The green (5577Å) and red-doublet (6300, 6364Å) lines are prompt emissions of metastable oxygen atoms in the 1 S and 1 D states, respectively, that have been observed in several comets. The value of intensity ratio of green to red-doublet (G/R ratio) of 0.1 has been used as a benchmark to identify the parent molecule of oxygen lines as H 2 O. A coupled chemistry-emission model is developed to study the production and loss mechanisms of O( 1 S) and O( 1 D) atoms and the generation of red and green lines in the coma of C/1996 B2 Hyakutake. The G/R ratio depends not only on photochemistry, but also on the projected area observed for cometary coma, which is a function of the dimension of the slit used and geocentric distance of the comet. Calculations show that the contribution of photodissociation of H 2 O to the green (red) line emission is 30 to 70% (60 to 90%), while CO 2 and CO are the next potential sources contributing 25 to 50% (<5%). The ratio of the photo-production rate of O( 1 S) to O( 1 D) would be around 0.03 (± 0.01) if H 2 O is the main source of oxygen lines, whereas it is ∼0.6 if the parent is CO 2 . Our calculations suggest that the yield of O( 1 S) production in the photodissociation of H 2 O cannot be larger than 1%. The model calculated radial brightness profiles of the red and green lines and G/R ratios are in good agreement with the observations made on comet Hyakutake in March 1996.

Aims. We study the formation of the [OI] lines -that is, 5577.339 Å (the green line), 6300.304 Å and 6363.776 Å (the two red lines) -in the coma of comets and determine the parent species of the oxygen atoms using the ratio of the green-to-red-doublet emission intensity, I 5577 /(I 6300 + I 6364 ), (hereafter the G/R ratio) and the line velocity widths. Methods. We acquired high-resolution spectroscopic observations at the ESO Very Large Telescope of comets C/2002 T7 (LINEAR), 73P-C/Schwassmann-Wachmann 3, 8P/Tuttle, and 103P/Hartley 2 when they were close to Earth (<0.6 au). Using the observed spectra, which have a high spatial resolution (<60 km/pixel), we determined the intensities and widths of the three [OI] lines. We spatially extracted the spectra to achieve the best possible resolution of about 1−2 , that is, nucleocentric projected distances of 100 to 400 km depending on the geocentric distance of the comet. We decontaminated the [OI] green line from C 2 lines blends that we identified.Results. The observed G/R ratio in all four comets varies as a function of nucleocentric projected distance (between ∼0.25 to ∼0.05 within 1000 km). This is mainly due to the collisional quenching of O( 1 S) and O( 1 D) by water molecules in the inner coma. The observed green emission line width is about 2.5 km s −1 and decreases as the distance from the nucleus increases, which can be explained by the varying contribution of CO 2 to the O( 1 S) production in the innermost coma. The photodissociation of CO 2 molecules seem to produce O( 1 S) closer to the nucleus, while the water molecule forms all the O( 1 S) and O( 1 D) atoms beyond 10 3 km. Thus we conclude that the main parent species producing O( 1 S) and O( 1 D) in the inner coma is not always the same. The observations have been interpreted in the framework of the previously described coupled-chemistry-emission model, and the upper limits of the relative abundances of CO 2 were derived from the observed G/R ratios. Measuring the [OI] lines might provide a new way to determine the CO 2 relative abundance in comets.

We have recently developed a coupled chemistry-emission model for the green (5577Å) and red-doublet (6300, 6364 A) emissions of atomic oxygen on comet C/1996 B2 Hyakutake. In the present work we applied our model to comet C/1995 O1 Hale-Bopp, which had an order of magnitude higher H 2 O production rate than comet Hyakutake, to evaluate the photochemistry associated with the production and loss of O( 1 S) and O( 1 D) atoms and emission processes of green and red-doublet lines. We present the wavelength-dependent photo-attenuation rates for different photodissociation pro 5577Å emission line is controlled by photodissociation of both H 2 O and CO 2 . The calculated mean excess energy in various photodissociation processes show that the photodissociation of CO 2 can produce O( 1 S) atoms with higher excess velocity compared to the photodissociation of H 2 O. Thus, our model calculations suggest that involvement of multiple sources in the formation of O( 1 S) could be a reason for the larger width of green line than that of red-doublet emission lines observed in several comets.

Context. The Rosetta encounter with comet 67P/Churyumov-Gerasimenko provides a unique opportunity for an in situ, up-close investigation of ion-neutral chemistry in the coma of a weakly outgassing comet far from the Sun. Aims. Observations of primary and secondary ions and modeling are used to investigate the role of ion-neutral chemistry within the thin coma. Methods. Observations from late October through mid-December 2014 show the continuous presence of the solar wind 30 km from the comet nucleus. These and other observations indicate that there is no contact surface and the solar wind has direct access to the nucleus. On several occasions during this time period, the Rosetta/ROSINA/Double Focusing Mass Spectrometer measured the low-energy ion composition in the coma. Organic volatiles and water group ions and their breakup products (masses 14 through 19), CO + , and CO + 2 (masses 28 and 44) and other mass peaks (at masses 26, 27, and possibly 30) were observed. Secondary ions include H 3 O + and HCO + (masses 19 and 29). These secondary ions indicate ion-neutral chemistry in the thin coma of the comet. A relatively simple model is constructed to account for the low H 3 O + /H 2 O + and HCO + /CO + ratios observed in a water dominated coma. Results from this simple model are compared with results from models that include a more detailed chemical reaction network. Results. At low outgassing rates, predictions from the simple model agree with observations and with results from more complex models that include much more chemistry. At higher outgassing rates, the ion-neutral chemistry is still limited and high HCO + /CO + ratios are predicted and observed. However, at higher outgassing rates, the model predicts high H 3 O + /H 2 O + ratios and the observed ratios are often low. These low ratios may be the result of the highly heterogeneous nature of the coma, where CO and CO 2 number densities can exceed that of water.

