Energetic HHe intensity ratio under solar maximum and solar minimum conditions: Ulysses observations Original Research Article

We study the solar cycle variability of the heliospheric energetic proton-to-helium abundance ratios. We use 0.5–1.0 MeV nucleon−1 proton and helium intensities observed by the Ulysses spacecraft at both high and low heliographic latitudes. Ulysses observations show that during solar maximum the 0.5–1.0 MeV nucleon−1 HHe intensity ratios are, on average, higher than during solar minimum. Under solar minimum conditions the interaction between slow and fast solar wind streams is strong, producing long-lasting and stable corotating interaction regions (CIRs) which are efficient accelerators of pickup He+. During solar maximum, transient events of solar origin (characterized by high HHe ratios) are able to globally fill the heliosphere. In addition, the absence of large and stable coronal holes results in a lack of recurrent strong corotating solar wind interactions, and consequently a less efficient acceleration of pickup He+. Even when solar wind stream interaction regions (SIRS) are observed, the HHe intensity ratio during solar maximum rarely decreases to the low (∼6) values typically observed during solar minimum CIR events. The latest data collected by Ulysses during its solar maximum descent from northern polar latitudes (mid-2002) show nearly-recurrent CIR events, which occasionally decrease the H/He ratios to these low (∼6) values. The still frequent occurrence of SEP events, however, produces increases of the H/He intensity ratios to high (∼30) values. Although SEP events still dominate the particle population in the inner heliosphere, the low HHe ratios observed in these specific CIR events suggest an efficient acceleration of pickup He+ rather than the acceleration of remnant SEP material.