Can core‐surface flow models be used to improve the forecast of the Earth's main magnetic field?

[1] Geomagnetic main field models used for navigation are updated every 5 years and contain a forecast of the geomagnetic secular variation for the upcoming epoch. Forecasting secular variation is a difficult task. The change of the main magnetic field is thought to be principally due to advection of the field by flow at the surface of the outer core on short timescales and when large length scales are considered. With accurate secular variation (SV) and secular acceleration (SA) models now available from new satellite missions, inverting for the flow and advecting it forward could lead to a more accurate prediction of the main field. However, this scheme faces two significant challenges. The first arises from the truncation of the observable main field at spherical harmonic degree 13. This can however be handled if the true core flow is large scale and has a rapidly decaying energy spectrum. The second is that even at a given single epoch the instantaneous SV and SA cannot simultaneously be explained by a steady flow. Nevertheless, we find that it may be feasible to use flow models for an improved temporal extrapolation of the main field. A medium-term (≈10 years) hindcast of the field using a steady flow model outperforms the usual extrapolation using the presently observed SV and SA. On the other hand, our accelerated, toroidal flow model, which explains a larger portion of the observed average SA over the 2000–2005 period, fails to improve both the short-term and medium-term hindcasts of the field. This somewhat paradoxical result is related to the occurrence of so-called geomagnetic jerks, the still poorly known dynamical nature of which remains the main obstacle to improved geomagnetic field forecasts.

[1]  W. Menke.  Geophysical data analysis , 1984 .

[2]  Nils Olsen,et al.  Core surface flow modelling from high‐resolution secular variation , 2006 .

[3]  Gauthier Hulot,et al.  Worldwide wavelet analysis of geomagnetic jerks , 1996 .

[4]  Nils Olsen,et al.  The Present Field , 2007 .

[5]  G. Hulot,et al.  MECHANICAL CORE-MANTLE COUPLING AND DYNAMO MODELLING , 1996 .

[6]  Gauthier Hulot,et al.  On core surface flows inferred from satellite magnetic data , 2005 .

[7]  J. Wahr,et al.  Taking into account truncation problems and geomagnetic model accuracy in assessing computed flows at the core—mantle boundary , 1992 .

[8]  Hermann Lühr,et al.  Third generation of the Potsdam Magnetic Model of the Earth (POMME) , 2006 .

[9]  Jeremy Bloxham,et al.  The origin of geomagnetic jerks , 2002, Nature.

[10]  Mioara Mandea,et al.  The 9th-Generation International Geomagnetic Reference Field , 2003 .

[11]  Jeremy Bloxham,et al.  Fluid flow near the surface of Earth's outer core , 1991 .

[12]  Paul H. Roberts,et al.  On Analysis of the Secular Variation , 1965 .

[13]  Jeremy Bloxham,et al.  Geomagnetic field analysis—III. Magnetic fields on the core—mantle boundary , 1985 .

[14]  J. Mouël.  Outer-core geostrophic flow and secular variation of Earth's geomagnetic field , 1984, Nature.

[15]  Alexandre Fournier,et al.  A case for variational geomagnetic data assimilation: insights from a one-dimensional, nonlinear, and sparsely observed MHD system , 2007, 0705.1777.

[16]  Brian Hamilton,et al.  The US/UK World Magnetic Model for 2010-2015 , 2010 .

[17]  B. Buffett,et al.  Mechanisms of core‐mantle angular momentum exchange and the observed spectral properties of torsional oscillations , 2005 .

[18]  P. Roberts.  Electromagnetic Core-Mantle Coupling , 1972 .

[19]  Mioara Mandea,et al.  Rapidly changing flows in the Earth's core , 2008 .

[20]  Gauthier Hulot,et al.  Secousses (jerks) de la variation séculaire et mouvements dans le noyau terrestre , 1993 .

[21]  Gauthier Hulot,et al.  Length of day decade variations, torsional oscillations and inner core superrotation: evidence from recovered core surface zonal flows , 2000 .

[22]  A. Böhmer,et al.  Macrolides and community-acquired pneumonia: is quorum sensing the key? , 2010, Critical care.

