  We present far ultraviolet spectra of a fast (~ 300 km/s) nonradiative shock
in the Cygnus Loop supernova remnant. Our observations were performed with the
Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, covering the
wavelength range 905-1187 Angstroms. The purpose of these observations was to
estimate the degree of electron-ion equilibration by (1) examining the
variation of O VI 1032, 1038 intensity with position behind the shock, and (2)
measuring the widths of the O VI lines. We find significant absorption near the
center of the O VI 1032 line, with less absorption in O VI 1038. The absorption
equivalent widths imply an O VI column density greatly exceeding that of
interstellar O VI along the Cygnus Loop line of sight. We suggest that the
absorption may be due to resonant scattering by O VI ions within the shock
itself. The widths of the O VI emission lines imply efficient ion-ion
equilibration, in agreement with predictions from Balmer-dominated spectra of
this shock.
