Extended 3He-rich periods of solar energetic particles in structured solar wind

We have surveyed helium isotope abundance data (1997-2005) measured with the Ultra-Low-Energy Isotope Spectrometer (ULEIS) on the Advanced Composition Explorer (ACE) and have found multiday periods of He-3-rich solar energetic particles (SEPs) in the energy range of similar to 0.4-10 MeV nucleon(-1) with He-3/He-4 similar to 0.1-1. We analyzed 17 periods where there were good counting statistics, also using in situ measurements of solar wind by ACE and Solar and Heliospheric Observatory (SOHO) along with solar observations by SOHO telescopes, to study coronal and interplanetary magnetic field (IMF) structures associated with multiday periods of He-3-rich SEPs. The analysis indicates that all events are associated with slow solar wind and most of them with significant compressions in solar wind at 1 AU. About half of the events are associated with a deep well in solar wind speed that is followed by a fast rise of the wind speed, typically by about 200 km s(-1). The corresponding coronal structure consists of an active region, leading coronal hole and trailing coronal hole situated not far from the solar equator. We have employed a simplified model of corotating compression in the solar wind to estimate the possible effect of the rising speed wind on confinement of He-3-rich SEPs in IMF structures. Numerical modeling indicates that a fast increase of the wind speed by 200 km s(-1) can significantly affect the SEP time-intensity profiles at 1 AU and results in higher SEP intensities than would be expected based on a standard model of solar wind.