Top: average He⁺ counts observed per φ bin, α bin, and 5 min in a velocity range of 1.8 <w < 2.2 plotted as a function of φ_B (x-axis) and α (y-axis). Centre: probability density η(φ_B) plotted as a function of α and φ_B in a spacecraft frame of reference. Bottom: same as centre panel, except for He⁺ pickup ions with velocities of 0.8 <w_sw < 1.2 coming from certain direction β in a solar wind frame of reference. The solid black lines denote the expected positions of the He⁺ ring beam distributions.