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H+ 3 has been detected at all of the solar system giant planets aside from Neptune. Current observational upper limits imply that there is far less H+ 3 emission at Neptune than rudimentary modelling would suggest. Here, we explore via modelling a range of atmospheric conditions in order to find some that could be consistent with observational constraints. In particular, we consider that the upper atmosphere might be much cooler than it was during the 1989 Voyager 2 encounter, and we examine the impact of an enhanced influx of external material that could act to reduce H+ 3 density. Resulting ionosphere models that are consistent with existing H+ 3 observational constraints have an exospheric temperature of 450K or less, 300K lower than the Voyager 2 value. Alternatively, if a topside CO influx of 2 × 108 cm-2 s-1 is imposed, the upper atmospheric temperature can be higher, up to 550 K. The potential cooling of Neptune's atmosphere is relevant for poorly understood giant planet thermospheric energetics, and would also impact aerobreaking manoeuvers for any future spacecraft. Such a large CO influx, if present, could imply Triton is a very active moon with prominent atmospheric escape, and/or that Neptune's rings significantly modify its upper atmosphere, and the introduction of so much exogenic material would complicate interpretation of the origin of species observed in Neptune's lower atmosphere. [ABSTRACT FROM AUTHOR]

Since its detection in the aurorae of Jupiter approximately 30 years ago, the H3+ ion has served as an invaluable probe of giant planet upper atmospheres. However, the vast majority of monitoring of planetary H3+ radiation has followed from observations that rely on deriving parameters from column-integrated paths through the emitting layer. Here, we investigate the effects of density and temperature gradients along such paths on the measured H3+ spectrum and its resulting interpretation. In a non-isothermal atmosphere, H3+ column densities retrieved from such observations are found to represent a lower limit, reduced by 20% or more from the true atmospheric value. Global simulations of Uranus' ionosphere reveal that measured H3+ temperature variations are often attributable to well-understood solar zenith angle effects rather than indications of real atmospheric variability. Finally, based on these insights, a preliminary method of deriving vertical temperature structure is demonstrated at Jupiter using model reproductions of electron density and H3+ measurements. The sheer diversity and uncertainty of conditions in planetary atmospheres prohibits this work from providing blanket quantitative correction factors; nonetheless, we illustrate a few simple ways in which the already formidable utility of H3+ observations in understanding planetary atmospheres can be enhanced. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'. [ABSTRACT FROM AUTHOR]

The upper atmosphere of Uranus has been observed to be slowly cooling between 1993 and 2011. New analysis of near-infrared observations of emission from H3+ obtained between 2012 and 2018 reveals that this cooling trend has continued, showing that the upper atmosphere has cooled for 27 years, longer than the length of a nominal season of 21 years. The new observations have offered greater spatial resolution and higher sensitivity than previous ones, enabling the characterization of the H3+ intensity as a function of local time. These profiles peak between 13 and 15 h local time, later than models suggest. The NASA Infrared Telescope Facility iSHELL instrument also provides the detection of a bright H3+ signal on 16 October 2016, rotating into view from the dawn sector. This feature is consistent with an auroral signal, but is the only of its kind present in this comprehensive dataset. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'. [ABSTRACT FROM AUTHOR]

Recently H 3 + was detected at Saturn’s low- and mid-latitudes for the first time (O’Donoghue et al. [2013]. Nature 496(7444), 193–195), revealing significant latitudinal structure in H 3 + emissions, with local extrema in one hemisphere mirrored at magnetically conjugate latitudes in the opposite hemisphere. The observed minima and maxima were shown to map to regions of increased or decreased density in Saturn’s rings, implying a direct ring–atmosphere connection. Here, using the Saturn Thermosphere Ionosphere Model (STIM), we investigate the “ring rain” explanation of the O’Donoghue et al. (O’Donoghue et al. [2013]. Nature 496(7444), 193–195) observations, wherein charged water group particles from the rings are guided by magnetic field lines as they “rain” down upon the atmosphere, altering local ionospheric chemistry. Based on model reproductions of observed H 3 + variations, we derive maximum water influxes of (1.6–16) × 10 5 H 2 O molecules cm −2 s −1 across ring rain latitudes (∼23–49° in the south, and ∼32–54° in the north), with localized regions of enhanced influx near −48°, −38°, 42°, and 53° latitude. We estimate the globally averaged maximum ring-derived water influx to be (1.6–12) × 10 5 cm −2 s −1 , which represents a maximum total global influx of water from Saturn’s rings to its atmosphere of (1.0–6.8) × 10 26 s −1 . The wide range of global water influx estimates stems primarily from uncertainties regarding H 3 + temperatures (and consequently column densities). Future ring rain observations may therefore be able to reduce these uncertainties by determining H 3 + temperatures self consistently. [ABSTRACT FROM AUTHOR]

On 21 April 2013, during a co-ordinated Saturn auroral observing campaign, the northern and southern poles of the planet were observed from the Earth using the NASA Infrared Telescope Facility (IRTF), Keck, and Hubble Space Telescope (HST) simultaneously with the Cassini infrared, visible, and ultraviolet remote sensing instruments. We present simultaneous multi-scale and multi-wavelength analysis of the morphology of auroral emissions at Saturn. The visible main auroral emission vary between ∼ 2 and 10 kR on timescales of minutes and across spatial scales of down to ∼ 14 km on the planet. The H 2 Far Ultraviolet (FUV) brightness varies by a factor of ∼ 10, from ∼ 4–40 kR, over timescales of 1 min and spatial scales of 720 km. H 3 + infrared emissions vary less than the H 2 emissions, from ∼ 5–10 μ W m −2 sr −1 , over similar spatial scales ( ∼ 300 km) and timescales of a few seconds to a few hours. The fine-scale temporal and spatial features seen in the main oval show that complex structures are present even during quiet solar wind conditions. Diffuse ultraviolet emissions southward of the southern midnight main oval that are not seen in the infrared, implying a steep temperature gradient of ∼ 50 K over 2–4 ° latitude equatorward of the main oval. Dynamics on scales of ∼ 100 km at the poles are revealed by lower spatial resolution observations, the morphologies of which are partly consistent with overlapping local-time fixed and co-rotating current systems. We also present the first direct comparison of simultaneous infrared, visible, and ultraviolet auroral emissions at Saturn. Finally, the main auroral emissions are found to be approximately co-located in the midnight sector, forming an arc with a width of ∼ 0.5–1 ° , at 72–74° southern latitude, moving slightly equatorward with increasing local-time. [ABSTRACT FROM AUTHOR]