Atmospheric monitoring for the H.E.S.S. experiment using a single scattering lidar

Abstract

The High Energy Stereoscopic System (H.E.S.S.) is an array of 4 telescopes located in Namibia, which use the imaging atmospheric Cherenkov technique (IACT) to study astrophysical emission of gamma radiation in the energy window from 100 GeV to 50 TeV. The calorimetric nature of the technique means that the sensitivity and energy resolution of the instrument are highly dependent on atmospheric parameters. This thesis presents the findings of atmospheric measurements taken using a 355 nm single scattering lidar. The lidar wavelength is well matched to the maximum in the Cherenkov spectrum seen by the telescopes. Monte Carlo simulation software is presented which has been developed to calculate the integral vertical lidar ratio (the ratio of extinction to backscatter) for Mie scattering by aerosols assumed to be at the H.E.S.S. site. This is found to be 29 ± 3 steradians. This ratio is used with the Fernald method to derive the probability of transmission profile, and is also compared to other lidar analysis techniques; the Klett method and the multi-angle method. The results of all 3 methods are compared to the lidar manufacturer's closed-source analysis software, with which the Klett method is found to be in strongest agreement. A model that describes the relationship between the lidar ratio and the extinction is presented. Using this with the lidar manufacturer's extinction values provides a vertical lidar ratio profile which, for the first time, provides insight into the aerosol scattering layers present at the H.E.S.S. site in Namibia. Recommendations for improvement of this research, and suggestions for incorporation of data into the H.E.S.S. analysis, have been made