Similarities and Distinctions in Cosmic-Ray Modulation During Different Phases of Solar and Magnetic Activity Cycles

Abstract

We study the solar-activity and solar-polarity dependence of galactic cosmic-ray intensity (CRI) on the solar and heliospheric parameters playing a significant role in solar modulation. We utilize the data for cosmic-ray intensity as measured by neutron monitors, solar activity as measured by sunspot number (SSN), interplanetary plasma/field parameters, solar-wind velocity [V] and magnetic field [B], as well as the tilt of the heliospheric current sheet [Λ] and analyse these data for Solar Cycles 20 -24 (1965Cycles 20 -24 ( _ 2011. We divide individual Solar Cycles into four phases, i.e. low, high, increasing, and decreasing solar activity. We perform regression analysis to calculate and compare the CRI-response to changes in different solar/interplanetary parameters during (i) different phases of solar activity and (ii) similar activity phases but different polarity states. We find that the CRI-response is different during negative (A<0) as compared to positive (A>0) polarity states not only with SSN and Λ but also with B and V. The relative CRI-response to changes in various parameters, in negative (A<0) as compared to positive (A>0) state, is solar-activity dependent; it is ≈ 2 to 3 times higher in low solar activity, ≈ 1.5 to 2 times higher in moderate (increasing/decreasing) activity, and it is nearly equal in high solar-activity conditions. Although our results can be ascribed to preferential entry of charged particles via the equatorial/polar regions of the heliosphere as predicted by drift models, these results also suggest that we should look for, any polarity-dependent response of solar wind and transport parameters in modulating CRI in the heliosphere.

Time and Charge-Sign Dependence of the Heliospheric Modulation of Cosmic Rays

“…All these physical processes are considered important in modulating GCRs, although their relative importance and how this changes with solar activity and with reversals of the HMF polarity (e.g. Web-ber et al 1990;Heber et al 2002Heber et al , 2003McDonald et al 2010;Aslam & Badruddin 2014) is still intensively investigated. Potgieter (2014b) concluded that the heliospheric modulation of GCRs is always an intriguing interplay of these four basic physical mechanisms that changes with solar activity and the polarity cycles.…”