Detection of H₃ ⁺ auroral emission in Jupiter's 5-micron window

We use high-resolution ground-based observations from the VLT CRIRES instrument in November 2012 to identify sixteen previously undetected H₃⁺ emission lines from Jupiter's ionosphere. These emission lines are located in Jupiter's 5-micron window (4.5-5.2 μm), an optically-thin region of the planet's spectrum where the radiation mostly originates from the deep troposphere. The H₃⁺ emission lines are so strong that they are visible even against this bright background. We measure the Doppler broadening of the H₃⁺ emission lines in order to evaluate the kinetic temperature of the molecules, and we obtain a value of 1390 ± 160 K. We also measure the relative intensities of lines in the ν₂ fundamental in order to calculate the rotational temperature, obtaining a value of 960 ± 40 K. Finally, we use the detection of an emission line from the 2ν₂(2)-ν₂ overtone to measure a vibrational temperature of 925 ± 25 K. We use these three independent temperature estimates to discuss the thermodynamic equilibrium of Jupiter's ionosphere.