Dependence of the parallel mean free path of high-energy galactic cosmic rays in the heliosphere on their rigidity, and solar activity as measured by the GRAPES-3 experiment

In recent times the role of space weather on the functioning of the Earth- and space-based technological systems is attracting increasing attention due to the recognition of its potential to adversely impact those systems, especially during the extreme geomagnetic storms. On the other hand, the effects of space weather on the propagation of the galactic cosmic rays (GCRs) can be used to probe the parameters characterizing the space weather. Within the heliosphere, the embedded turbulent magnetic field in the solar wind creates a radial density gradient in an otherwise isotropic distribution of the GCRs. The diffusion-convection framework predicts that the variations in the GCR intensity, and the solar wind velocity should be anticorrelated. In the work presented here, these measurements are extended to the GRAPES-3 data collected over a long interval of 22 years from 2000 to 2021. The "$HK$" parameter is defined as the magnitude of ratio of the rate of change of muon intensity relative to that of the solar wind velocity. Here, the radial diffusion coefficient ${\kappa }_{rr}$, and the parallel mean free path ${\lambda }_{\parallel }$ have been inferred based on several simplifying assumptions, along nine independent directions each of which corresponds to a different median GCR rigidity. Each of these parameters including the $HK$ show a strong correlation with the F10.7 Index, which is an excellent proxy for the 11 year solar activity. This possibly is the first measurement of the parameters that influence the transport of GCRs in the heliosphere, and their dependence on the solar activity at relatively high-median rigidities (64–92 GV) by a ground-based instrument. These measurements should put the link between the space weather, and the high-rigidity GCRs on a much sounder footing, and sheds some light on the complex relationship among the parameters that characterize the space weather.