Energetic [formula omitted] intensity ratio under solar maximum and solar minimum conditions: Ulysses observations

Abstract

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 H He 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 H He 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 H He 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 H He ratios observed in these specific CIR events suggest an efficient acceleration of pickup He + rather than the acceleration of remnant SEP material.