Uranus’ hydrogen upper atmosphere: Insights from pre- and post-equinox HST Lyman-α images

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Volume 693 (January 2025)

A&A, 693 (2025) A231

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Table 3

Atmosphere profiles giving good fit to the 1998 and 2011 data brightness profiles.

Year	H^*			Ambient H	χ²_ν	Remark

	T	H	% of V2 base	Scale factor to V2 base
	(eV)	(km)	ambient H abundance	ambient H profile
	0.15	2500	0.45%	0.45	1.85	Requires reduction in Voyager 2 base ambient H by a factor of ~2
	0.17	3000	0.3%	0.8	1.75
1998	0.2	3500	0.24%	1.0	2.0	For an ambient H abundance = V2 base ambient H abundance
	0.23	4000	0.2%	1.1	2.8
	0.23	4000	0.15%	1.3	2.25
	0.3	5000	0.075%	1.5	3.0
	0.35	6000	0.03%	1.7	3.5	$χ_{ν}^{2}$ $\chi _v^2$ increases rapidly at higher T

	0.17	3000	0.3%	0.6	1.1	Ambient H abundance 40% lower than Voyager 2 base ambient H abundance
	0.23	4000	0.1%	1.0	1.1
	0.3	5000	0.05%	1.0	1.0
	0.4	7000	0.02%	1.0	1.0
2011	0.52	9000	0.01%	1.0	1.1	Most probable Maxwellian speed is $\frac{1}{2} V_{\infty - Uranus}$ ${1 \over 2}{V_{\infty - {\rm{Uranus}}}}$
	0.64	12 000	0.0075%	1.0	1.15
	0.88	15 000	0.006%	1.0	1.65
	1.05	18 000	0.004%	1.05	1.35
	1.46	25 000	0.003%	1.05	1.8	Brightness profiles disagree 2 RU onwards.
	2.0	35 000	0.0025%	1.05	2.6	Most probable Maxwellian speed is close to
						to V_∞–Uranus. Half of the population is escaping

Notes. For all the H^* profiles we used 𝓏_max = 3000 km. Except ambient H, rest of the atmosphere profile is same as that of the Voyager 2 base profile. The Voyager 2 base ambient H abundance is 2.8 × 10¹⁷ cm⁻².

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