Atmospheric Monitoring
 
The faint fluorescence light that is emitted along the track of development of the shower must be used to interpret how many particles produced the light and at what altitude. This allows the development of the shower cascade in the atmosphere to be studied and is a powerful method used to find the energy of the cosmic ray that initiates an event. However, to make these inferences, careful monitoring of the atmosphere is required. We need to know the pressure and temperature as a function of height above the Pampa and what the aerosol content of the atmosphere is.
 
Several instruments have been developed to help us understand the atmosphere is great detail. One of these is a LIDAR (Light Detection and Ranging) which uses the same principles as RADAR to give information about the atmosphere. A schematic diagram of a LIDAR system is shown in figure 1.
 
 
Figure 1: A schematic diagram of the principles of a LIDAR system. A pulsed- laser is fired upwards into the atmosphere and light reflected from molecule and dust is collected on a mirror system. The light is collected by a photomultiplier using a data acquisition system.
  
 
At the Auger Observatory, LIDARs are operated adjacent to each of the four fluorescence stations (Pierre Auger Observatory). The laser beam is fully steerable and the laser is a frequency tripled Nd:Yag laser emitting pulses of 6 mJ energy and 7 ns duration. The back-scattered laser light is collected with three 80 cm parabolic mirrors and focussed onto photomultiplier tubes. A photograph of the LIDAR system at Los Leones is shown in figure 2.
 
One of the parameters that is derived from the LIDAR observations is the optical depth profile which can be measured up to elevations of 20 km.
 
 
Figure 2: The LIDAR system at Los Leones. The three 80 cm mirrors can be seen.
Photomultipliers not shown) can be housed in the mounting at the focus of the mirror. The laser beam comes from the region just below the left hand mirror.
  
 
There are other instruments that are used as part of the atmospheric monitoring program at the Auger Observatory. Measurements of the total horizontal atmospheric attenuation are made by observing two light sources at 14.79 and 43.94 km using a CCD camera. Results from these measurements agree with those made with the LIDAR. There is also a program of star monitoring using a 20 cm telescope and a photomultiplier. This instrument is used in a fully robotic mode to observe a set of standard stars. From these observations extinction coefficients can be obtained. Recently the robotic telescope was used to make a unique observation of the optical flash from GRB06117 (Jelinek et al. Astronomy and Astrophysics, in press).
 
Some of the atmospheric monitoring information that is available may be of interest to scientists in other disciplines.