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Dynamics of large-scale solar-wind streams obtained by the double superposed epoch analysis. 4. Helium abundance

This work is a continuation of our previous articles (Yermolaev et al. in J. Geophys. Res. 120, 7094, 2015; Yermolaev et al. in Solar Phys. 292, 193, 2017; Yermolaev et al. in Solar Phys. 293, 91, 2018), which describe the average temporal profiles of interplanetary plasma and field parameters in large-scale solar-wind (SW) streams: corotating interaction regions (CIRs), interplanetary coronal mass ejections (ICMEs including both magnetic clouds (MCs) and ejecta), and sheaths as well as interplanetary shocks (ISs). In this work we analyze the average profile of helium abundance Nα/Np for interval of 1976-2016. Our results confirm main early obtained results: Nα/Np is higher in quasi-stationary fast streams than in slow ones; it slowly changes in compression regions CIRs and Sheaths from values in undisturbed solar wind to values in corresponding type of piston (HSS or ICME); in Ejecta it is closed to abundance in undisturbed streams and it is maximal in MCs. We obtained for first time that Nα/Np correlates with plasma β-parameter in compression regions CIRs and Sheaths and anti-correlates in ICMEs. The Nα/Np vs. β dependence is stronger in MCs than in Ejecta and may be used as an indicator of conditions in CME formation place on the Sun.