Observation of Variations in Cosmic Ray Muon Flux During Thunderstorms and Implications for Large-Scale Electric Field Changes

Abstract:

This work presents the first observation by the Telescope Array Surface Detector (TASD) of the effect of thunderstorms on the development of the cosmic ray showers. Observations of variations in the muons intensity, using the TASD, allows us to study the electric field inside thunderstorms on a large scale without dealing with all the limitation of narrow exposure in time and space using balloons and aircraft detectors.

The Telescope Array detector is located in the southwestern desert of the State of Utah. Currently it is the largest Ultra High Energy Cosmic Ray (UHECR) experiment in the Northern Hemisphere. The surface detector array part of the TA experiment, is composed of 507 scintillator detectors on a 1.2 km square grid covering a 700 km² in area, 1400 m above sea level. The TASD combination of size and elevation makes it a unique tool that allows the study thunderstorms as they progress across the detector.

In this work, observations of changes in muon intensity ΔN/N using the Telescope Array Surface Detector, was studied and found to be at the 1-3 % level. These observations where found to be both positive and negative in polarity. They were also found to be correlated with lightning but mostly with thunderstorms. The size of the footprint of these variations on the ground ranged from 4-14 km in diameter and lasted for 10s of minutes. Preliminary results for the dependence of ΔN/N on the potential inside thunderstorms via simulations from CORSIKA is investigated.

With the muon variations observations, the TASD contributes, in a unique way, to solving decades of mysteries related to lightning initiation and propagation. This is one of the several and significant contributions of the TASD to atmospheric electricity physics.