Dipolar elementary current systems for ionospheric current reconstruction at low and middle latitudes

Abstract

The technique of spherical elementary current systems (SECS) is a powerful way to determine ionospheric and field-aligned currents (FAC) from magnetic field measurements made by low-Earth-orbiting satellites, possibly in combination with magnetometer arrays on the ground. The SECS method consists of two sets of basis functions for the ionospheric currents: divergence-free (DF) and curl-free (CF) components, which produce poloidal and toroidal magnetic fields, respectively. The original CF SECS are only applicable at high latitudes, as they build on the assumption that the FAC flow radially into or out of the ionosphere. The FAC at low and middle latitudes are far from radial, which renders the method inapplicable at these latitudes. In this study, we modify the original CF SECS by including FAC that flow along dipolar field lines. This allows the method to be applied at all latitudes. We name this method dipolar elementary current systems (DECS). Application of the DECS to synthetic data, as well as Swarm satellite measurements are carried out, demonstrating the good performance of this method, and its applicability to studies of ionospheric current systems at low and middle latitudes.

Keywords: Ionospheric current; Magnetic field; SECS method.

Fig. 1

The current density in the 2D-CF DECS on the left and 2D-DF SECS/DECS on the right. Color represents dipole latitude. In the illustrated case ${\theta }_0={55}^{\circ }$

Fig. 2

Test T8. Red and blue solid lines show the magnetic field components measured by Swarm-A/C, respectively, while the dashed lines show the fit from the Swarm/DECS analysis

Fig. 3

Test T8. Latitude profiles of the the current components between the Swarm-A/C satellites. Black solid lines show the simulated current, while red dashed lines show the Swarm/DECS analysis result. In the top panel upward current is positive

Fig. 4

Test T8. Map of the E-region horizontal current. From left to right: the TIE-GCM model current, result from the Swarm/DESC analysis, and the difference between the two. Same scale is used in each panel. Swarm-A/C orbits are shown as black lines. Compare with the latitude profiles shown in Fig. 3

Fig. 5

Test T8. Map of the radial current. From left to right: The TIE-GCM model current, result from the Swarm/DESC analysis, and difference between the two. Swarm-A/C orbits are shown as black lines. Compare with the latitude profiles shown in the top panel of Fig. 3

Fig. 6

Event E2. Red and blue solid lines show the magnetic field components measured by Swarm-A/C, respectively, while the dashed lines show the fit from the Swarm/DECS analysis

Fig. 7

Event E2. Latitude profiles of the the current components between the Swarm-A/C satellites obtained from the Swarm/DECS analysis. Black solid line in the upper panel shows the radial current from the quad method (Ritter et al. 2013)

Fig. 8

Event E2. Maps of the E-region horizontal and radial currents. From left to right: the horizontal current from Swarm/DECS analysis the radial current from the Swarm/DECS analysis and the radial current from the quad method (Ritter et al. 2013). Swarm-A/C orbits are shown as black lines. Compare with the latitude profiles shown in Fig. 7