Saturation of Ionospheric Currents under Extreme Environments

Strong solar wind conditions can drive large electric potentials across planetary polar caps, but previous studies have shown that the cross polar cap potential reaches a saturation point during periods of high interplanetary magnetic field (IMF), limiting the efficiency of solar wind–magnetosphere coupling. By analogy, field-aligned currents (FACs) have been expected to show similar saturation, but observational studies to date provide no clear evidence. Here we present global magnetohydrodynamic simulations of Earth and Jupiter that demonstrate saturation of ionospheric currents, including FACs, at high IMF strengths. This current saturation reduces auroral radio power, in contrast to the monotonic scaling predicted by the radio-magnetic scaling law model. Our results suggest that FAC saturation is an important feature of magnetosphere–ionosphere coupling and may impose intrinsic limits on planetary radio emissions, with implications for interpreting radio observations of exoplanets.