Lognormal intensity distribution of the FUV continuum background shortward of Ly-alpha

Abstract: The diffuse far-ultraviolet (FUV) continuum radiation "longward" of Ly-alpha (1216A) is well known to correlate with the dust emission at 100 um. However, it has been claimed that the FUV continuum background "shortward" of Ly-alpha shows very weak or no correlation with the 100 um emission. In this paper, the observational data of the diffuse FUV radiation by the Far Ultraviolet Spectroscopic Explorer is reexamined in order to investigate the correlation between the diffuse FUV radiation shortward of Ly-alpha and the 100 um emission. Large fluctuations were confirmed in the linear-linear correlation plots, but good correlations were found in the log-log plots. The large fluctuations in the linear-linear plots, and thus poor correlations, between the FUV and 100 um intensities were attributed to the lognormal property of the FUV intensity distribution. The standard deviation of the intensity distribution of the FUV radiation shortward of Ly-alpha was found to be sigma_log I = 0.16-0.25. The result is consistent with that obtained not only for the FUV radiation longward of 1216A, but also with the dust column density measurements of various molecular clouds. This implies that most of the diffuse FUV radiation shortward of Ly-alpha is dust-scattered light in the turbulent interstellar medium. The diffuse FUV data obtained from the Voyager missions was also investigated. However, much wider random fluctuations were found compared with the FUSE data, which is most likely due to the systematic difficulties in data reduction of the Voyager data.