(a) Average velocity distribution functions of He²⁺ observed by the SWS during “D1,” “D2,” and “U2” indicated in Figure 2. (b)–(c) PSD averaged over w_sw ∼ 1.5–4 (dashed lines in (a)) as a function of PA, normalized to its maximum among all PA bins, shown as a line plot with estimated statistical errors for the three time periods. The PSDs are obtained with applying the base rate weight in (c) and without in (b).