Nightglow excitation and maintenance of the nighttime ionosphere by low-energy protons

The effects of 4-kev protons on the earth's upper atmosphere at midlatitudes have been investigated. A Monte Carlo calculation of the flux distribution in the atmosphere, assuming that the initially monoenergetic protons are uniformly mirroring at altitudes where neutralization is important, shows that two thirds of the incident protons are lost to space and none penetrate to altitudes lower than 135 km. The largest energy deposition occurs near 200 km. An initial flux of 5×107 protons cm−2 sec−1 gives up 10−1 erg cm−2 (column) sec−1 to neutral heating and about 10−3 erg cm−2 (column) sec−1 to electron heating. The calculated intensity of 3914 A radiation is 1.0 R at a magnetic field inclination of 45°; protons and neutrals produce 5.3 R of 6300 A light and 2.0 R of 5577 A light, but the secondary electrons may increase these latter numbers. Equilibrium solutions to the five coupled diffusion equations involving the major atmospheric ions have been found both with and without the proton neutral flux. To maintain the nighttime ionosphere above 250 km, 1.2×108 ions cm−2 sec−1 must diffuse down from the exosphere; the additional 4-kev proton flux has little effect. Below 250 km diffusion alone can account for only about 2% of the observed ion concentration; the proton neutral flux can support the ionosphere down to about 150 km, depending somewhat on the magnetic latitude.