Abstract
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'.
Highlights
Hydrogen, the most abundant cosmic element, dominates the composition of giant planets
In particular, is difficult to determine remotely, and yet it plays a primary role in identifying the relevant physical processes at work in planetary upper atmospheres
These errors are expected because standard reduction of H+3 observations, which typically have nadir viewing geometries, starts by assuming a uniform layer of constant density and temperature, whereas we know that neither density nor temperature are constant within giant planet H+3 layers
Summary
The most abundant cosmic element, dominates the composition of giant planets. Further detections at Saturn and Uranus, and continued ground-based monitoring of H+3 at Jupiter, Saturn and Uranus in the subsequent decades, have demonstrated its remarkable effectiveness as a probe of giant planet upper atmospheres Derived H+3 temperatures suffer from a key ambiguity: the vast majority of ground-based observations are column integrations through the entire ionosphere, from top-to-bottom, and measured emissions result from convolution of the vertical structures in both H+3 density and temperature. We investigate how giant planet atmospheric models can help supplement interpretation of H+3 spectroscopic observations. These calculations are concentrated on unravelling the column-integrated density and temperature degeneracy behind the observed spectra, and on minimizing—or at least understanding—the effect of thermospheric gradients on analysis of H+3 datasets. We combine the insights from these sections in order to demonstrate a preliminary method of retrieving altitude profiles of H+3 temperature from nadir viewing geometry observations
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