Interaction of singly charged interstellar helium ions with the solar wind

Because of the long time scales for recombination and charge exchange in the solar wind, helium ions remain in the charge state characteristic of their source region. Ions identified in the solar wind with energy per charge 4 times that of the protons [Wolfe et al., 1966; Bame et al., 1968] have been ascribed to singly charged helium traveling at the same bulk speed as the solar wind. These ions are identified as interstellar helium that has entered the heliosphere, penetrated to a heliocentric distance of <1 AU, been ionized there, and been swept back past 1 AU again. The initial velocities of such ions can be resolved into components, one directed toward the sun parallel to the interplanetary magnetic field B with a magnitude equal to the corresponding component of the parent helium atom. Owing to the presence of the electric field E = −V×B/c in the infinitely conducting solar wind plasma of velocity V, the remaining initial velocity components correspond to the usual cycloidal trajectory in a plane normal to B with a guiding center drift velocity vD = cE×B/B2. This letter suggests and describes two mechanisms that together should lead to He+ ions that are assimilated with the solar wind at 1 AU, i.e., ions thermalized and moving with bulk speed equal to that of the solar wind. The first mechanism we consider is the collective plasma instability recently identified theoretically by Wu and Davidson [1972] in which the transverse motion of the newly born ions is rapidly thermalized. The second mechanism is that of cyclotron resonance scattering by the intrinsic magnetic field fluctuations in the solar wind. This second mechanism will be shown to be important for scattering the He+ ions into the bulk motion of the wind.