Solar Cycle Variations in the Composition of the Suprathermal Heavy-Ion Population near 1 AU

Abstract

We have measured the annual variation in the quiet-time composition of interplanetary suprathermal ions between 0.04 and 1 MeV nucleon-1 from 1994 November 20 through 2006 January 1. Our results show the following: (1) The C/O and Fe/O ratios are similar to the corresponding values measured in the solar wind and corotating interaction regions (CIRs) during solar minimum conditions of 1994-1997 and 2005. (2) During periods of increased solar activity between 1998 and 2002, the C/O ratio is similar to that measured in solar energetic particle (SEP) events, while the Fe/O ratio lies between the values measured in coronal mass ejection-driven interplanetary shock and SEP events. (3) The 3He/4He ratio lies in the 1.5%-5% range during 1997-2005 August, but it dropped by an order of magnitude during 2005. We conclude that the composition of the suprathermal ion population between ~6 and 30 times the solar wind speed near 1 AU is dynamic and is essentially dominated by ions accelerated in SEP events during solar maximum conditions and by suprathermal solar wind ions and/ or those accelerated in CIRs during solar minimum conditions. © 2006. The American Astronomical Society, All rights reserved.