On the variability of He+ suprathermal tails

Interstellar neutral helium penetrates to well within 1 AU where it is ionized predominantly by UV from the Sun. These freshly created $He+$ pickup ions (PUI) are convected outward with the solar wind and subject to acceleration processes, for example in compression regions, corotating interaction regions, or CME-driven interplanetary shocks. These acceleration processes result in the formation of suprathermal tails in the PUI velocity distribution. The PLASTIC instruments on the STEREO spacecraft cover for pickup He the energy range of ∼1-20 keV/nuc. They allow the determination of PUI velocity distribution functions from the source distribution to suprathermal energies. In this paper we study the suprathermal tails of He pickup ions during the period January to December 2008 that featured many corotating interaction regions. The PLASTIC observations for the $He+$ suprathermal tail show a large variability of the spectral index γ of the distribution function f(w)∼w^γ, with w = V/V_sw, where V and V_sw are the particle velocity and the solar wind velocity, respectively. Incorporating a transformation from the spacecraft frame to the solar wind frame, the spectral index at velocities 1.5 ≤ w ≤ 5 (in the solar wind frame) ranges from γ ∼ −5.7 at a forward shock to γ = −9 in the slow solar wind.