At mid- and low-latitudes and during quiet geomagnetic times, electrical currents flowing in the ionosphere generate magnetic variations of a few tens of nT on the ground and at low-Earth orbit altitudes. The Sq ("solar quiet") current system takes the form of two large-scale vortices on the dayside of the Earth, one in each hemisphere. Due to a local enhancement of conductivity, 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.
Figure 1: Normalized equivalent current function, DIFI-4 model, for October 1, UT=0, 6, 12 and 18 and F10.7=100 SFU. A 14.1 kA current flows between the contours. The dip equator is depicted as a thick black line..
The Dedicated Ionospheric Field Inversion (DIFI) model is a time-varying, spherical harmonic representation of the quiet-time Sq and EEJ fields between +/- 55 degrees quasi-dipole latitudes. It is derived from a combination of Swarm satellite and magnetic observatory data. Time variations are represented by Fourier series with periods of 24h, 12h, 8h and 6h, modulated by annual and semiannual periodicities. The spherical harmonic expansion goes 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. The primary field originating in the ionosphere and the secondary field generated by induced currents in the mantle are separated by using an Earth conductivity model (provided by Alexei Kuvshinov, ETH) with a surface shell of laterally variable conductance and a 1-D conductivity profile underneath.
DIFI-4 is the most recent DIFI model. It includes 0503 L1b Swarm data and observatory data between Dec 1, 2013 and Dec 31, 2017.
Earlier versions of the model are available from the DIFI-2 and DIFI-3 pages.
The DIFI model is developed by CIRES in collaboration with the Institut de Physique du Globe de Paris (IPGP) through Swarm's Satellite Constellation Application and Research Facility (SCARF), a project funded by the European Space Agency (ESA). DIFI is an official level 2 product of the Swarm mission. DIFI models and other Swarm level 2 products are also available at https://earth.esa.int/web/guest/swarm/data-access.
| Available DIFI-4 Downloads | ||||
|---|---|---|---|---|
| Type | Format | Mbyte | Contents | |
 | Model Coefficients | ASCII | 2.2 | DIFI-4 spherical harmonic coefficients |
| Article | 3.5 | "First results from the Swarm Dedicated Ionospheric Field Inversion chain" (2016 paper describing an earlier DIFI model) | |
| Article | 2.1 | "Swarm SCARF Dedicated Ionospheric Field Inversion chain” (2013 paper describing the DIFI algorithm and the coefficient file format) | |
| Python Module | Python | ... | Software available upon request. Please email geomag.models@noaa.gov |
| Animation | AVI | .8 | Time evolution of the DIFI current system on January 1 |
| Animation | AVI | .8 | Time evolution of the DIFI current system on April 1 |
 | Animation | AVI | .8 | Time evolution of the DIFI current system on July 1 |
| Animation | AVI | .8 | Time evolution of the DIFI current system on October 1 |