Solar Cycle and Seasonal Dependences of Field‐Aligned Currents

Enhancements of Birkeland field‐aligned currents (FACs) have been reported during magnetospheric substorms and geomagnetic storms. While FACs related to geomagnetic disturbances have been extensively studied, the long‐term variations of FACs remain less explored. Here, based on ∼15 years of FAC measurements from the Active Magnetosphere and Planetary Electrodynamics Response Experiment, we present quantitative studies on the solar cycle and seasonal dependences of FACs. Yearly mean FACs (both dayside and nightside) are found to increase with increasing F 10.7 solar flux, peaking ∼1 year after the F 10.7 maximum and reaching a minimum ∼1 year after the F 10.7 minimum, with a maximum‐to‐minimum FAC ratio of ∼2–3. The dayside FAC distributions (median) are characterized by a summer hemispheric peak, ∼99% and ∼35%–38% stronger than in local winter and equinoxes, respectively, while the nightside FAC in winter is only ∼14%–17% weaker than in other seasons. These seasonal patterns are consistent across all years and levels of solar activity. Nightside FACs show a strong correlation (with Spearman's rank correlation coefficients r = 0.78–0.82) with solar wind‐magnetosphere energy coupling functions, the association is weaker ( r = 0.58–0.61) for dayside FACs. These results plausibly indicate that the contribution of ionospheric conductivity dominates dayside FACs. Nightside FACs, with weaker ionospheric conductivity contributions, are “directly driven” by energy coupling processes. These findings, based on long‐term observations, will be valuable for improving FAC modeling.