Modulation of Galactic Cosmic Rays from Helium to Nickel in the Inner Heliosphere

Galactic cosmic-ray (GCR) helium and heavier ions are important sources of space radiation, and their elemental spectra and composition can help us better understand the transport in both the galaxy and the heliosphere. A model based on the numerical solution of Parker's transport equation is used to study the modulation of GCR helium and heavier ions in the inner heliosphere. The model incorporates a modified Parker heliospheric magnetic field, time-dependent diffusion and drift model, time-delayed heliosphere, and randomly determined solar magnetic polarity during solar maximum. We set the outer boundary of modulation at 85 au, and the reference unmodulated GCR energy spectra for GCR helium and heavier ions, which are assumed to have a general form, are determined by fitting the numerical results to the selected GCR measurements, e.g., BESS, ACE/CRIS, HEAO-3-C2, etc. In addition, we use the Sun's polar magnetic field data from NSO/NISP to determine the possibility of A > 0 during the recent solar maximum, and it gives an improved numerical result during the period 2013-2015. Finally, the fitted unmodulated GCR energy spectra are used to study the long-term modulation of GCRs of helium and heavier ions, and the computed results show good agreement with various GCR measurements.