Modulation of ACR Oxygen at 1 AU and the Topology of the Heliospheric Current Sheet

Between late 1997 and mid-2001, the flux of ~ 10 MeV/nuc anomalous cosmic ray (ACR) oxygen at 1 AU has decreased by a factor of over 100. Comparison of the time profiles of the oxygen intensity at ~ 7-16 MeV/nuc at 1 AU and in the outer heliosphere suggests that there are two processes affecting this modulation. Between 1997 and 2000, there are three structures that cause step decreases at 1 AU but not in the outer heliosphere. In 2000, these steps are followed by a major decrease and the effective disappearance of ACR oxygen at 1 AU. Although smaller, this decrease is also seen in the outer heliosphere as the onset of durable modulation. We compare the timing of this decrease and subsequent changes in the ACR oxygen intensity with changes in the increasingly complex topology of the heliospheric current sheet during the reversal of the solar magnetic field. This work was supported by NASA under grant NAG5-6012 and contract NAS7-1407.