[23]  Mioara Mandea,et al.  GRIMM: the GFZ Reference Internal Magnetic Model based on vector satellite and observatory data , 2008 .

[24]  Gauthier Hulot,et al.  On the characteristics of successive geomagnetic jerks , 1998 .

[25]  J. Bloxham,et al.  On the dynamical implications of models of Bs in the Earth’s core , 1999 .

[26]  R. Holme.  Electromagnetic core—mantle coupling—I. Explaining decadal changes in the length of day , 2002 .

[27]  Ulrich R. Christensen,et al.  Core flow inversion tested with numerical dynamo models , 2000 .

[28]  D. Jault,et al.  Westward drift, core motions and exchanges of angular momentum between core and mantle , 1988, Nature.

[29]  W. Menke.  Geophysical data analysis : discrete inverse theory , 1984 .

[30]  K. Whaler,et al.  Does the whole of the Earth's core convect? , 1980, Nature.

[31]  Gauthier Hulot,et al.  An estimate of average lower mantle conductivity by wavelet analysis of geomagnetic jerks , 1999 .

[32]  George E. Backus,et al.  Steady flows at the top of the core from geomagnetic field models: The steady motions theorem , 1985 .

[33]  C. Voorhies.  Time‐varying fluid flow at the top of Earth's core derived from definitive geomagnetic reference field models , 1995 .

[34]  A. Jackson,et al.  TIME-DEPENDENCY OF TANGENTIALLY GEOSTROPHIC CORE SURFACE MOTIONS , 1997 .

[35]  Nils Olsen,et al.  The 10th generation international geomagnetic reference field , 2005 .

[36]  Susan Macmillan,et al.  Evaluation of candidate geomagnetic field models for the 10th generation of IGRF , 2005 .

[37]  Don Liu,et al.  Observing system simulation experiments in geomagnetic data assimilation , 2006 .

[38]  Nils Olsen,et al.  Extending comprehensive models of the Earth's magnetic field with Ørsted and CHAMP data , 2004 .

[39]  Mioara Mandea,et al.  Investigation of a secular variation impulse using satellite data: The 2003 geomagnetic jerk , 2007 .

[40]  Edward Crisp Bullard,et al.  Kinematics of geomagnetic secular variation in a perfectly conducting core , 1968, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[41]  David Gubbins,et al.  Geomagnetic field analysis—V. Determining steady core-surface flows directly from geomagnetic observations , 1995 .

[42]  Gauthier Hulot,et al.  Reply to comment by V. Lesur et al. on “Can core‐surface flow models be used to improve the forecast of the Earth's main magnetic field” , 2009 .

[43]  Mioara Mandea,et al.  CHAOS—a model of the Earth's magnetic field derived from CHAMP, Ørsted, and SAC‐C magnetic satellite data , 2006 .

[44]  Richard J. Blakely,et al.  The Magnetic Field of the Earth's Lithosphere: The Satellite Perspective , 1999 .

[45]  J. Cain,et al.  Geomagnetic field analysis , 1989 .

[46]  Christopher C. Finlay,et al.  Geomagnetic Secular Variation and Its Applications to the Core , 2007 .

[47]  Catherine Constable,et al.  Foundations of geomagnetism , 1996 .

[48]  S. I. Braginskii.  Torsional Magnetohydrodynamic Vibrations in the Earth's Core and Variations in Day Length , 1970 .

[49]  Mioara Mandea,et al.  Small-scale structure of the geodynamo inferred from Oersted and Magsat satellite data , 2002, Nature.

[50]  M. Alexandrescu,et al.  Geographical Distribution of Magnetic Observatories and Field Modelling , 1994 .

[51]  Jeremy Bloxham,et al.  Time‐dependent mapping of the magnetic field at the core‐mantle boundary , 1992 .

[52]  V. Lesur,et al.  Comment on “Can core-surface flow models be used to improve the forecast of the Earth's main magnetic field?” by Stefan Maus, Luis Silva, and Gauthier Hulot , 2009 .