Mid and low-latitudinal ionospheric field-aligned currents derived from the Swarm satellite constellation and their variations with local time, longitude, and season

Abstract

We use the Field Aligned Currents (FAC) data of the 10 quietest days of each month of 2014–2018, the first four years of FAC observational data, derived from Swarm A and C satellites observations to study their characteristics within 15–55° latitude in the northern and −15° to −55° latitude in the southern hemispheres. Results show that in summer months, the noon time FAC have the largest variations. Over the same sector region and the same local time, the FAC have the same trend irrespective of season.Examinations of seasonal patterns indicate that, both dawn and dusk times FACs in both hemispheres always have negative (southbound) variations during summer months. In winter months the dusk time FAC becomes positive only in the southern hemisphere, while the dawn time FAC is ambiguous as it becomes negative only at low latitudes. The noon time FAC is strongly positive (northbound), in both hemispheres in summer months, but during winter months it becomes southbound at mid-latitudes and remains northbound at low latitudes. Over the African, Asian and American sectors, the summer months FAC have identical structure, while in winter months the direction of the dawn and noon times FACs are ambiguous.At the foci points of the solar quiet (SQ) current, the northern hemispheric FAC has the same variation as those observed by Park et al. (2011) and Luhr et al. (2015) during summer months, while during winter months the noon time FAC is still weakly positive. The latitudinal variation of the FAC with respect to longitudes shows that the southern hemispheric FAC at mid-latitudes is always negative at eastern longitudes irrespective of season. It suggests that the FAC is longitudinally dependent.Unlike previous ground based studies, the satellite data not only shows a single prominent negative (southbound) FAC over the African sector region around equinoxes, but it shows that during the March equinox the dusk time FAC is always negative in both hemispheres except at western longitudes 45°, where it has positive patterns. In the middle of summer and winter (months 7 and 1), the noon time FAC varies from positive in summer to negative in winter and the dawn time FAC varies from negative in summer to positive in winter which agrees with the theoretical prediction. The dusk time FAC in the northern hemisphere remains negative which agrees the observations of Park et al. (2011) and Luhr et al. (2015), while in the southern hemisphere it changes from negative in summer to positive in winter.