  Ultraviolet spectrographs aboard the Hubble Space Telescope (HST) and the Far
Ultraviolet Spectroscopic Explorer (FUSE) have proved their value as sensitive
probes of the low-density intergalactic medium (IGM) at low redshifts (z <
0.1). Recent observations in Ly-alpha, Ly-beta, and occasional higher Lyman
lines show that warm photoionized gas in the low-z IGM may contain 20-25% of
the baryons, with a N(HI)^-1.8 distribution in column density. Measurements of
resonance lines of Si III, C III, C IV, and O VI suggest that the metallicity
of these absorbers ranges from 1-10% of solar abundance down to values below
0.003 Z(solar). A comparison of Ly-beta/Ly-alpha ratios (FUSE and HST) yields a
distribution of Doppler parameters with mean b = 31.4 +/- 7.4 km/s and median
28 km/s, comparable to values at z = 2-3. The curve-of-growth (CoG) b-values
are considerably less than widths derived from Ly-alpha profile fitting, with
mean b(CoG)/b(width) = 0.52, which suggests that low-z absorbers contain
sizable non-thermal motions or velocity components arising from cosmological
expansion and infall. A challenge for future UV spectroscopic missions (HST/COS
and SUVO) is to obtain precision measurements of Omega(IGM) and metallicities
for the strong Ly-alpha absorbers that dominate the IGM baryon content. This
program will require accurate determinations of: (1) curves of growth using
higher Lyman series lines; (2) the ionizing radiation field at 1-5 Ryd; and (3)
characteristic sizes and shapes of the absorbers.
