Observation of Variations in Cosmic Ray Single Count Rates During Thunderstorms and Implications for Large-Scale Electric Field Changes

TL;DR

This study reports the first large-scale observation of how thunderstorms influence cosmic ray count rates, revealing electric field effects that can be modeled and simulated to understand thunderstorm electric structures.

Contribution

It provides the first large-area measurements of cosmic ray variations during thunderstorms and models the electric fields responsible for these effects using CORSIKA simulations.

Findings

Cosmic ray count rates vary by 0.5-2% during thunderstorms.

Variations correlate with lightning and thunderstorm movement.

Electric fields of approximately 0.2-0.4 GV can reproduce observed effects.

Abstract

We present the first observation by the Telescope Array Surface Detector (TASD) of the effect of thunderstorms on the development of cosmic ray single count rate intensity over a 700 km$^{2}$ area. Observations of variations in the secondary low-energy cosmic ray counting rate, using the TASD, allow us to study the electric field inside thunderstorms, on a large scale, as it progresses on top of the 700 km$^{2}$ detector, without dealing with the limitation of narrow exposure in time and space using balloons and aircraft detectors. In this work, variations in the cosmic ray intensity (single count rate) using the TASD, were studied and found to be on average at the $\sim(0.5-1)\%$ and up to 2\% level. These observations were found to be both in excess and in deficit. They were also found to be correlated with lightning in addition to thunderstorms. These variations lasted for tens of minutes; their footprint on the ground ranged from 6 to 24 km in diameter and moved in the same direction as the thunderstorm. With the use of simple electric field models inside the cloud and between cloud to ground, the observed variations in the cosmic ray single count rate were recreated using CORSIKA simulations. Depending on the electric field model used and the direction of the electric field in that model, the electric field magnitude that reproduces the observed low-energy cosmic ray single count rate variations was found to be approximately between 0.2-0.4 GV. This in turn allows us to get a reasonable insight on the electric field and its effect on cosmic ray air showers inside thunderstorms.