Time variations of the Magnetic Field at the Heliospheric Termination Shock – Galactic Cosmic-Ray modulation

In this work we apply Parker's model for the solar wind expansion to estimate the magnetic field at the heliospheric termination shock, using in-ecliptic measurements from different spacecraft at 1 AU near the Earth, and study the effects of its temporal variation on the galactic cosmic-ray long-term modulation. We find that, for a time period of approximately two 11-year solar cycles from 1970 to 1995, the magnetic field at 1 AU extrapolated to the heliospheric termination shock anticorrelates very well with the cosmic-ray intensity measured from cosmic-ray ground stations. The correlation between the cosmic-ray intensity and the interplanetary magnetic field at 1 AU is generally lower. From this study it becomes evident that the modulation of cosmic-rays is substantially governed by the shielding of the heliosphere by the heliospheric termination shock magnetic field.