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Understanding the magnetic field-aligned currents (FAC) is the key to understanding magnetosphere-ionosphere coupling. Unfortunately, FAC are very difficult to observe. Satellites can measure FAC using magnetometers, but these are point measurements rapidly moving in space. Statistical average distributions of FAC are available from satellite data but these are unsuitable for detailed real time studies. Arrays of ground-based magnetometers (e.g. IMAGE) can estimate FAC but rely on knowing the full conductance distribution, which is generally not available. Should the full conductance distributions become available, the technique still suffers from an inherent low spatial resolution of > 120 km. Incoherent radars (e.g. EISCAT) can in theory directly measure FAC but nobody has done so convincingly in practice. This technique also suffers from a very low temporal resolution because of the need to scan the radar. Coherent radars (e.g. CUTLASS and STARE) can measure the ionospheric electric field distribution with high spatial and temporal resolution. However, they lack the ability to measure conductances. Computing FAC from electric field distributions is theoretically well established and requires knowledge of the Pedersen and Hall conductances as well as their spatial gradients. It is exactly this key information, which the multi- wavelength imager can supply in calibrated quantitative units. By combining data from the CUTLASS or STARE radars and the new optical instrument, the quantitative FAC can be computed with a spatial (20-45 km) and temporal (10-30 s) resolution determined by the radar used. This technique would allow the detailed morphology of FAC over a large area (520x520 km) to be studied for the first time. This is significant because FAC are the single most important coupling mechanism between the ionosphere and magnetosphere, and carry the majority of the energy deposited from the magnetosphere into the ionosphere. Fundamental questions such as "When is the magnetosphere a current or voltage generator in the ionosphere?" can be answered. In addition, the spatial arrangement of FAC associated with aurora, and their variations with time, can be studied for the first time with unprecedented resolution.
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