At mid- and low-latitudes during quiet geomagnetic times, electrical currents flowing in the ionosphere generate magnetic variations of a few tens of nanoteslas (nT) at the Earth's surface and at low-Earth orbit altitudes. The Sq ("solar quiet") current system forms two large-scale vortices on the dayside, one in each hemisphere. A stronger current, the equatorial electrojet (EEJ), flows along the dip equator and generates magnetic signatures that can reach up to 150 nT at some locations.
The Extended Dedicated Ionospheric Field Inversion (xDIFI) model is a time-varying, spherical harmonic representation of the quiet-time Sq and EEJ magnetic fields between +/- 55 degrees quasi-dipole latitudes. It is derived from a combination of CHAMP and Swarm satellite data, as well as ground-based magnetic observatory data. Temporal variations are represented using Fourier series with periods of 24, 12, 8 and 6 hours, modulated by annual and semiannual periods. The spherical harmonic expansion extends to degree 60 and order 12 in geomagnetic dipole coordinates. Solar activity (represented by the F10.7 index) dependence is also included in the model. DIFI provides both the primary (external) and induced (internal) magnetic fields components.
xDIFI-1 is not the most recent xDIFI model; xDIFI-2 is a more recent version. xDIFI-1 includes L3 CHAMP data between January 1, 2001 and December 31, 2009; 0602/0603 L1b Swarm data between December 1, 2013 and June 30, 2021; and observatory data between January 1, 2001 and June 30, 2021.
Model coefficients and Python software are available upon request. Please email ciresgeomag@colorado.edu.
The xDIFI model is developed by CIRES in collaboration with the Institut de Physique du Globe de Paris (IPGP) through the Swarm DISC (Swarm Data, Innovation, and Science Cluster), a project funded by the European Space Agency (ESA). xDIFI is an official level 2 product of the Swarm mission. xDIFI models and other Swarm level 2 products are also available at http://swarm-diss.eo.esa.int.