of a non-magnetic lower crust overlain by a more magnetic upper crust that is, however, not magnetized strongly enough to produce significant long-wavelength magnetic anomalies. Canada
How Damaging are Time Gaps in Vector Data Coverage to Global Field Models?
University of Liverpool, UK
Danish National Space Center, Copenhagen, Denmark
Our understanding of all aspects of the geomagnetic field has been advanced enormously by recent satellite missions, providing global three-component vector magnetic data. The Swarm mission should continue this coverage, but with a launch scheduled for 2009, and the end of the CHAMP mission likely to come in 2008, any delay in launch (or early failure of CHAMP) would leave a period without data coverage. Here, we consider the implications of such a gap, either with no data, or a scalar-measurement only mission. The effect on the model of the data gap is surprisingly small, perhaps because the main field is not varying rapidly over the two years of the gap. Scalar data applied simply do not improve the model greatly over no data, but eigenvector evidence of Backus effect suggests that more careful modelling using these data could yield an improvement.
Evidence for a Geomagnetic Jerk after 2003 in LOD
. University of Liverpool, UK
Olivier de Viron, IPGP, France
Geomagnetic jerks are well-known, but arguably still poorly understood, features of the temporal behaviour of the geomagnetic field. They consist of sharp changes in the secular acceleration of the field, in some cases seen globally, while in other cases only at some locations at the Earth's surface. Recently (Holme, 2004), we have provided evidence for a correlated feature in the rotation of the Earth. Removing a detailed calculation of atmospheric angular momentum (AAM) from the geodetic signal produces a much smoother signal to examine for decadal changes in length of day (LOD), assumed to result from core-mantle angular momentum exchange. Fitting penalised least-squares splines to a simple yearly running average of the smoothed signal allows its easy numerical differentiation, yielding features preceding and correlated with jerks, consistent with a sharp change in the core-mantle torque. This observation provides support for recent claims that jerks are associated with torsional oscillations in the Earth's core.
Here we extend analysis of the rotation signal to the start of 2006. We find evidence of a jerk-like feature centred approximately in 2003, suggesting that a geomagnetic jerk may be visible in the time following this. Our study was motivated by suggestions (Olsen and Mandea, pers. comm.) that such a jerk can be detected in satellite observations of the geomagnetic field. Thus, our results provide support for the existence of a geomagnetic jerk in 2004, and also for the detection of such jerks in satellite magnetic data.
An update on the comparison of Cluster data with the Tsyganenko 2001 model and a summary of future research directions within Geospace.
Emma E. Woodfield1, M.W. Dunlop2, R.T. Holme1, J.A. Davies2, M. Hapgood2, R. Ayliffe1
Liverpool,2Rutherford Laboratory Appleton
I will start by presenting an update on the work we have been doing comparing the Tsyganenko 2001 model to magnetic data from the 4 Cluster spacecraft. A frequently observed feature in the residuals is a bipolar signature in the individual components combined with little or no change in the residual of the field magnitude. This indicates the presence of a field-aligned current and we investigate its location using particle data from other instruments on the spacecraft.
We have also been looking at Comprehensive Model behaviour during an isolated substorm to assess the effectiveness of using indices to remove such active data, particularly in relation to the timing of the various indices. We compared the results of the model when the magnetospheric index used was Dst and also when we used SymH.
Finally, I will describe the beginnings of an investigation in collaboration with UCL looking into the preconditioning effects the thermosphere has on magnetic fields.