Galactic cosmic ray transport in the heliosphere: Forbush ion chamber data analysis

Abstract

The study of ratio α (=λ⊥/λ||) of galactic cosmic ray (GCR) free paths normal (λ⊥) and parallel (λ||) to the mean interplanetary magnetic field (IMF) B is continued with annual mean GCR solar diurnal anisotropy (SDA) data of the Cheltenham/Fredericksburg shielded ion chamber (CH-IC) for 1936–1977 previously analyzed by Bieber and Chen (1991) but not corrected for atmospheric temperature effect. This study uses CH-IC data corrected for temperature effect to calculate α for the first time assuming negligible latitudinal density gradient. Solar activity dependence of α shown by Bieber and Chen (BC) remains qualitatively unchanged. Present GCR modulation theories do not provide guidance for α, its rigidity or solar cycle dependence. Flat heliosphere current sheet (HCS) and Master equations derived by Ahluwalia and Dorman (1997) are used for computing α and study its rigidity dependence and correlation with solar activity for positive (p) and negative (n) polarity intervals of B at 1 AU; B points outward/ inward from the Sun during p/n interval in Northern hemisphere, IMF polarity changes sign near sunspot number (SSN) maximum. CH-IC median rigidity of response (Rm) for GCR differential spectrum is 67 GV. We show that computed α is high for p- and low for n-intervals, also α is low near SSN maximum and high near SSN minimum when effective limiting GCR rigidity (Rc) for SDA is known to be <100 GV. The results are consistent with those reported earlier by Ahluwalia and Modzelewska (2020) for NagV-MT (Rm = 60 GV) for 1971–2017; they challenge validity of present conceptual understanding of GCR modulation processes in the heliosphere, pointing to a great need to develop an acceptable modulation theory to explain empirical results reported in this paper.