Context. In comets, the atomic oxygen green (5577 Å) to red-doublet (6300, 6364 Å) emission intensity ratio (G/R ratio) of 0.1 has been used to confirm H 2 O as the parent species producing forbidden oxygen emission lines. The larger (>0.1) value of G/R ratio observed in a few comets is ascribed to the presence of higher CO 2 and CO relative abundances in the cometary coma. Aims. We aim to study the effect of CO 2 and CO relative abundances on the observed G/R ratio in comets observed at large (>2 au) heliocentric distances by accounting for important production and loss processes of O( 1 S) and O( 1 D) atoms in the cometary coma. Methods. Recently we have developed a coupled chemistry-emission model to study photochemistry of O( 1 S) and O( 1 D) atoms and the production of green and red-doublet emissions in comets Hyakutake and Hale-Bopp. In the present work we applied the model to six comets where green and red-doublet emissions are observed when they are beyond 2 au from the Sun. Results. The collisional quenching of O( 1 S) and O( 1 D) can alter the G/R ratio more significantly than that due to change in the relative abundances of CO 2 and CO. In a water-dominated cometary coma and with significant (>10%) CO 2 relative abundance, photodissociation of H 2 O mainly governs the red-doublet emission, whereas CO 2 controls the green line emission. If a comet has equal composition of CO 2 and H 2 O, then ∼50% of red-doublet emission intensity is controlled by the photodissociation of CO 2 . The role of CO photodissociation is insignificant in producing both green and red-doublet emission lines and consequently in determining the G/R ratio. Involvement of multiple production sources in the O( 1 S) formation may be the reason for the observed higher green line width than that of red lines. The G/R ratio values and green and red-doublet line widths calculated by the model are consistent with the observation. Conclusions. Our model calculations suggest that in low gas production rate comets the G/R ratio greater than 0.1 can be used to constrain the upper limit of CO 2 relative abundance provided the slit-projected area on the coma is larger than the collisional zone. If a comet has equal abundances of CO 2 and H 2 O, then the red-doublet emission is significantly (∼50%) controlled by CO 2 photodissociation and thus the G/R ratio is not suitable for estimating CO 2 relative abundance.

The CO 2 production rate has been derived in comets using Cameron-band (a 3 → X 1 ) emission of CO molecules, assuming that photodissociative excitation of CO 2 is the main production mechanism of CO in the a 3 metastable state. We have developed a model for the production and loss of CO(a 3 ), which has been applied to comet 103P/Hartley 2: the target of the EPOXI mission. Our model calculations show that photoelectron impact excitation of CO and dissociative excitation of CO 2 can together contribute about 60-90 per cent to Cameronband emission. The modelled brightness of (0-0) Cameron-band emission on comet Hartley 2 is consistent with Hubble Space Telescope observations for 3-5 per cent CO 2 (depending on the model input solar flux) and 0.5 per cent CO relative to water, where the photoelectron impact contribution is about 50-75 per cent. We suggest that estimation of CO 2 abundances on comets using Cameron-band emission may be reconsidered. We predict a height-integrated column brightness of the Cameron band of ∼1300 Rayleigh during the EPOXI mission encounter period.

Remote observation of spectroscopic emissions is a potential tool for the identification and quantification of various species in comets. The CO Cameron band (to trace CO 2 ) and atomic oxygen emissions (to trace H 2 O and/ or CO 2 , CO) have been used to probe neutral composition in the cometary coma. Using a coupled-chemistryemission model, various excitation processes controlling the CO Cameron band and different atomic oxygen and atomic carbon emissions have been modeled in comet 67P/Churyumov-Gerasimenko at 1.29 AU (perihelion) and at 3 AU heliocentric distances, which is being explored by ESAʼs Rosetta mission. The intensities of the CO Cameron band, atomic oxygen, and atomic carbon emission lines as a function of projected distance are calculated for different CO and CO 2 volume mixing ratios relative to water. Contributions of different excitation processes controlling these emissions are quantified. We assess how CO 2 and/or CO volume mixing ratios with respect to H 2 O can be derived based on the observed intensities of the CO Cameron band, atomic oxygen, and atomic carbon emission lines. The results presented in this work serve as baseline calculations to understand the behavior of low out-gassing cometary coma and compare them with the higher gas production rate cases (e.g., comet Halley). Quantitative analysis of different excitation processes governing the spectroscopic emissions is essential to study the chemistry of inner coma and to derive neutral gas composition.

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.