# Spatial and temporal structure of EAS reflected Cherenkov light signal

## TL;DR

This paper analyzes the spatial and temporal features of Cherenkov light reflected from snow, using simulations to evaluate the SPHERE-2 detector's response and its potential for cosmic ray composition studies around 10 PeV.

## Contribution

It introduces a detailed Monte Carlo simulation framework and a modular detector response model for the SPHERE-2 experiment, assessing its performance and uncertainties.

## Key findings

- The detector's acceptance was quantified for various conditions.
- Reflected Cherenkov LDF distortions were characterized and compared to ideal detection.
- The method's performance is comparable to other techniques in cosmic ray composition analysis.

## Abstract

A compact device lifted over the ground surface might be used to observe optical radiation of extensive air showers (EAS). Here we consider spatial and temporal characteristics of Vavilov-Cherenkov radiation ("Cherenkov light") reflected from the snow surface of Lake Baikal, as registered by the SPHERE-2 detector. We perform detailed full direct Monte Carlo simulations of EAS development and present a dedicated highly modular code intended for detector response simulations. Detector response properties are illustrated by example of several model EAS events. The instrumental acceptance of the SPHERE-2 detector was calculated for a range of observation conditions. We introduce the concept of "composite model quantities", calculated for detector responses averaged over photoelectron count fluctuations, but retaining EAS development fluctuations. The distortions of EAS Cherenkov light lateral distribution function (LDF) introduced by the SPHERE-2 telescope are understood by comparing composite model LDF with the corresponding function as would be recorded by an ideal detector situated at the ground surface. We show that the uncertainty of snow optical properties does not change our conclusions, and, moreover, that the expected performance of the SPHERE experiment in the task of cosmic ray mass composition study in the energy region $\sim$10 PeV is comparable with other contemporary experiments. Finally, we compare the reflected Cherenkov light method with other experimental techniques and briefly discuss its prospects.