5.1.5
Models of the internal and
external geomagnetic fields
The magnetic field is commonly modelled by expressing the field as a series of spherical harmonic terms. At the present time (2000-2010) there is much scientific activity in the field modelling community and in source field reconstruction. This is due to several successful satellite magnetometer surveys, such as those of Ørsted and Champ.
5.1.5.1 Models of the internal field
Scientific models of the internal field now extend to around spherical harmonic degree 100, equivalent to a minimum wavelength of 400 km at the Earth’s surface. However the effective amplitudes of these high degree terms decrease rapidly with increasing altitude above the Earth’s surface. The core field, which dominates for degrees less than about 14 at the Earth’s surface, can be readily simplified to that of a simple inclined and offset dipole or a low-degree model for spacecraft operation above a few hundred km altitude.
Although they are by no means the only
recognised high quality models, the IGRF
models are products of the
international community of geomagnetic scientists, updated on a five-year
basis, assuming a linear secular variation between updates.
The descriptive annex to this Chapter (Annex E) describes in
more detail the current revision of the IGRF, and further information on other
internal field models.
5.1.5.2 Models of the external field
There are a number of dynamic models of the external field. These are typically either empirical (in the sense of a simplified mathematical model fit to measured data), analytic (based on physical models of the various external current systems), or magneto-hydrodynamic (or ‘general circulation models’). Of the latter type, the science is rapidly evolving and whilst, in principle, they should fully capture the details of magnetospheric processes, such models are judged not yet mature enough to serve as a reliable standard.
Of those models that are currently maintained and updated in the light of new data and new field parameterisations, the model of Tsyganenko (revised repeatedly between 1987 and 2006 [RN.31] [RD.98] [RD.99] [RD.111] [RD.112]) is well used and is empirical in nature. The model of Alexeev et al (2001) [RN.30] is described as an analytic model. This model is at the core of a proposed International Standards Organisation (ISO) standard external field model [RD.113].
The availability of suitable inputs to external models is a factor that affects whether they can be useful in common applications. Alexeev et al (2001) [RN.30] has inputs of date, time, solar wind density and velocity, and Tsyganenko 1996 [RN.31]has inputs of solar wind pressure, Dst and IMF By and Bz components. Tsyganenko 2002 to 2005 [RD.98] [RD.99] models require a more complex set of inputs.
Further discussion of these models and of their applications are dealt with in Annex E.