Comparison of large-scale field-aligned currents under sunlit and dark ionospheric conditions

[1] The present study statistically compares large-scale field-aligned currents (FACs) under sunlit and dark ionospheric conditions. A total of similar to 74,000 auroral oval crossings are selected from magnetic field measurements from the DMSP F7 and F12 to F15 satellites. For the dayside FAC it is reconfirmed that both current intensity and density are statistically larger in the illuminated events than in the unilluminated events. As for the nightside FAC, in contrast, a few important features become clear for the first time, which can be summarized as follows: ( 1) At 20 <= MLT < 02 both R1 and R2 intensities tend to be larger when the ionosphere is dark than when it is sunlit. ( 2) Although dependence on the ionospheric condition is less clear for the FAC density, a systematic preference of the occurrence of strong FACs for the dark ionosphere can be found for both R1 and R2 currents in the dusk-to-premidnight ( 16 <= MLT < 22) sector and for the R1 current in the postmidnight sector ( 00 <= MLT < 02). ( 3) For both FAC intensity and density the difference between the illuminated and unilluminated events tends to increase with increasing geomagnetic activity as measured by the IMF B-Z component. Result 1 can be partially explained in terms of the interhemispheric asymmetry of the magnetospheric configuration, which, however, should not affect the FAC density. Therefore result 2 strongly suggests that the solar-induced conductivity controls the M-I coupling in a different way in certain nightside MLT sectors than in dayside sectors.