E.3
Models of the external
geomagnetic field
Spherical
harmonic models of external fields, e.g. CM4, are typically no higher than
degree two (i.e. quadrupolar) and capture only the quasi-static elements of the
external field. There is usually some dependence on the Dst
(or similar) geomagnetic index, which parameterises variations in the symmetric
ring current intensity. Neither the IGRF
nor WMM have an external
field component.
For satellite operations closer to the Earth, the spherical harmonic models of Olsen et al [RD.138] or Sabaka et al [RD.140] can be useful. However, these models are inaccurate on the dayside of the Earth (approximately 0700-1700 local solar time), at geomagnetic latitudes above about 55 degrees, or during more active geomagnetic conditions (approximately Kp>3). They are also likely to be degraded in accuracy beyond low-Earth orbit (>800km altitude), where the magnetic survey satellites that measure the base data for these models are flown.
Non-spherical
harmonic models use a variety of geomagnetic and solar wind data to
parameterise the various external current systems. Dynamic models
are widely used and combined with internal models such as IGRF. The models of
Tsyganenko and of Alexeev and co-workers both use the IGRF as the core field.
They therefore both provide a consistent model of the total field measured by
any in-situ spacecraft, from approximately 2-70 Earth radii.
Older
dynamic models also exist, dating back to the 1960s, and these are occasionally
used in the space science and engineering community, e.g. when using radiation
belt models of that era. These include models due to Mead and