Galactic cosmic ray transport in the heliosphere: Study with muon data

Abstract

We use the galactic cosmic ray (GCR) solar diurnal anisotropy (SDA) data of Nagoya vertical muon telescope (NagV-MT) to compute the ratio α of GCR free paths normal and parallel to mean interplanetary magnetic field (IMF) B. The values are compared to those computed earlier for the neutron monitors (NMs) of the global network for three sunspot cycles (21–23) and parts of the other two (19, 24). Current GCR modulation theories do not provide any guidance for α, its rigidity and solar cycle dependence. We use a flat heliosphere current sheet and the Master equations derived by Ahluwalia and Dorman [1997] 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 north hemisphere; IMF polarity changes sign near SSN maximum. For NagV-MT (Rm = 60 GV), Rm being the median value of response to GCR differential rigidity spectrum, we find that α is high for p- and low for n-intervals, compared to being nearly flat for NMs; α for NM2 (Rm = 25 GV) is larger than that for NM1 (Rm = 15 GV), implying an increase in α with GCR rigidity. The α values for NagV-MT are low near SSN maximum as is the case for NMs and high near SSN minimum, when effective limiting GCR rigidity (Rc) for SDA is known to be lower than 100 GV. The results challenge the validity of current conceptual understanding of GCR modulation processes in the heliosphere, pointing to a great need to develop an acceptable theory to explain the empirical results reported in this paper.