  We analyse the near-infrared interline sky background, OH and O2 emission in
19 hours of H band observations with the GNOSIS OH suppression unit and the
IRIS2 spectrograph at the 3.9-m AAT. We find that the temporal behaviour of OH
emission is best described by a gradual decrease during the first half of the
night followed by a gradual increase during the second half of the night
following the behaviour of the solar elevation angle. We measure the interline
background at 1.520 microns where the instrumental thermal background is very
low and study its variation with zenith distance, time after sunset, ecliptic
latitude, lunar zenith angle and lunar distance to determine the presence of
non-thermal atmospheric emission, zodiacal scattered light and scattered
moonlight. Zodiacal scattered light is too faint to be detected in the summed
observations. Scattered moonlight due to Mie scattering by atmospheric aerosols
is seen at small lunar distances (< 11 deg), but is otherwise too faint to
detect. Except at very small lunar distances the interline background at a
resolving power of R~2400 when using OH suppression fibres is dominated by a
non-thermal atmospheric source with a temporal behaviour that resembles
atmospheric OH emission suggesting that the interline background contains
instrumentally-scattered OH. However, the interline background dims more
rapidly than OH early in the night suggesting contributions from rapid dimming
molecules. The absolute interline background is 560 +/- 120 photons s^-1 m^-2
micron^-1 arcsec^-2 under dark conditions. This value is similar to previous
measurements without OH suppression suggesting that non-suppressed atmospheric
emission is responsible for the interline background. Future OH suppression
fibre designs may address this by the suppression of more sky lines using more
accurate sky line measurements taken from high resolution spectra.
