Solar cycle variations of the energetic H/He intensity ratio at high heliolatitudes and in the ecliptic plane

We study the variability of the heliospheric energetic proton- to-helium abundance ratios during different phases of the solar cycle. We use energetic particle, solar wind and magnetic field data from the Ulysses, ACE and IMP-8 spacecraft to compare the H/He intensity ratio at high heliographic latitudes and in the ecliptic plane. During Ulysses' first orbit (1992-1996), the HI-SCALE instrument measured corotating energetic particle flux enhancements characterized by low values (~10) of the 0.5-1.0 MeV/ nucleon H/He intensity ratio. During Ulysses' second orbit (1999-2002) the more frequent occurrence of solar energetic particle events resulted in almost continuously high (>~20) values of the H/He ratio, even at the highest heliolatitudes reached by Ulysses. Comparison with in-ecliptic measurements from an identical instrument on the ACE spacecraft showed similar H/He values at ACE and Ulysses, suggesting a remarkable uniformity of energetic particle intensities in the solar maximum heliosphere at high heliolatitudes and in the ecliptic plane. In-ecliptic observations of the H/He intensity ratio from the IMP-8 spacecraft show variations between solar maximum and solar minimum similar to those observed by Ulysses at high heliographic latitudes. We suggest that the variation of the H/He intensity ratio throughout the solar cycle is due to the different level of transient solar activity as well as the different structure and duration that corotating solar wind structures have under solar maximum and solar minimum conditions. During solar minimum the interactions between the two different types of solar wind streams (slow vs. fast) are strong and long-lasting, allowing a continuous and efficient acceleration of interstellar pickup He-. During solar maximum, transient events of solar origin (characterized by high values of the H/He ratio) are able to globally fill the heliosphere. In addition, during solar maximum, slow and variable solar wind occupies most of space that, together with the dynamic variations in the heliosphere, lead to weak and short-lived solar wind stream interactions. This results in a less efficient acceleration of pickup He+, and thus a higher value of the H/He intensity ratio.