Propagation of the Onset of Modulation in Cycle 23 from 1 to 72 AU

Abstract

In late 1997, the modulation of galactic cosmic rays at 1 AU began to increase in response to increasing solar activity. We report observations of the onset of modulation from spacecraft ranging from IMP-8 and U l y s s e s near the ecliptic at 1 AU and 5 AU respectively out to Voyager 1 and 2 , in the northern and southern solar hemispheres at radii beyond 55 AU, to test whether specific features of the modulation onset can be identified and traced from IMP to Voyager. We find that, though the onset of modulation occurs throughout the heliosphere at times consistent with outward propagation between 400 and 800 km/s, specific features are difficult to trace from IMP and Ulysses to Voyager.