FUV spectrum in the polar region during slightly disturbed geomagnetic conditions

We present the results of simultaneous high resolution observations of far ultraviolet (FUV) spectra/images and precipitating electrons made for the polar region during a period of slightly disturbed geomagnetic conditions. The polar region was divided into five subregions: the dayside subauroral region, dayside auroral zone, polar cap, nightside auroral zone, and nightside subauroral region. Precipitation in the dayside subauroral region was dominated by soft electrons, and the intensity of the OI 135.6 nm line relative to the NI 149.3 nm line was significantly enhanced, while electrons of a few keV energies also existed. On the other hand, the nightside subauroral region showed the hardest electron spectrum among the five subregions, and the FUV intensity was the brightest there, with substantial increase in the long Lyman-Birge-Hopfield (LBH) band. The auroral zones showed series of inverted-V events with more energetic electrons in the nightside than in the dayside, and the corresponding FUV auroral features were also brighter and broader in the nightside. In the polar cap region, discrete polar arcs were seen, with corresponding electron beams of similar to keV, embedded in the diffuse low-intensity FUV background caused by polar rain. The relative intensity of OI 135.6 nm to the NI 149.3 nm line was evaluated for the five subregions and it was found to decrease with increasing electron characteristic energy when energy was smaller than similar to 2 keV, and remained more or less the same above similar to 2 keV, which was confirmed by imulations. On the other hand, the relative intensity of the long LBH band to the short LBH band increased with increasing characteristic energy over the entire energy range up to similar to 4 keV. The OI 135.6 nm line and long LBH band intensities were compared with the energy flux of precipitating electrons, and they were found to have good correlations. When the energy flux was fitted as a function of the long LBH intensity by a power law, the power index was found to be 1.37 for the entire polar region with similar to 75 km spatial bins, similar to the value obtained previously for inverted-V events with similar spatial averages.