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Ann. Geophys., 21, 1229-1243, 2003
www.ann-geophys.net/21/1229/2003/
© European Geosciences Union 2003
Solar cycle variations of the energetic H/He intensity ratio at high heliolatitudes and in the ecliptic plane
D. Lario1, E. C. Roelof1, R. B. Decker1, G. C. Ho1, C. G. Maclennan2, and J. T. Gosling3
1Applied Physics Laboratory. The Johns Hopkins University, Laurel, Maryland, USA
2Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey, USA
3Los Alamos National Laboratory, Los Alamos, New Mexico, USA
Abstract. 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 the first out-of-ecliptic excursion of Ulysses (1992–1996), the
HI-SCALE instrument measured corotating energetic particle intensity
enhancements characterized by low values (< 10) of the 0.5–1.0 MeV
nucleon-1 H/He intensity ratio. During the second out-of-ecliptic
excursion of Ulysses (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 for 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, the lack of strong
recurrent high-speed solar wind streams, together with the dynamic character of
the Sun, 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.
Key words. Interplanetary physics
(energetic particles, interplanetary shocks; solar wind plasma)
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