Low-latitude effects of high-latitude field-aligned currents

High-latitude field-aligned currents (FACs) reflect, in steady-state, the force balance between magnetospheric plasma dynamics and the collisional coupling of plasma to the neutral atmosphere in the ionosphere. Assessing the impact of high-latitude FACs at low latitudes is difficult for at least two reasons. First, FACs are primarily inferred from magnetometer measurements in low-Earth orbit by estimating the radial current using horizontal magnetic field perturbations and converting it to a FAC using a geometric factor. While this yields a locally correct estimate of the FAC density, the magnetic field generated by a radial current system differs from that generated by the corresponding FAC system when field lines are not radial. As a result, the magnetic field of the horizontal component of FACs, including their remote magnetic field observed at low latitudes, are neglected. Second, in many numerical simulations, FACs are coupled to the ionosphere only at high latitudes, while boundary conditions are imposed at lower latitudes, arguably making it difficult, from a fundamental physics perspective, to trace how high-latitude forcing influences low latitudes.
Here we use AMPERE estimates of high-latitude FACs at 10-min resolution derived from magnetometer measurements on the Iridium satellite constellation to quantify their low-latitude impact. FACs in both polar regions are used to calculate the remote magnetic field using the integration method of Engels and Olsen (1998, https://doi.org/10.1016/S1364-6826(98)00094-7). A recently developed magnetosphere-ionosphere coupling model (Laundal et al. 2025, https://doi.org/10.5194/angeo-43-803-2025) is used to compute the associated penetration electric field. The resulting magnetic and electric fields are compared with observations at low latitudes.