Scaling Features of Diurnal Variation of Galactic Cosmic Rays

Renata Modzelewska,Agata Krasińska,Anna Wawrzaszek,Agnieszka Gil

doi:10.1007/s11207-021-01866-6

Abstract

We analyze the scaling properties of the diurnal variation of galactic cosmic rays (GCRs) in Solar Cycle 24 and the solar minima between Solar Cycles 23/24 and 24/25 for 2007 – 2019 based on the count rates of the Oulu, Newark, Hermanus, and Potchefstroom neutron monitors. The scaling features of the GCR diurnal variation are studied by evaluating the Hurst exponent, a quantitative parameter used as an indicator of the state of the randomness of a time series. We estimate the Hurst exponents for GCR diurnal-variation parameters amplitude and phase using structure-function and detrended-fluctuation-analysis methods. Results show that the Hurst exponents for the GCR diurnal variation vary in the range from approx0.3 to approx0.9, with a general tendency of being systematically above 0.5. It suggests that the GCR diurnal variation reveals a more persistent structure than Brownian motion. However, the time series of GCR diurnal-variation amplitude and phase evolve from a more persistent structure in the solar minimum between Solar Cycles 23/24 in 2007 – 2009 to a more random character in and near the solar maximum 2012 – 2014. This observation seems to be in agreement with the general configuration of the heliosphere through the 11-year solar-activity cycle. Moreover, the temporal profile of the Hurst exponent for GCR diurnal amplitude and phase around the beginning of the solar minimum between Solar Cycles 24/25 (2018 – 2019) differs from the solar minimum between Solar Cycles 23/24 in 2007 – 2009, suggesting a dependence on solar-magnetic polarity. These findings could shed more light on GCR particle transport in the turbulent heliosphere over the solar cycle.

Highlights

Primary cosmic rays, which are charged particles, reach the Earth with energies at relativistic levels
The panels correspond to data from Oulu, Newark, Hermanus, and Potchefstroom neutron monitors (NMs) measured during Solar Cycle 24 and the solar minima between Solar Cycles 23/24 and 24/25, for the period 2007 – 2019
The results obtained show that the Hurst exponents [H ] for the galactic cosmic rays (GCRs) diurnal variation parameters vary in the range from ≈ 0.3 to ≈ 0.9 through Solar Cycle 24 and the solar minima between Solar Cycles 23/24 and 24/25

Summary

Introduction

Primary cosmic rays, which are charged particles, reach the Earth with energies at relativistic levels. Anisotropy for the part of the time interval studied here, i.e. 2001 – 2014, was discussed by Tezari and Mavromichalaki (2016) for two neutron monitors having various cut-off rigidities located at the same longitude and different latitudes: Athens and Oulu They have shown that the GCR diurnal anisotropy behaves in a different way during the opposite phases of the solar cycle, depending on the solar magnetic-field polarity, confirming previous results Tiwari, Singh, and Agrawal (2012) investigating Solar Cycles 20 – 23 showed clear dependencies on the level of the solar-activity cycle, revealing that annual-average anisotropy reaches maximal values during the declining phases for all solar cycles, while during the minimum epochs, anisotropy took the lowest values They presented evidence of substantial differences between the descending epochs of the odd (21 and 23) and even (20 and 22) solar cycles. Studies of the GCR diurnal anisotropy prove its utility in the recognition of other aspects of heliospheric changes, for instance analyzes of Forbush-decrease features (Gololobov et al, 2017), or a recurrence of the GCR anisotropy as a unique proxy of the level of solar activity, as well as solar-wind states (Mavromichalaki, Papageorgiou, and Gerontidou, 2016; Modzelewska and Alania, 2018)

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Conclusion