Confirmation of existence of the small-scale field-aligned currents in middle and low latitudes and an estimate of time scale of their temporal variation

The magnetic data obtained by the SWARM (the Earth's Magnetic Field and Environment Explorers) satellites in middle or low latitudes during the initial 2months after launch were analyzed, when they flew nearly on the same orbit with variable time separation ranging from 5 to 100s. It was confirmed that the small-scale magnetic fluctuations having period around 10-30s are the manifestation of spatial structure of small-scale field-aligned currents along the orbits. From the statistical relation between correlation coefficients and two satellite separation in time, the typical time scale of temporal variation of the field-aligned current system is estimated to be around 200s for meridional component and 340s for zonal components of the magnetic fluctuations, respectively. Existence of shorter time scale around 30-50s was also found. These results suggest that the main source of current generation is the acoustic mode of atmospheric gravity waves.