Coupling a photochemical model of Triton's atmosphere with an electron transport code
Authors: Benne Benjamin , Benmahi Bilal , Dobrijevic Michel , Cavalié Thibault , Loison Jean-Christophe , Hickson Kevin , Barthélémy Mathieu , Lilensten Jean 
Published: February 2023

IntroductionDuring the only flyby of Triton by Voyager 2 in 1989, a dense ionosphere was observed (Tyler et al. 1989). Results were surprising as the solar irradiation of this satellite is ten times lower than on Titan, and yet its ionosphere is denser. Thus, electronic precipitation from Neptune&#8217;s magnetosphere was hypothesized to bring the needed extra input energy (Krasnopolsky et al. 1993), as high energy electrons have been observed by the spacecraft in this area (Krimigis et al. 1989).&#160; To understand how this precipitation could impact the composition of Triton&#8217;s atmosphere, we coupled an electron transport code to a photochemical model of this atmosphere.Methodology&#160;We used the electron transport code TRANS that was utilized to compute the transport of electrons in various planetary atmospheres (see Gronoff et al. 2009 and references therein). We adapted it to Triton&#8217;s conditions and used the results from Strobel et al. (1990) and Sittler and Hartle (1996) to compute the input precipitation. This led us to calculate the mean magnetic field and the mean precipitation before adjusting it depending on energy, as detailed in Sittler and Hartle (1996). We then coupled TRANS with our most recent photochemical model of Triton&#8217;s atmosphere (Benne et al. 2022) by using TRANS outputs to compute the reaction rates of the electro-dissociation and electro-ionization reactions. Iterations were performed between the two codes until steady state was reached. After determining the nominal composition of the atmosphere, we ran a Monte Carlo simulation to characterize the effect of chemical uncertainties on the model results.ResultsWith our previous model presented in Benne et al. (2022), we found a peak electronic number density larger by a factor of 2.5 to 5 compared to the one derived from Voyager 2 observations. By coupling the photochemical model with TRANS, we find that our electronic profile is now in agreement with these measurements, resulting from a significant decrease of the electro-ionization rate. In contrast with the results of Benne et al. (2022), Krasnopolsky and Cruikshank (1995) and Strobel and Summers (1995), the main ionization source is solar EUV radiation instead of magnetospheric electrons. This work also allows us to better understand how the varying magnetic environment impacts the atmospheric chemistry.References[1] Tyler, G. L. et al. Science 246, no. 4936 (December 15, 1989): 1466&#8211;73.[2] Krasnopolsky, V. A. et al. Journal of Geophysical Research 98 (February 1, 1993): 3065&#8211;78.[3] Krimigis, S. M. et al. Science 246, no. 4936 (December 15, 1989): 1483&#8211;89.[4] Gronoff, G. et al. Astronomy & Astrophysics 506, no. 2 (November 2009): 955&#8211;64.[5] Strobel, Darrell F. et al. Geophysical Research Letters 17, no. 10 (1990): 1661&#8211;64.[6] Sittler, E. C., and R. E. Hartle. Journal of Geophysical Research: Space Physics 101, no. A5 (May 1, 1996): 10863&#8211;76.[7] Benne, B. et al. Astronomy & Astrophysics 667 (November 2022): A169.[8] Krasnopolsky, Vladimir A., and Dale P. Cruikshank. Journal of Geophysical Research 100, no. E10 (1995): 21271.[9] Strobel, D. F., and M. E. Summers. 1995, 1107&#8211;48. Cruikshank, Dale P., Mildred Shapley Matthews, et A. M. Schumann. &#171; Neptune and Triton &#187;, 1995.