Concurrent ultraviolet and infrared observations of the north Jovian aurora during Juno's first perijove

The UltraViolet Spectrograph (UVS) and the Jupiter InfraRed Auroral Mapper (JIRAM) observed the north polar aurora before the first perijove of the Juno orbit (PJ1) on 27 August 2016. The UVS bandpass corresponds to the H2 Lyman and Werner bands that are directly excited by collisions of auroral electrons with molecular hydrogen. The spectral window of the JIRAM L-band imager includes some of the brightest H3+ thermal features between 3.3 and 3.6 µm. A series of spatial scans obtained with JIRAM every 30 s is used to build up five quasi-global images, each covering ∼12 min. of observations. JIRAM's best spatial resolution was on the order of 50 km/pixel during this time frame, while UVS has a resolution of about 750 km. Most of the observed large-scale auroral features are similar in the two spectral regions, but important differences are also observed in their morphology and relative intensity. Only a part of the UV-IR differences stems from the higher spatial resolution of JIRAM, as some of them are still present following smoothing of the JIRAM images at the UVS resolution. For example, the JIRAM images show persistent narrow arc structures in the 100°-180° SIII longitude sector at dusk not resolved in the ultraviolet, but consistent with the structure of in situ electron precipitation measured two hours later. The comparison between the H2 intensity and the H3+ radiance measured along two radial cuts from the center of the main emission illustrates the complex relation between the electron energy input, their characteristic energy and the H3+ emission. Low values of the H3+ intensity relative to the H2 brightness are observed in regions of high FUV color ratio corresponding to harder electron precipitation. The rapid loss of H3+ ions reacting with methane near and below the homopause appears to play a significant role in the control of the relative brightness of the two emissions. Cooling of the auroral thermosphere by H3+ radiation is spatially variable relative to the direct particle heating resulting from the precipitated electron flux.