The High Velocity Clouds (HVCs) detected in HI are an important constituent of the infalling gas fueling the Galaxy. Our understanding the HVCs is limited by the use of HI as the only velocity resolved gas tracer. One of the critical issues is the temperature and densities of these HVCs, and whether there is H2 molecular gas in them. The [C II] 158-micron line is the strongest infrared line emitted by the interstellar gas and is an excellent tracer and probe of both the diffuse gas in the cold neutral medium (CNM) and the warm ionized medium (WIM). Members of our team have used [CII] is an efficient tracer of the dark H2 gas in the clouds in the Galactic plane that are not traced by CO or HI. Here we propose HIFI [CII] observations of the HVCs in Complex C observed by Spitzer, and in the DRACO and UMAEAST fields, selected from the early results of diffuse interstellar dust observations by Planck. It is important to search for the hidden gas using other tracers such as [C II] because it gives us information about the transfer of mass from the halo to the disk. In addition to the C+ emission in the HVCs, our observation will characterize the WIM and the CNM clumps in a few intermediate velocity clouds (IVCs) and high latitude local HI gas with high sensitivity.
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.