Observation of the Far-ultraviolet Continuum Background with SPEAR/FIMS
  We present the general properties of the far-ultraviolet (FUV; 1370-1720A)
continuum background over most of the sky, obtained with the Spectroscopy of
Plasma Evolution from Astrophysical Radiation instrument (SPEAR, also known as
FIMS), flown aboard the STSAT-1 satellite mission. We find that the diffuse FUV
continuum intensity is well correlated with N_{HI}, 100 $\mu$m, and H-alpha
intensities but anti-correlated with soft X-ray. The correlation of the diffuse
background with the direct stellar flux is weaker than the correlation with
other parameters. The continuum spectra are relatively flat. However, a weak
softening of the FUV spectra toward some sight lines, mostly at high Galactic
latitudes, is found not only in direct-stellar but also in diffuse background
spectra. The diffuse background is relatively softer that the direct stellar
spectrum. We also find that the diffuse FUV background averaged over the sky
has about the same level as the direct-stellar radiation field in the
statistical sense and a bit softer spectrum compared to direct stellar
radiation. A map of the ratio of 1400-1510A to 1560-1660A shows that the sky is
divided into roughly two parts. However, this map shows a lot of patchy
structures on small scales. The spatial variation of the hardness ratio seems
to be largely determined by the longitudinal distribution of spectral types of
stars in the Galactic plane. A correlation of the hardness ratio with the FUV
intensity at high intensities is found but an anti-correlation at low
intensities. We also find evidence that the FUV intensity distribution is
log-normal in nature.
