The Gaseous ISM: Observations with the Wisconsin H-alpha Mapper (WHAM)
  The Wisconsin H-Alpha Mapper (WHAM) is a new facility dedicated to the study of faint optical emission lines from diffuse interstellar gas. During its first 18 months of operation, WHAM carried out a survey of the interstellar H-alpha emission associated with the warm, ionized component of the interstellar medium. The observations consisted of 37,000 spectra obtained with a one degree diameter beam on a 0.98 X 0.85 degree grid (lxb), covering the sky above declination -30 degrees. This survey provides for the first time a detailed picture of the distribution and kinematics of the diffuse ionized hydrogen through the H-alpha line comparable to surveys of the neutral hydrogen obtained through the 21 cm line. Preliminary reduction of the data from selected portions of the sky reveal that the interstellar H II has a complex distribution, with long filaments and loop-like structures extending to high Galactic latitudes and superposed on a more diffuse background. Apart from the H-alpha sky survey, WHAM has also detected for the first time faint diagnostic emission lines in selected directions, [O I] 6300, [O III] 5007, and He I 5876, which provide information about the physical state of the gas and clues about the source of the ionization. Maps of [S II] 6716 and [N II] 6584 over limited regions of the sky are providing information about variations in the temperature and ionization conditions within the Galactic disk, and the detection of faint optical emission lines from high velocity clouds is probing conditions in the halo. 
R. J. Reynolds, L. M. Haffner, and S. L. Tufte