Ionospheric density as a function of cometocentric distance from two models (solid and dashed lines, see text) and assuming that the density starts to decrease as 1/r² at 10⁴ km. The density is calculated for two outgassing rates (red and blue). The bow shock distance at the terminator (which is roughly twice the subsolar standoff distance) for the two outgassing rates is indicated (vertical dash-dotted lines). The grey shaded region indicates the solar wind density at 1 AU. The ion density measured by Giotto at comet 1P/Halley is also included (green dots).