Minor helium components co-moving with the solar wind

We revisit the problem of the minor 4He components in the solar wind. It is shown that due to interactions of neutral interstellar atoms or interplanetary dust-desorbed atoms and molecules with the solar wind ions and with solar EUV radiation various sorts of secondary neutral and ion helium populations are produced, enriching the original content of helium species in the solar wind. Basing on numerical modeling we show that besides the well known He+ pickup population one can expect measurable amounts of other minor constituents, such as: He++ pickup ions, He+ ions of solar wind characteristics, and energetic (∼ 4 keV) neutral He atoms. We provide estimates of their expected fluxes and discuss their radial variations. It is shown that the doubly charged He++ pickup population may typically contribute ∼ 2–3% to the total helium pickup flux and it demonstrates (similarly to the He+ pickup component) a pronounced downwind–upwind anisotropy resulting from the gravitational focusing of the interstellar helium. Similar fluxes, of the order of ∼ 102–103 cm−2 s−1 at 1 AU, are expected for the energetic neutral helium component, created due to the double charge-exchange between the solar wind alpha particles and the interstellar helium atoms. According to our calculations, this component may be the dominant constituent of the Neutral Solar Wind (NSW) up to distances of ∼ 0.4–0.6 AU on the upwind side, ∼ 0.5–1.0 AU in the sidewind direction, and even up to ∼ 2–4 AU in the downwind region, depending on the phase of the solar cycle. Another minor component discussed in the paper are the ‘solar’ He+ ions, with the properties inherited from the former solar wind alpha particles after their transcharge on the neutral H and He interstellar atoms or on the neutral dust-desorbed H atoms and H2 molecules, or after their radiative recombination. It is shown that in the outer regions (beyond ∼ 3–4 AU) the decharging of alphas on neutral hydrogen is the dominant source of the ‘solar’ He+ ions. Their predicted abundance in the solar wind due to this mechanism remains in good agreement with recent estimates of the upper limit of the solar He+ /He++ ratio determined from SWICS measurements on Ulysses carried out in 1991–1993. At smaller distances (R < 1–2 AU) a significant contribution to the solar He+ abundance is expected also from other mechanisms, mainly from the radiative recombination, and closer to the Sun Send offprint requests to: D. Ruciński possibly from the decharge of alphas on dust-desorbed hydrogen atoms and molecules. Extending our calculation to the outer heliospheric regions (R ∼ 70–100 AU) we conclude that the content of the most abundant He+ pickup component may reach a noticeable fraction up to ∼ 15–35% of the abundance of the original solar wind alphas, the ‘solar’ He+ ions may contribute at ∼ 1–3% of the solar wind alpha level, and the content of He++ pickup and neutral energetic He atoms is typically between ∼ 0.5% and 1.0% of the alphas abundance.