  We present direct estimates of the mean sky brightness temperature in
observing bands around 99GHz and 242GHz due to line emission from distant
galaxies. These values are calculated from the summed line emission observed in
a blind, deep survey for specrtal line emission from high redshift galaxies
using ALMA (the 'ASPECS' survey). In the 99 GHz band, the mean brightness will
be dominated by rotational transitions of CO from intermediate and high
redshift galaxies. In the 242GHz band, the emission could be a combination of
higher order CO lines, and possibly [CII] 158$\mu$m line emission from very
high redshift galaxies ($z \sim 6$ to 7). The mean line surface brightness is a
quantity that is relevant to measurements of spectral distortions of the cosmic
microwave background, and as a potential tool for studying large-scale
structures in the early Universe using intensity mapping. While the cosmic
volume and the number of detections are admittedly small, this pilot survey
provides a direct measure of the mean line surface brightness, independent of
conversion factors, excitation, or other galaxy formation model assumptions.
The mean surface brightness in the 99GHZ band is: $T_B = 0.94\pm 0.09$ $\mu$K.
In the 242GHz band, the mean brightness is: $T_B = 0.55\pm 0.033$ $\mu$K. These
should be interpreted as lower limits on the average sky signal, since we only
include lines detected individually in the blind survey, while in a low
resolution intensity mapping experiment, there will also be the summed
contribution from lower luminosity galaxies that cannot be detected
individually in the current blind survey.
