Fast Auroral Snapshot observations of the dependence of dayside auroral field-aligned currents on solar wind parameters and solar illumination

The solar illumination dependence of solar wind-magnetosphere coupling has been studied by examining the dependence of auroral zone field-aligned currents on solar wind parameters. A database of Region I currents from ∼900 crossings of the auroral zone by Fast Auroral Snapshot (FAST) was parameterized by magnetic local time, invariant latitude, and whether the foot point was illuminated. The magnitudes of dayside currents (∼200 events) were correlated with solar wind parameters using the technique of rank correlation. Very significant, strong correlation between dayside sunlit currents and solar wind parameters such as B_tsin(θ/2)v² and B _tsin(θ/2)P_d^1/2 was observed, consistent with a reconnection source. There was no correlation with upstream parameters for dayside Region I currents when the ionospheric foot point was in darkness. This is the first experimental evidence showing that the correlation of dayside Region I currents to upstream parameters depends on ionospheric illumination. Both the strong dependence of the correlation on solar illumination (ionospheric conductivity) and the weakness of observed correlations, compared to those obtained in previous studies of the cross polar cap potential, may be interpreted to imply that the reconnection process is a voltage, rather than a current, source. However, several theoretical studies have suggested that reconnection does not act as either a pure voltage source or a current source. In addition, because the mapping of the field-aligned currents was not examined, the observed difference between the sunlit and dark events could be explained by current preferentially flowing in the hemisphere with the higher conductivity if the source were on closed field lines.