EAS time structures with ARGO-YBJ experiment

Shower-by-shower fluctuations play a key role for the understanding of EAS morphology. Each observed event keeps track of the propagation through the atmosphere. This factor can influence the shape of time front and the sequence of arrival times. The design of the ARGO-YBJ detector offers a unique chance to have high resolution pictures of shower footprints at the ground. The time structure of the shower disc has been studied as a function of the distance to the shower axis. The curvature of the shower front, defined as the mean of time residuals with respect to a planar fit, has also been investigated in the energy range between 300 GeV and 100 TeV and its features employed to characterize the standard surface array observables. After a study on the shower time profile and on the lateral distribution at different time delay from the shower front, showers with large time residual rms with respect to the shower front have been investigated. The longitudinal time structures in data could help to better define selection criteria for particular analysis, such as gamma/hadron separation, composition or "exotic" physics, and allow a better determination of EAS disc structure and correlations between front profile, front thickness and core distance