MINOR HELIUM COMPONENTS COMOVING WITH THE SOLAR-WIND

We revisit the problem of the minor He-4 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 a nd 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 i ons, He+ ions of solar wind characteristics, and energetic (similar to 4 keV) neutral He atoms. We provide estimates of their expected fluxe s and discuss their radial variations. It is shown that the doubly cha rged He++ pickup population may typically contribute similar to 2-3% t o the total helium pickup flux and it demonstrates (similarly to the H e+ pickup component) a pronounced downwind-upwind anisotropy resulting from the gravitational focusing of the interstellar helium. Similar f luxes, of the order of similar to 10(2)-10(3) cm(-2) s(-1) at 1 AU, ar e 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 com ponent may be the dominant constituent of the Neutral Solar Wind (NSW) up to distances of similar to 0.4-0.6 AU on the upwind side, similar to 0.5-1.0 AU in the sidewind direction, and even up to similar 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+ io ns, with the properties inherited from the former solar wind alpha par ticles after their transcharge on the neutral H and He interstellar at oms or on the neutral dust-desorbed H atoms and H-2 molecules, Or afte r their radiative recombination. It is shown that in the outer regions (beyond similar to 3-4 AU) the decharging of alphas on neutral hydrog en is the dominant source of the 'solar' He+ ions. Their predicted abu ndance in the solar wind due to this mechanism remains in good agreeme nt with recent estimates of the upper limit of the solar He+/He++ rati o determined from SWICS measurements on Ulysses carried out in 1991-19 93. At smaller distances (R < 1-2 AU) a significant contribution to th e solar He+ abundance is expected also from other mechanisms, mainly f rom the radiative recombination, and closer to the Sun possibly from t he decharge of alphas on dust-desorbed hydrogen atoms and molecules. E xtending our calculation to the outer heliospheric regions (R similar to 70-100 AU) we conclude that the content of the most abundant He+ pi ckup component may reach a noticeable fraction up to similar to 15-35% Of the abundance of the original solar wind alphas, the 'solar' He+ i ons may contribute at similar to 1-3% Of the solar wind alpha level, a nd the content of He++ pickup and neutral energetic He atoms is typica lly between similar to 0.5% and 1.0% of the alphas abundance.