Search for mid-IR rotational and ν1→ν2 difference band H3+ emission in Jupiter's northern aurora

Using TEXES at the IRTF telescope, we obtained high-resolution, mid-infrared, longslit spectra of Jupiter's northern auroral “hot spot” near System III longitude 180° to search for selected pure-rotational and ν1→ν2 difference band lines of H3+. Because these lines have not been investigated in the laboratory, we used theoretically predicted frequencies and line intensities to guide this search. No pure rotational H3+ line emission was detected near the predicted frequencies. However, two metastable ν1→ν2 difference band lines appear to have been marginally detected at the 68% confidence level (1 − σ). The non-detection of the pure rotational line sets a 1 − σ upper limit to the vertical column abundance of H3+ in the northern jovian aurora of 8.4 × 1012  cm−2, which is independent of any assumption concerning the departure of the vibrational H3+ energy level populations from thermal equilibrium. This is consistent with the column 6 × 1012 cm−2 inferred by Melin et al. [Melin, H., Miller, S., Stallard, T., Grodent, D., 2005. Icarus 178, 97-103] from their non-LTE H3+ emission model, in which the fundamental band ro-vib levels are underpopulated by a factor of 6-10 relative to LTE. We find that the IR-inactive ν1 levels of H3+, from which the jovian ν1→ν2 difference band emission originates, are populated in thermal equilibrium. The difference band lines thus serve as proxies for the rotational lines in establishing the total auroral H3+ column. As a result, the marginally detected 943.953 cm−1 ν1→ν2 difference band line implies a vertical H3+ column abundance in the range (4.5 ± 3.1) × 1012 cm−2, consistent with the upper limit from the rotational line. The difference band line constrains the vibrational relaxation of the IR-active ν2 fundamental band in Jupiter's aurora to a factor of 4.5-7.5, with an uncertainty ±68%, which supports the model-dependent result of Melin et al.