Ionospheric and induced field leakage in geomagnetic field models, and derivation of candidate models for DGRF 1995 and DGRF 2000

As part of the 9th generation of the IGRF defined by IAGA, we proposed a candidate model for DGRF 1995 and two candidate models for DGRF 2000. These candidate models, the derivation of which is described in the present note, are based on the "Comprehensive Model, Version 4 (CM4)", and on the "Orsted Main and Secular Variation Model (OSVM)"; two parent models that have been published elsewhere (Olsen, 2002; Sabaka et al., 2004; Lowes and Olsen, 2004). However, the main field part of OSVM is contaminated by "leakage" of the ionospheric field and its induced counterpart, which affects mainly the zonal coefficients g(1)(0), g(3)(0),..., by 1-2 nT. We describe the reason for this contamination, and present a method to correct for it. Since not only OSVM but probably all main field models that are derived primarily from data around local midnight suffer from this effect, the presented scheme can also be applied to approximately correct these models.