Energetic H/He intensity ratio under solar maximum and solar minimum conditions: Ulysses observations

Details Energetic H/He intensity ratio under solar maximum and solar minimum conditions: Ulysses observations Energetic H/He intensity ratio under solar maximum and solar minimum conditions: Ulysses observations

Aug 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003adspr..32..585l&link_type=abstract

Advances in Space Research, Volume 32, Issue 4, p. 585-590.

Physics

Scientific paper

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.

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