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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.02.005 Byline: Faez Bakalian Keywords: Mars; atmosphere; Aeronomy; Atmospheres; evolution; Atmospheres; structure; Ionospheres Abstract: The energy and altitude distributions for nitrogen atoms produced from photodissociation of N.sub.2 and dissociative recombination of N.sup.+.sub.2 have been computed for the low and high solar activity martian thermospheres. We find that the fraction of nitrogen atoms with EE.sub.esc at the exobase is [approximately equal to]2.5% for photodissociation as compared to [approximately equal to]22.6% for dissociative recombination. However, the peak magnitudes of the production rate profiles for dissociative recombination are a factor of [approximately equal to]1000 and [approximately equal to]200 times smaller than those of photodissociation at low and high solar activities, respectively. Thus, our findings suggest that photodissociation of N.sub.2 is the dominant escape mechanism for atomic nitrogen for both the low and high solar activity martian thermospheres. The general consensus in the field prior to these research results was that dissociative recombination of N.sup.+.sub.2 was the main escape mechanism. We find that it is the dissociation of N.sub.2 along repulsive states that results in the production of very energetic nitrogen atoms. Author Affiliation: Institute for Terrestrial and Planetary Atmospheres, Stony Brook University, Stony Brook, NY 11777, USA Article History: Received 15 November 2005; Revised 1 February 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.02.004 Byline: Faez Bakalian (a), Richard E. Hartle (b) Keywords: Mars; atmosphere; Aeronomy; Atmospheres; evolution; Atmospheres; structure; Ionospheres Abstract: Monte Carlo simulations were carried out to compute the escape flux of atomic nitrogen for the low and high solar activity martian thermospheres. The total escape of atomic nitrogen at low and high solar activities was found to be 3.03x10.sup.5 and 1.32x10.sup.6cm.sup.-2s.sup.-1, respectively. The escape flux of atomic nitrogen at low and high solar activities from photodissociation of N.sub.2 was found to be 2.75x10.sup.5 and 9.90x10.sup.5cm.sup.-2s.sup.-1, respectively. The remainder of the contribution is from dissociative recombination, which is only important at high solar activity were it comprises about 25% of the total escape. The relative contributions to the total N escape flux from thermal motion of the background atmosphere, winds and co-rotation, and photoionization and subsequent solar wind pickup are also considered here. We find that the total predicted escape fluxes are observed to increase by 20 and 25% at low and high solar activities owing to thermal motion of the background atmosphere. At low and high solar activities, we find that the co-rotation and wind velocities combined translate to a maximum transferable energy of [approximately equal to]0.0103 and 0.0181 eV, respectively, and that the total escape flux contribution from winds and co-rotation is negligible. Photoionization was found to be a minor process only impacting those source atoms produced with energies close to the escape energy, between 1.5 and 2 eV. The contributions to the total escape fluxes at low and high solar activities from photoionization and subsequent solar wind pickup are found to be about 8 and 13%, respectively. Author Affiliation: (a) Institute for Terrestrial and Planetary Atmospheres, Stony Brook University, Stony Brook, NY 11777, USA (b) Goddard Space Flight Center, NASA, Greenbelt, MD 20771, USA Article History: Received 15 November 2005; Revised 1 February 2006