RADIOMETRIC AND RADIO OCCULTATION STUDIES OF JUPITER’S AURORAE AT MICROWAVE FREQUENCIES
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Understanding the composition and dynamics of Jupiter's aurorae is one of the main objectives for the Juno spacecraft, which has been orbiting Jupiter since 2016. One of the most surprising outcomes of the mission is the ability to observe the planet's northern and southern aurorae using the spacecraft's MicroWave Radiometer (MWR). Cold microwave emission associated with the Jovian aurorae have been observed on the MWR's 0.6, 1.24, and 2.6 GHz channels, where the lowest frequency channel at 0.6 GHz has the most prominent auroral signature. This inverse frequency dependence is typically caused by radiowave interactions with plasmas. The implication of these MWR observations suggest that Jupiter's aurorae have significantly higher electron densities compared to the predicted electron densities of the Jovian aurorae based on the ion densities derived from ultraviolet and infrared observations. Even though the particle densities between ions and electrons are now postulated to have a one or two order magnitude difference, the MWR maps of the northern aurora display morphology similar to ultraviolet images from the UltraViolet Spectrograph (UVS). To conceptualize the plasma composition and structure related to the MWR observations, three radiative transfer models were developed. (1) A cold plasma modelled as a single slab that retrieved the electron density required to recreate the observed cold microwave emission. (2) A cold plasma model with an assumed electron density of 108 cm-3 that retrieved the number of layers required to recreate the observed cold microwave emission. (3) A warm plasma model with an assumed electron density of 108 cm-3 that retrieved the thickness of the layer and electron temperature required to recreate the observed cold microwave emission. The cold and warm plasma models were also included in forward models of potential radio occultations that will be conducted with the Juno spacecraft starting in 2023. The forward models will be used to retrieve vertical electron density profiles of the auroral and non-auroral ionosphere.