Fig. 4.

Observations of the He-focusing cone, as seen in the ${\text{He}}^{+}$ PUI phase space density observed near 1 au in 1984, 1985, and 1998–2002. (a) AMPTE-IRM SULEICA and (b) ACE SWICS made observations near Earth and L1, respectively. (c) ACE and Nozomi observations are also compared for observations in 2000. The peak location of the He focusing cone is found on DOY 339.75, corresponding to Earth’s crossing of the cone center at longitude $\lambda ={74.6}^{\circ }$. From Gloeckler et al. (2004). © ESO. Reproduced with permission