Field-aligned currents (FACs) play an important role in the magnetosphere-ionosphere coupling system for transporting electromagnetic energy. Although large-scale region 1 (R1) and region 2 (R2) FACs have been statistically investigated for a few decades, there have been relatively fewer investigations of the fundamental characteristics of mesoscale (ranging from about 10 to 100 km) FACs and the relationship between them and large-scale FACs. Here we report some fundamental characteristics of mesoscale FACs on the basis of a statistical analysis using long-term Akebono satellite data from March 1989 to December 1996. We found that the current density of mesoscale FACs with a spatial scale of 10-80 km at ionospheric altitude is on average the largest in the dayside region of 75°-82° in corrected geomagnetic latitude and 0600-1500 in corrected geomagnetic local time. The current density is especially enhanced under the sunlit condition in summer. The intensities and distributions of mesoscale FACs change with interplanetary magnetic field conditions. We also found altitude dependence of mesoscale FACs for the first time. Current densities of mesoscale FACs gradually increase with the altitude. Mesoscale FACs tend to distribute in the R1 current region, and the current density in the morning to noon sector (downward Rl FAC region) is stronger than that in the noon to evening sector (upward R1 FAC region). Further, the seasonal variations of mesoscale FACs correspond to those of large-scale FACs on the dayside but are opposite to those of large-scale FACs on the nightside.
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