Muon Excess at Sea Level during the Progress of a Geomagnetic Storm and High-Speed Stream Impact Near the Time of Earth’s Heliospheric Sheet Crossing

Abstract

In this article we present results of studying the association between the muon flux variation at ground level, registered by the New-Tupi muon telescopes ($22^{\circ} 53'00''~\mbox{S}, 43^{\circ} 06'13'~\mbox{W}$; 3 m above sea level), and the geomagnetic storm on 25 – 29 August 2015 that has raged for several days as a result of a coronal mass ejection (CME) impact on Earth’s magnetosphere. A sequence of events started with an M3.5 X-ray class flare on 22 August 2015 at 21:19 UTC. The New-Tupi muon telescopes observed a Forbush decrease (FD) triggered by this geomagnetic storm, which began on 26 August 2015. After Earth crossed the heliospheric current sheet (HCS), an increase in particle flux was observed on 28 August 2015 by spacecraft and ground-level detectors. The observed peak was in temporal coincidence with the impact of a high-speed stream (HSS). We study this increase, which has been observed with a significance above 1.5% by ground-level detectors in different rigidity regimes. We also estimate the lower limit of the energy fluence injected on Earth. In addition, we consider the origin of this increase, such as acceleration of particles by shock waves at the front of the HSS and the focusing effect of the HCS crossing. Our results show possible evidence of a prolonged energetic (up to GeV energies) particle injection within the Earth atmosphere system, driven by the HSS. In most cases, these injected particles are directed to the polar regions. However, the particles from the high-energy tail of the spectrum can reach mid-latitudes, and this could have consequences for the atmospheric chemistry. For instance, the creation of NO_x species may be enhanced, and this can lead to increased ozone depletion. This topic requires further study.