Table 1
Input parameters (simulation box size, upstream parameters, neutral atmosphere and physico-chemistry) for the hybrid model simulations.
| Input parameters | Value | Unit | Description |
|
|
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| R h | 1.3 | AU | 67P/C-G perihelion distance |
| (x,y,z)box | ±25 000 | km | Boundary of square simulation box |
| maxΔ(x,y,z)box | 100 | km | Grid mesh size close to the comet (maximum resolution) |
| minΔ(x,y,z)box | 1600 | km | Grid mesh size far from the comet (minimum resolution) |
| Δt | 7.4 × 10-3 | s | Time step of the simulation |
|
|
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| Solar wind upstream | |||
|
4.14 | cm-3 | H+ density, np(1 AU) = 7 cm-3 |
|
|
cm-3 | He2+ density |
| B = (Bx,By,Bz) | (0,7.69,0) | nT | IMF B(1 AU) = 10 nT, with By = B(1 AU) |
| (Tp,Te) | (105,1.5 × 105) | K | Proton and electron temperatures |
| | Usw | | 400 | km s-1 | Solar wind speed, velocity along the − x axis |
|
|
|||
| Neutral atmosphere | |||
| r c | 2 × 103 | m | Average radius of comet |
| Q 0 | 4.89 × 1027 | s-1 | Neutral outgassing rate (H2O) |
Snodgrass et al. (2013),  |
|||
| v 0 | 1000 | m s-1 | Radial velocity of outgassing neutrals |
|
1.95 × 10-7 | s-1 | Photoionisation rate, low solar activity, scaled to Rh |
| H2O → H2O+, Huebner & Mukherjee (2015) | |||
|
7.13 × 10-6 | s-1 | Total photodestruction rate, low solar activity, scaled to Rh |
| Huebner & Mukherjee (2015) | |||
|
|
|||
| Ionosphere | |||
| Q i | 3.03 × 1022 | s-1 | Photoionisation rate in simulation box |
| k i,s | 1.5 × 10-15 | m3 s-1 | H2O+-H2O ion-neutral drag rate, Gombosi et al. (1996) |
|
|
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| Chemistry reactions | |||
|
Polynomial fit, as in Table 2 | m2 | Charge-exchange cross section for H+ + H2O → H + H2O+ |
|
Polynomial fit, as in Table 2 | m2 | Charge-exchange cross section for He2+ + H2O → He+ + H2O+ |
|
2.86 × 10-14 | m3 s-1 | Electron ionisation reaction rate constant,  K |
| H2O + e → H2O+ + 2e, Cravens et al. (1987) | |||
|
1.38 × 10-14 | m3 s-1 | Electron recombination reaction rate constant, K |
| Hollenbach et al. (2012), McElroy et al. (2013) | |||
H2O+ + e → neutrals,  , K |
|||
Notes. All values are given at a Sun-comet distance of 1.3 AU, corresponding to perihelion conditions for comet 67P/Churyumov-Gerasimenko. Charge-transfer reactions are fitted with a polynomial or order 4, the coefficients of which are given in Table 2.
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