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-2 model, for April 1, UT=0, 6, 12 and 18 and F10.7=100 SFU. A 13.7 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-2 is the most recent DIFI model. It includes 0408 L1b Swarm data between Nov 23, 2013 and Jan 30, 2016, and observatory data between Nov 27, 2013 and Nov 2, 2015.

DIFI-2015b is the model described in the Earth, Planets and Space article (see below). It includes 0405 L1b Swarm data between Dec 1, 2013 and July 17, 2015, and observatory data between Dec 1, 2013 and Feb 22, 2015. An earlier version of the model, DIFI-2015a, was derived from a smaller Swarm dataset and using a less recent mantle conductivity model.

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-2, DIFI-2015a and other Swarm level 2 products are also available at https://earth.esa.int/web/guest/swarm/data-access.

Available DIFI-2 Downloads
	Type	Format	Mbyte	Contents
	Model Coefficients	ASCII	2.2	DIFI-2 spherical harmonic coefficients (recommended version)
	Model Coefficients	ASCII	2.2	DIFI-2015b spherical harmonic coefficients (version described in the EPS paper)
	Model Coefficients	ASCII	2.2	DIFI-2015a spherical harmonic coefficients (earlier version)
	Article	PDF	3.5	"First results from the Swarm Dedicated Ionospheric Field Inversion chain" (2016 EPS paper describing the 2015b model and how it was calculated)
	Article	PDF	2.1	"Swarm SCARF Dedicated Ionospheric Field Inversion chain” (2013 EPS paper describing the DIFI algorithm and the coefficient file format)
	Matlab Software	MATLAB	...	Matlab code to calculate the ionospheric field at a given time and location (TBA)
	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