  Air fluorescence detectors measure the energy of ultra-high energy cosmic
rays by collecting fluorescence light emitted from nitrogen molecules along the
extensive air shower cascade. To ensure a reliable energy determination, the
light signal needs to be corrected for atmospheric effects, which not only
attenuate the signal, but also produce a non-negligible background component
due to scattered Cherenkov light and multiple-scattered light. The correction
requires regular measurements of the aerosol attenuation length and the aerosol
phase function, defined as the probability of light scattered in a given
direction. At the Pierre Auger Observatory in Malargue, Argentina, the phase
function is measured on an hourly basis using two Aerosol Phase Function (APF)
light sources. These sources direct a UV light beam across the field of view of
the fluorescence detectors; the phase function can be extracted from the image
of the shots in the fluorescence detector cameras. This paper describes the
design, current status, standard operation procedure, and performance of the
APF system at the Pierre Auger Observatory.
