Variability of Mid-Infrared Aurora on Jupiter: 1979 to 2016

We report on long term measurements of Jupiter's thermal infrared aurora covering spectral and altitude regions that will complement Juno observational capabilities. Results from spectral data acquired (1979 - 2016) by ground-based infrared heterodyne spectroscopy (IRHS) and Fourier transform spectroscopy (FTS) by Voyager IRIS and Cassini CIRS during Jupiter flybys will be presented. Measurements with ultra-high spectral resolution IRHS were made over 30 years, primarily of ethane near 12 micrometer wavelength. They provide fully resolved individual spectral lines whose shape provides unique information on variability of temperature and abundance. CIRS and IRIS data at lower spectral resolution provide extended spatial distributions covering a broad spectral region including abundances and auroral response of hydrocarbon constituents in the 8-13 micrometer spectral region (ethane, methane, ethylene, and acetylene). Spatial distribution of ethane and ethylene line emission intensity near polar auroral regions was measured and variability was correlated to possible changes in temperature distribution and molecular abundances. In the 30+ year IRHS ethane data, significant (10-fold) periodic variability of auroral emission was observed over 11 year cycles, consistent with solar activity. Short-term (less than a day) changes in emission were evident at the highest intensity periods, near solar maxima. Consistency was found between results retrieved from measurements by Cassini CIRS during its flyby of Jupiter in 2000-2001 and contemporaneous IRHS ground based measurements, illustrating the complementarity of space/ground-based observations. Based on our long-term studies the expectation is that the thermal IR auroral emission may be low during the Juno-Jupiter encounter. Confirmation of this prediction will be addressed by results form our IRHS measurements in April 2016 at the IRTF. Results will be useful for the Juno mission, since this spectral region and this work provide complementary information and diagnostic for studying Jupiter in a spectral region and altitude range not directly probed by Juno.