Model for the production of CO Cameron band emission in Comet 1P/Halley

Abstract

The abundance of CO 2 in comets has been derived using CO Cameron band (a 3 Π → X 1 Σ + ) emission assuming that photodissociative excitation of CO 2 is the main production process of CO(a 3 Π). On comet 1P/Halley the Cameron (1-0) band has been observed by International Ultraviolet Explorer (IUE) on several days in March 1986. A coupled chemistry-emission model is developed for comet 1P/Halley to assess the importance of various production and loss mechanisms of CO(a 3 Π) and to calculate the intensity of Cameron band emission on different days of IUE observation. Two different solar EUV flux models, EUVAC of Richards et al. (1994) and SOLAR2000 of Tobiska (2004), and different relative abundances of CO and CO 2 , are used to evaluate the role of photon and photoelectron in producing CO molecule in a 3 Π state in the cometary coma. It is found that in comet 1P/Halley 60-70% of the total intensity of the Cameron band emission is contributed by electron impact excitation of CO and CO 2 , while the contribution from photodissociative excitation of CO 2 is small (20-30%). Thus, in the comets where CO and CO 2 relative abundances are comparable, the Cameron band emission is largely governed by electron impact excitation of CO, and not by the photodissociative excitation of CO 2 as assumed earlier. Model calculated Cameron band 1-0 emission intensity (40 R) is consistent with the observed IUE slit-averaged brightness (37 ± 6 R) using EUVAC model solar flux on 13 March 1986, and also on other days of observations. Since electron impact excitation is the major production mechanism, the Cameron emission can be used to derive photoelectron density in the inner coma rather than the CO 2 abundance.

Model for atomic oxygen visible line emissions in Comet C/1995 O1 Hale-Bopp

“…The degradation of solar EUV-generated photoelectrons is accounted by using Analytical Yield Spectrum (AYS) technique which is based on the Monte-Carlo method (Singhal and Bhardwaj, 1991;Bhardwaj and Singhal, 1993;Bhardwaj and Michael, 1999a,b;Bhardwaj and Jain, 2009). Details of the AYS approach and the method to calculate photoelectron flux and excitation rates are given in our earlier papers (Bhardwaj et al, 1990(Bhardwaj et al, , 1996Bhardwaj, 1999Bhardwaj, , 2003Haider and Bhardwaj, 2005;Bhardwaj and Raghuram, 2011;Raghuram and Bhardwaj, 2012;Bhardwaj and Jain, 2012).…”

“…The photodissociative excitation cross sections for the production of O( 1 D) and O( 1 S) from H 2 O, CO 2 , and CO used in the model are presented in Figure 1. The attenuation of solar radiation and solar UV-EUV generated photoelectrons in the cometary coma are described in our previous works (Bhardwaj et al, 1990;Bhardwaj, 1999Bhardwaj, , 2003Bhardwaj and Haider, 1999;Raghuram and Bhardwaj, 2012).…”

CO+first-negative band emission: A tracer for CO in the Martian upper atmosphere

“…where σ lT (E) is total inelastic cross section of lth species and U(E, E 0 ) is the two-dimensional AYS, which contains the nonspatial electron degradation information. More details about the AYS method can be found in our earlier work (Singhal & Haider 1984;Bhardwaj et al 1990Bhardwaj et al , 1996Bhardwaj 1999;Jain & Bhardwaj 2011Bhardwaj & Jain 2013;Raghuram & Bhardwaj 2012.…”

CO prompt emission as a CO2 marker in comets and planetary atmospheres

“…However, the 75% fraction of the Cameron bands that are claimed to arise from 331 photoelectrons on CO 2 [Raghuram and Bhardwaj, 2012] do not exclude similar CO 332 products, considering that the energy threshold for CO(a) production from CO 2 is 333 11.5 eV, whereas that for CO(a') production is 12.3 eV. Therefore the 10 R estimate for 334 the intensity of the a'-a 5-0 band is a lower limit, an upper limit being larger by a factor 335 of four.…”