On the short term modulation of cosmic rays by high-speed streams at the Pierre Auger surface array detectors

TL;DR

This paper analyzes how high-speed solar wind streams modulate galactic cosmic rays on short timescales using Pierre Auger Observatory data, and introduces a semi-empirical Toy-model to describe this modulation.

Contribution

It presents the first detailed analysis of short-term cosmic ray modulation by high-speed streams using Pierre Auger data and proposes a semi-empirical Toy-model for this phenomenon.

Findings

Cosmic ray diurnal variation amplitude is anti-correlated with solar wind speed.

High-speed streams cause a baseline fall in cosmic ray diurnal variation.

The Toy-model satisfactorily reproduces observed modulation patterns.

Abstract

We present an analysis of the short-term modulation (one rotation of Bartels-27 days) of the galactic cosmic rays (GCR) by the solar wind, based on the cosmic ray rates observed by the Pierre Auger Observatory (PAO) on their surface detectors in scaler mode. The incidence of GCR with energies below $\sim$ 50 TeV, at the top of the atmosphere, produces more than 90\% of the secondary particles registered at ground level, i.e., they are subject to solar modulation. The modulation is consistent with at least two components: The first is the modulation of the amplitude of the cosmic rays diurnal variation, anti-correlated with the solar-wind speed. The second one occurs during the high-speed stream (HSS), the baseline of the cosmic rays diurnal variation train falls, following the time profile of the solar-wind speed inversely. Based on the radial gradient of the cosmic ray diffusion theory and under some other premises, such as the latitude dependence on diurnal variation and the inclusion of drift processes in the propagation of GCR, a semi-empirical description of the modulation is possible to do, and it hereafter is called as Toy-model. Although the Toy-model does not include fluctuations due to propagation in the atmosphere, it provides satisfactory results when compared with the PAO scaler mode data. We present details of these observations as well as the Toy-model validation.