Modelling H3+ in planetary atmospheres: effects of vertical gradients on observed quantities

Moore, L., Melin, H., O'Donoghue, J. ORCID: https://orcid.org/0000-0002-4218-1191, Stallard, T. S., Moses, J. I., Galand, M., Miller, S. and Schmidt, C. A. (2019) Modelling H3+ in planetary atmospheres: effects of vertical gradients on observed quantities. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 377 (2154). 20190067. ISSN 1471-2962 doi: 10.1098/rsta.2019.0067

Abstract/Summary

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’.