Time structure analysis of extensive air showers using the Telescope Array Data

In this paper, we report results of the study of temporal shower front structure based on the data taken by the Telescope Array Surface Detector from 2008 to 2019. By analyzing the waveforms measured by FADCs, we defined two quantities characterizing the shower structure, the curvature of the shower front and the thickness of the shower disk. These analyses were performed by selecting the data sample of high-quality events at zenith angles up to 60 degrees. Firstly we used an empirical form of the shower front curvature in term of residual time, and found a clear zenith angle dependence but no significant energy dependence based on the 3 energy intervals in log(E/eV) bins from 18.90 to 20.00. Secondly, the thickness of the disk is analyzed by the rising time of the FADC waveform. We found that the thickness of the shower disk has a dependency on angles and energy.