Photochemistry of Pluto's atmosphere and ionosphere near perihelion

We consider Pluto's photochemistry using a background model for a hydrodyna mically escaping atmosphere by Krasnopolsky [1999], Some adjustments are ma de in the basic continuity equation and in the boundary conditions to accou nt for hydrodynamic flow in the atmosphere. We model the photochemistry for 44 neutral and 23 ion species. Because of the high methane mixing ratio, P luto's photochemistry is more similar to that of Titan than that of Triton. Charge exchange between N-2(+), and CH, significantly reduces the producti on of atomic nitrogen. The most abundant photochemical products are C2H2 (3 x10(17)), C4H2 (10(17)), HCN (6x10(16)), H-2 (4x10(16)), C2H4 (4x10(16)), H C3N (3.4x10(16)), C2H6 (2x10(16)), C3H2 (9x10(15)), and C3H4 (8x10(15), all in cm(-2)). In addition to the parent N-2, CH4, and CO molecules which abs orb photons with lambda<145 nm, these products absorb almost completely pho tons with lambda<185 nm, therefore significantly increasing the number of d issociation events. Photochemical losses of the parent species are much sma ller than their escape. Precipitation rates are the highest for C2H2, C4H2, HC3N, HCN, C2H6, and C2H4 (65, 58, 23, 14, 9, and 6 g cm(-2) Byr(-1), resp ectively, reduced by a factor of 3 to account for seasonal variations). Esc ape of photochemical products is highest for H-2, H, C2H2, C2H4, HCN, and N (2x10(26), 1.4x10(26), 6x10(24), 3.6x10(24), 2.3x10(24), and 1.8x10(24) s( -1), respectively). The electron density reaches a maximum of 800 cm(-3) at 2250 km. The most abundant ions are HCNH+, C3H3+, and C3H5+. Some of the p hotochemical products might be detected using the technique of UV solar occ ultation spectroscopy from a spacecraft flyby.