Time Structure of the Shower Front as Measured at Haverah Park above 1019 eV

The time structure of the air shower front observed with any particle detector is largely defined by the development of the shower in the atmosphere. Shower front structure can thus be correlated with the mass of the initiating primary particle. We have extended previous work on this topic, using the Haverah Park array, to explore these features in events of mean energy 2 × 1019 eV. We compare the measurements with Monte Carlo calculations made using the CORSIKA/QGSGET model. Data and simulations show clear azimuthal asymmetries in the time structure, which relate to the cosmic ray mass composition. The observed time structure can be best understood if iron primaries are dominant at these energies, but this conclusion is model dependent.