COSMIC-RAY MODULATION BY THE GLOBAL MERGED INTERACTION REGION IN THE HELIOSHEATH

It was found by Voyager 1 and Voyager 2 that the Global Merged Interaction Region (GMIR) still has effects on cosmic-ray transport in the heliosheath. The GMIR produced by the intense solar activities of 2005 September propagated into the heliosheath in 2006, causing several decreases in the cosmic-ray flux at Voyager 1. Motivated by these observations, we investigate cosmic-ray modulation by the GMIR in the heliosheath using a three-dimensional Galactic cosmic-ray modulation code that includes the drift effect, termination shock, and a propagating GMIR. The simulation reproduces all major modulation features found by previous cosmic-ray observations in the supersonic solar wind region. However, when the simulation is made for the heliosheath region during a passage of the GMIR, the cosmic-ray modulation shows a new feature. The cosmic-ray fluxes at many locations in the heliosheath region experience two distinguished decreases. One coincides with the GMIR's arrival at the termination shock, the other with its arrival at the observing points. Based on this feature, we use the Voyager observations of the 2006 GMIR modulation events to derive the radial distance of the termination shock of similar to 91 AU and the GMIR shock propagation speed inside the heliosheath of similar to 172 km s(-1) for this event.