Fig. 11.

(a) Spectra of ${\text{H}}^{+}$, ${\text{He}}^{2+}$, and ${\text{He}}^{+}$ ions measured by Ulysses SWICS as a function of particle speed in the spacecraft reference frame. (b) Spectra in the solar wind plasma reference frame assuming that the phase density is only a function of particle speed in the plasma reference frame in each channel of the instrument. In some speed ranges, particles with the same speed are measured by sunward and anti-sunward facing channels separately, which results in two tracks of the distribution function. The low-speed parts of the ${\text{H}}^{+}$ and ${\text{He}}^{2+}$ spectra are dominated by the solar wind ions, which can be fit with a kappa distribution as shown by the red and green dashed curves. The low-speed part of the ${\text{He}}^{+}$ spectrum is fit with a spectrum for inner source PUIs (Schwadron et al. 2000). (c) Interstellar PUI spectra after subtraction of fitted spectra of the solar wind ${\text{H}}^{+}$ and ${\text{He}}^{2+}$ ions and inner source ${\text{He}}^{+}$ PUIs. It shows that the interstellar PUIs are not isotropic in the plasma reference frame. (d) Radial dependence of interstellar neutral H and He density derived using the PUI distribution formula from Vasyliunas and Siscoe (1976). We note that the radial dependence of He density derived here likely needs a modification because the anisotropy of freshly produced ${\text{He}}^{+}$ PUIs causes a significant underestimation of the PUI flux