Ionization by cosmic rays of the atmosphere of Titan

Cosmic ray radiation is the main mechanism for ionizing the lower atmospher e of Titan. Their higher penetration power, in comparison with solar photon s, allows cosmic rays to penetrate deep into the atmosphere of Titan, ioniz ing the neutral molecules and generating an ionosphere with an electron den sity peak, placed at around 90 km, similar in magnitude to the ionospheric peak produced by solar radiation in the upper atmosphere. In the lower atmo sphere, the electron density profile, in the absence of a magnetic held, de pends mainly on the modulation of cosmic rays by the solar wind and on the nature of the ionizable particles. We present here the first results of a n ew numerical model developed to calculate the concentration of electrons an d most abundant ions in the Titan lower atmosphere. The present knowledge o f Titan's atmosphere permits us to include new neutral and ionic species, s uch as oxygen derivates, in a more detailed ion-chemistry calculation than previous lower ionospheric models of Titan. The electron density peaks at 9 0 km with a magnitude of 2150 cm(-3). The ion distribution obtained predict s that cluster cations and hydrocarbon cations are the most abundant ions b elow and above the electron density peak, respectively. We also discuss the effect of solar activity at the distance of the Saturn orbit on the spectr um of the cosmic particles. We obtain that from solar minimum to solar maxi mum the ionization rate at the energy deposition peak changes by a factor o f 1.2 at 70 km, and by a factor of 2.6 at altitudes as high as 400 km. The electron density at the concentration peak changes by a factor of 1.1 at 90 km, and by a factor of 1.6 at 400 km. (C) 1999 Elsevier Science Ltd. All r ights reserved.