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[54.03] Spatial Variations of Jupiter's H₃⁺ Emission Deduced From Analysis of IRTF/ProtoCAM Images

T. Satoh (FRCCS, Science University of Tokyo), J.E.P. Connerney (GSFC)

\newcommand{\hhhplus}{H₃⁺}

As well as the intense polar {\hhhplus} aurorae, spectroscopic observations detected weak emission of this ion covering Jupiter's disk (Ballester et al., Icarus 107, 189, 1994; Marten et al., Planet. Space Sci. 42, 391, 1994). We have analyzed intensity distribution of the {\hhhplus} emission across Jupiter's disk in 3.4-{\micron} images acquired using ProtoCAM at NASA's Infrared Telescope Facility (IRTF) in 1992.

The apparent disk emission is approximated using a mathematical function of the form, D(\mu, \mu₀, \theta) = D₀ \mu^a \mu₀^b \times f(\theta) , where \mu₀ and \mu are the direction cosines of incident (solar illumination) and emergent (towards the observer) angles relative to the local zenith, and \theta is the planetocentric latitude. The formulation is similar to the Minnaert's function which is a popular tool to characterize the limb-to-limb intensity variations in visible wavelengths. Three parameters of the above function characterize the dependence on the solar illumination angle (b), limb brightening due to increasing path length near the planetary limb (a), and the latitudinal variation (f(\theta)). We have found that the apparent limb brightening is consistent with the line-of-sight effect (a = -1.0), evidencing that the layer of {\hhhplus} emission is indeed optically thin.

We then adopt a simple atmospheric model to obtain the vertical distribution of the {\hhhplus} ion. The {\em true} disk emission (corrected for the line-of-sight effect) can be well reproduced with {\hhhplus} peak density 9.6 \times 10³ {cm^-3} at an altitude 700--750 km (7--5 nb pressure) above P = 600 mb level. For the first time, the observation of Jupiter's {\hhhplus} disk emission is proven useful to study the {\em vertical} distribution of this ion, although the assumptions in our model may be too simple. Images of higher signal-to-noise ratio should be obtained and compared with more sophisticated atmospheric models such as JIM (Achilleos et al., JGR 103, 20089, 1998).

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