Ultrahigh energy gamma rays in the geomagnetic field and atmosphere

The nature and origin of ultrahigh energy (UHE, referring to >10¹⁹eV) cosmic rays are great mysteries in modern astrophysics. The current theories for their explanation include the so-called top-down decay scenarios whose main signature is a large ratio of UHE gamma rays to protons. An important step in determining the primary composition at ultrahigh energies is the study of air shower development. UHE gamma ray induced showers are affected by the Landau-Pomeranchuk-Migdal (LPM) effect and the geomagnetic cascading process. In this work extensive simulations have been carried out to study the characteristics of air showers from UHE gamma rays. At energies above several times 10¹⁹eV the shower is affected by geomagnetic cascading rather than by the LPM effect. The properties of the longitudinal development such as the average depth of the shower maximum or its fluctuations depend strongly on both primary energy and incident direction. This feature may provide possible evidence of UHE gamma ray presence by fluorescence detectors.