. Background

  Atmospheric changes such as ozone depletion and global warming, which are attributed to human activities, are becoming a serious environmental issue. A major cause of the ozone depletion in the stratosphere is due to trace gases such as chlorine and bromine compounds originated from human-made chlorofluorocarbons (CFCs), halogen containing substances, and others. After the Montreal Protocol [United Nations Environmental Programme, 1987] and its amendments, the total combined abundance of ozone-depleting compounds in the lower atmosphere peaked around 1994, and is now slowly declining. The WMO report [2006] says, the recovery of the ozone abundance is estimated around 2060-2075, however the results calculated from chemistry-climate models for the future ozone amount vary considerably with each other. The ozone depletion has been observed not only in the polar latitudes, but also in the mid- and lower latitudes. Furthermore, stratospheric trace gases such as ozone, carbon dioxide and water vapor have serious effects on climate change.

  To perform quantitative assessment for the future we need three-dimensional and simultaneous measurements of ozone and trace gases in the stratosphere. Also important is global accurate data with high resolutions in space and time. A space-bone submillimeter-wave limb-emission sounding has major advantages for measuring these trace gases during day and night in the stratosphere globally.

  The objective of the SMILES (Superconducting Submillimeter-Wave Limb-Emission Sounder) mission is to demonstrate highly sensitive submillimeter-wave soundings and to monitor global distributions of the stratospheric trace gases. For these requirements a superconductive low-noise receiver with a mechanical 4-K refrigerator will be used in space for the first time.