We present a detailed case study of the electrodynamics of a high-latitude trough observed at ~12 UT (~1 MLT) on March 8, 2008 using multiple instruments, including incoherent scattering radar (ISR), GPS total electron content (TEC), magnetometers and auroral imager. The electron density within the trough dropped as much as 80% within 6 minutes. This trough was collocated with a counter-clockwise convection flow vortex, indicating divergent horizontal electric fields and currents. Together with a collocated dark area shown in auroral images, the observations provide strong evidence for an existence of downward field-aligned currents (FACs) collocated with the high-latitude trough. This is further supported by Assimilative Mapping of Ionospheric Electrodynamics (AMIE) results. In addition, the downward FACs formed at about the same time as a substorm onset and east of the Harang reversal, suggesting it is part of the substorm current wedge. It has long been a puzzle why this type of high-latitude trough predominantly occurs just east of the Harang reversal in the post-midnight sector. We suggest that the high-latitude trough is associated with the formation of downward FACs of the substorm current system, which usually occur just east of the Harang reversal. In addition, we find that the ionospheric electron temperature within the high latitude trough decreases in the F region while increasing in the E region. We discuss possible mechanisms responsible for the complex change in electron temperature, such as ion composition change and/or presence of downward FACs.