The Hot Halo

The detection in the 1950s of halo high-velocity clouds in Ca ii absorption lines led to the prediction of the presence of a “Galactic Corona”. Starting in 1973, the hot phase of the local ISM was probed with the Copernicus satellite, while X-ray sensors detected a di use X-ray background of, at that time, unknown origin. Since 1978, the hot gas in the vast stretches of halo has become accessible through absorption spectra obtained with the IUE, HST and FUSE satellites. This chapter reviews the essentials of both absorption and emission by hot halo gas as well as theories for origin and properties of the hot phase. The large body of results on the hot Galactic Halo obtained with FUSE using extragalactic sight lines is summarized at the end.