Since decades now the location and geometry of the solar wind termination shock has been a subject of intensive theoretical investigations (for a recent review see Zank 1999), though up to now no shock structure could yet be identified in direct or indirect signatures by the deep-space NASA spaceprobes. Meanwhile not only the location but even more the structure of the shock has become an object of prime interest since the multi-fluid character of this shock transition has clearly been recognized (Donohue & Zank 1993; Zank et al. 1993; Chalov & Fahr 1994,1995,1997; Le Roux & Fichtner 1997; Kausch & Fahr 1997; Fahr et al. 2000). For instance, dependent on the degree of the cosmic-ray-induced shock modulation and the pick-up ion (PUI) injection efficiency at the shock, density compression ratios between 1 and 6.5 can be expected. Furthermore not only will the shock be a non-classical one (i.e. classical Rankine-Hugoniot relations may not be applicable), it most likely also has the character of a weak shock with preshock solar wind Mach numbers much smaller than expected earlier (see Baranov & Malama 1993; Fahr & Rucinski 1999; Fahr et al. 2000). Contrary to earlier expectations (Potgieter & Moraal 1988) the spectrum of anomalous cosmic rays originating near the shock is likely not to be of a power-law type, not even at the shock position itself (see Le Roux & Fichtner 1997; Dworsky & Fahr 2000; Chalov & Fahr 2000). For a better control of this open field of theoretical speculations by means of observational data directly relevant to these features we shall develop a theoretical description for post-shock pick-up ions in the heliospheric sheath region here and shall derive PUI-related energetic neutral atoms (ENA) fluxes and discuss their detectability at the position of the Earth.
Copyright ESO 2001