INFRARED CIRRUS, WHAT HAVE WE LEARNED

IRAS discovered thermal emission from infrared cirrus in 1983 and COBE has extended observations of these interstellar dust clouds to additi onal IR wavelengths. IR studies of individual high galactic latitude c louds are combined with sensitive optical measurements of scattered li ght from the same clouds. At least 4 distinctly different types of gra ins are required to explain the complex relationships revealed by comb ining observations of various samples of clouds made on different spat ial scales and in a number of spectral bands. In addition to the domin ant 22 K component seen most prominently at 100 mum, many clouds are b right at 12 and 25 mum, indicating dust at temperatures well above 100 K. Optical studies suggest at least two more types of grains are need ed to explain the measured blue and red colors and the highly variable ratio of blue light to 100 mum thermal emission that is observed. We show a new display of 100 mum cirrus on a global scale and we determin e the power density in a typical cloud to spatial scales of a few arcs ec.