The longitudinal dependence of the local I_60μm-I_100μm flux density ratio at high galactic latitude

We have studied the local 60 μm and 100 μm emission for a region in the northern half of the first two quadrants of the Galaxy (42°≤l≤146° & 22°≤b≤50°), using the IRAS Sky Survey Atlas. We have unfolded the 100 μm data and have determined a dust layer vertical scale height of 117 pc. We find the average I_60μm-I_100μm flux density ratio in this region to be 0.19 (±0.01), in excellent agreement with values previously presented in the literature. We also find what appears to be a longitudinal dependence of the local I_60μm-I_100μm flux density ratio. For the galactic longitude range of 42°≤l≤ 100°, the average flux density ratio is found to be 0.25 (± 0.02) while for the longitude range of 101°≤l≤146° it is found to be 0.16 (± 0.02). We find that a change in the UV fraction of the interstellar radiation field can explain the change in the infrared flux density ratio. Further, we find the higher flux density ratio region to have a vertical scale height ∼ 20% less than the scale height associated with the lower flux density ratio region. A. change in the thickness of the dust layer may be the cause of the altered radiation field at these high latitudes.