  The sky photon background flux has been measured at the southern Auger site
in Malargue, Argentina, using the observatory's fluorescence detectors (FD).
The analysis is based on ADC variances of pixels not triggered by the First
Level Trigger. Photon fluxes are calculated for each individual pixel at each
telescope. The statistics from each night of data taking allows a study of
local variations in the photon flux. Results show a clear dependence of the
flux on elevation angle. Time variations, possibly related to different
atmospheric conditions, do not mask this dependence. In particular the flux
excess above the horizon shows a rather stable and reproducible behaviour with
elevation. Correlation of this dependence with atmospheric parameters can be of
interest as it offers the promise of extracting those parameters directly from
FD data, thus allowing cross checks with independent methods based on different
monitoring devices.
