This, combined with the fact that the external magnetic field perturbations were small due to a very long period of low solar activity, serve studies of internal fields.
Further, it is worth noting that whatever the bias sign and because of the presence of occupied and/or free surface states in the PS layer, a part of the electric field spreads systematically into it, resulting in the PS polarization and the screening of internal fields at the metal/PS interface, that is why the metal/PS contact can be considered as quasi-ohmic.
They provide a detailed picture of the internal field, of the core field secular variation and secular acceleration at Earth's surface.
To obtain the main (dynamo) field model, we first take the average of models of the internal field (sources within the sphere r = rperigee) computing on all the universal times (practically at 0 00 UT, 1 00 UT to 23:00 UT).
This will allow describing the evolution of the internal field and of its time change in a suitable way for map representation, as: (2 where Eint is the internal part of the geomagnetic element E considered, Δt is the time (in years) elapsed since the beginning of the given segment, and a0, a1, a2 are constants determined by fit.
The dominant component of the internal field is that of a centred dipole.
The panels in a column illustrate, from top to bottom, the evolution of the internal field (E11) and of its first and second time-derivatives, respectively.
Therefore, the internal induced magnetic fields on average have larger contributions to surface observations for larger storms, even if the increase of the contribution ratio of the internal field is not dramatic.
The geographical pattern of the secular variation of the internal field (Ė11) at much smaller scales (country size) could be decided by the combination of the 22-year and the ~80-year variations.
