H3+ Cooling in the Jovian Aurora: Model and Juno Remote Sensing Observations

The UltraViolet Spectrograph (UVS) and the Jupiter InfraRed Auroral Mapper (JIRAM) on board Juno have observed the Jovian polar aurora from polar orbit since August 2016. The UVS instrument measures the H2 Lyman and Werner bands whose brightness is related to the auroral electron flux. The 3.3-3.6 μm spectral window of the JIRAM L-band imager maps the H3+ thermal radiance with unprecedented spatial resolution. Combination of spatial scans obtained with both UVS and JIRAM makes it possible to build polar projections of both emissions sometimes observed concurrently. Cooling of the auroral thermosphere by H3+ radiation to space is a key process in the energy balance of the auroral latitude region. Earlier comparison study has indicated that the global features are similar in the two spectral regions, but significant differences were also observed in the morphology and spatial contrast intensity. A correlation between regions of low values of the H3+ intensity relative to the H2 intensity and areas of high FUV color ratio suggested that destruction of H3+ ions reacting with methane near and below the homopause play a significant role in the control of the relative brightness of the two emissions. In a first step, we use an auroral electron transport coupled with a photochemical model of ion composition to derive the H3+ ion density profile and cooling rate for different auroral energy fluxes and mean electron energies. Departure from LTE at high latitudes is accounted for. In a next step, we use the H2 brightness distribution and the ultraviolet color ratio observed with UVS to map the characteristics of the electron precipitation. We then compare the observed spatial distribution of the H3+ radiance with that predicted from the electron input derived from UVS observations.