Top left: phase space diagram in a spacecraft frame of reference. The white shaded area is the instrumental coverage of PLASTIC in phase space. The angle α denotes the azimuthal angle of an ion’s velocity vector in a resting frame of reference. Within the highlighted phase space volumes in red, yellow, and blue, only He⁺ particles with velocities of 1.9 <w < 2.0 and angles of α = (−15°,0°,15°) are measured. Top right: mean count rates of He⁺, C_He⁺ detected in the three different phase space volumes (in corresponding colours) as a function of φ_B. Bottom: probability density η(φ_B) (see Eq. (2)) plotted for the range −180°<φ_B < 180°. The vertical lines denote the expected magnetic field configurations in which the He⁺ ring beam distribution lies within the respective phase space volumes.