Multi-instrument analysis of far-ultraviolet aurora in the southern hemisphere of comet 67P/Churyumov-Gerasimenko

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Volume 647 (March 2021)

A&A, 647 (2021) A119

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Table 1

Cross-sections for the dissociative excitation of cometary molecules by electron impact.

Emission line	Species	Threshold energy [eV]	Reference and assumptions
Ly-β and OI1027	H₂O	17.21	Makarov et al. (2004). Assume the same shape as Ly-α with ratio 7∕55 at 200 eV. Includes coincident OI line.
	CO₂	21 ± 2 ⁽¹⁾	Kanik et al. (1993) Assume the same shape as OI1304. Use ratio at 200 eV.
	CO	23.17	James et al. (1992).
	O₂	20.6	Wilhelmi & Schartner (2000). Absolute value given at 200 eV. Assume same shape as OI1304.

OI1304	H₂O	15.2	Makarov et al. (2004).
	CO₂	21 ± 2	Mumma et al. (1972). Reduced by a factor of 0.59 due to updated measurements of H₂ Ly-α (McConkey et al. 2008).
	CO	20.6	Ajello (1971). Some uncertainty at energies >100 eV.
	O₂	14.6⁽²⁾	Kanik et al. (2003). Scaled up by factor 2.93∕2.90 in line with the recommendation by McConkey et al. (2008).

OI1356	H₂O	15.2	Makarov et al. (2004). Assume same threshold and shape as OI1304.
	CO₂	20.6 ⁽²⁾	Feldman et al. (2015). Excitation rate of OI1356 33 times larger for CO₂ than H₂O (Wells et al. 1972), after revision of the lifetime of the ⁵S° state (Wells & Zipf 1974). Assume the same shape as OI1304.
	CO	20.6	Ajello (1971). Ratio given at 100 eV (Wells & Zipf 1974; Wells et al. 1972). Assume the same shape as OI1304.
	O₂	14.6	Kanik et al. (2003). Scaled up by factor 6.47∕6.40 in line with recommendation by McConkey et al. (2008).

CI1657	CO₂	25 ± 2	Mumma et al. (1972).
CI1657	CO		Difficult to measure cross-section due to the strong overlapping CO4PG band. No contribution from dissociative excitation of CO considered for CI1657.

CO4PG	CO₂	25 ± 2	Contribution of (0,2) bands included in CI1657 cross-section of Mumma et al. (1972).
CO4PG	CO	8	Beegle et al. (1999). Absolute values for bands given at 100 eV. Shape of (0,1) band given. Scaled down by factor 0.925 due to remeasurement of calibrating NI line (Ajello et al. 2019).

CII1335	CO₂	44	Mumma et al. (1972).
CII1335	CO	33	Ajello (1971).

References. ⁽¹⁾ Mumma et al. (1972), ⁽²⁾ McConkey et al. (2008).

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