Panel a: two-dimensional histogram of He⁺ counts as a function of ecliptic longitude, λ, and V_r . The data have been filtered for IMF orientation according to − 15^° ≤ Θ_GC ≤ 15^° and − 15^° ≤ Φ_GC ≤ 15^° (see Sects. 3 and 6), and acceleration sites by requiring (see Sect. 4.3). The counts per bin are normalized to the maximum of each λ bin. Panel b: demonstration of the three cutoff determination techniques performed on the example of bin λ = 100^°. The upper panel shows the tanh-fit as used in Möbius et al. (2015), the middle panel shows the 50% threshold method, and the lower panel illustrates the maximization of the correlation coefficient with a reference curve. Panel c: resulting cutoff speeds from the threshold method as a function of λ. The interstellar flow longitude is determined by fitting Eq. (5) to the results. Panel d: mirror-correlation coefficients for every bin of λ calculated from the cutoff speeds. The interstellar flow longitude is determined by fitting a cos -function to the resulting correlation coefficients.