CHAOS-a model of the Earth's magnetic field derived from CHAMP, Ørsted, and SAC-C magnetic satellite data

Abstract

S U M M A R YWe have derived a model of the near-Earth magnetic field (up to spherical harmonic degree n = 50 for the static field, and up to n = 18 for the first time derivative) using more than 6.5 yr of high-precision geomagnetic measurements from the three satellites Ørsted, CHAMP and SAC-C taken between 1999 March and 2005 December.Our modelling approach goes in several aspects beyond that used for recent models: (i) we use different data selection criteria and allow for higher geomagnetic activity (index Kp ≤ 2o), thus we include more data than previous models; (ii) we describe the temporal variation of the core field by splines (for n ≤ 14); (iii) we take magnetometer vector data in the instrument frame and co-estimate the Euler angles that describe the transformation from the magnetometer frame to the star imager frame, avoiding the inconsistency of using vector data that have been aligned using a different (pre-existing) field model; (iv) we account for the bending of the CHAMP optical bench connecting magnetometer and star imager by estimating Euler angles in 10 day segments and (v) we co-estimate degree-1 external fields separately for every 12 hr interval.The model provides a reliable representation of the static (core and crustal) field up to spherical harmonic degree n = 40, and of the first time derivative up to n = 15.

NOAA/NGDC candidate models for the 11th generation International Geomagnetic Reference Field and the concurrent release of the 6th generation Pomme magnetic model

“…Recent examples include the Pomme magnetic model (Maus et al, 2006), the CHAMP, Ørsted and SAC-C based CHAOS model (Olsen et al, 2006) and the GFZ Reference Internal Magnetic Model, GRIMM (Lesur et al, 2008). Such more sophisticated main field models are widely used in geomagnetism for studies of the dynamics of the earth's core (e.g.…”

On the Usage of Geomagnetic Indices for Data Selection in Internal Field Modelling

“…Replacing magnetic londitudes with Magnetic Local Time (M LT ) helps in distinguishing such features in ionospheric currents that depend on the location of sub-solar point with respect to the magnetic pole (Baker and Wing 1989). QD-latitudes ± 60 or 55 are often used as the demarcation between polar and non-polar areas which are handled differently in data processing (Olsen et al 2006;Lesur et al 2010;Olsen et al 2015).…”

“…The Solar-Magnetic (SM) coordinate system has appeared to be the most suitable for ring current and magnetopause current parametrization while for magne-totail currents the Geocentric-Solar-Magnetospheric (GSM) coordinate frame is a more appropriate way for data modeling (Maus and Lühr 2005;Olsen et al 2006) (for the introduction of SM and GSM coordinates, see Kivelson and Russell (1995)). The Quasi-Dipolar (QD) coordinate system introduced by Richmond (1995) is a good framework to represent ionospheric currents and FACs which still at Low Earth Orbit (LEO) altitudes are organized according to the ambient magnetic field whose topology can be described accurately enough with IGRF models (Sabaka et al 2015).…”

“…Magnetically quiet times are associated with the conditions of |Dst| < 30 nT and |dDst/dt| or |dRC/dt| being less than 2 or 5 nT /h (Olsen et al 2014;Maus et al 2010;Sabaka et al 2015). CHAOS, CM-5 and SIFM characterize weak global activity with the condition Kp ≤ 2 o (Olsen et al 2006;Sabaka et al 2015;Olsen et al 2015). POMME-6 uses the am index instead of Kp with the limits am < 12 for mid latitudes and am < 27 for high latitudes (Maus et al 2010).…”

Ergodicity of the recent geomagnetic field

“…To avoid the problem of the evident model heterogeneity we finally resorted to two couples of more homogeneous models, in terms of model construction and data selection. In particular, we analysed the differences between POMME 3.0 and POMME 6.0 (Maus et al, 2006; and between CHAOS-1 and CHAOS-2 from 2006 to 2010 (Olsen et al, 2006;2009), where the older versions of each couple of models acts as a predictive model and the most recent one as the definitive one; POMME models and CHAOS-1 model include their MF, SV and SA terms, so we considered, together with MF and SV terms, all SA terms (case a -empty squares in Fig.4) or without them (case b -empty circles in Fig.4) for POMME analyses, and all SA terms for CHAOS analysis (case c-full squares in Fig.4). Both analyses of POMME models (cases a and b) were extended up to N=10, but no significant change appears with lower and higher degrees (verified for N=8 and 16); the analysis of CHAOS models was limited to N=14.…”