Guiding electron to neutral number density ratios in the coma of comet 67P/Churyumov-Gerasimenko throughout the pre-perihelion phase of the Rosetta mission
Abstract
We combine TIMED/SEE solar EUV spectra [1] with photoionization cross section sets [2] to derive partial photoionization frequencies of H2O, CO and CO2, the dominant molecules in the coma of comet 67P/Churyumov-Gerasimenko [3]. The values are in accord with earlier estimates [4] although with some differences regarding the yield of minor product ions. The computed H2O ionization frequency in the coma of 67P is estimated as a function of time (and heliocentric distance) from August 2014 to August 2015, from the rendezvous of Rosetta with 67P at ~3.6 AU to perihelion at ~1.25 AU. This timedependent ionization frequency is used together with an adopted radial speed of the cometary neutrals of 650 m/s [5] to generate a guiding electron to neutral number density ratio, Ge/N(r,d) as a function of cometocentric distance, r, and heliocentric distance, d. We present a parameterization of Ge/N(r,d) and argue that comparisons of observations with Ge/N(r,d) is a useful method to gain insights into physical and chemical processes at play in the cometary coma. Minor deviations, by up to a factor of 2 or so, can result from missing ionization processes and by variations in the chemical composition, in the impinging solar EUV irradiation, or in the neutral's outgassing speed profiles. Such effects are accounted for in Ref. [6]. Major deviations, with Ge/N(r,d) being significantly higher than ratios observed, can result e.g., from the effect of electric fields accelerating the ion population or, near perihelion, by attenuation of the solar EUV irradiation, by the increased importance of dissociative recombination as a plasma neutralizing mechanism [7] and possibly also, by nanograin charging [8]. Observations by LAP, MIP and ROSINA onboard Rosetta at r=10 km and d=3.2 AU indicate an electron to neutral number density ratio of 1-2x10-6 [9]. This is surprisingly well in resemblance with our Ge/N(r=10 km,d=3.2 AU) value of 1.0x10-6. Further comparisons with observations will be presented at the meeting.
- Publication:
-
European Planetary Science Congress
- Pub Date:
- October 2015
- Bibcode:
- 2015EPSC...10..714V