Analysis of the Effect of Jupiter's Northern Aurora on Juno Microwave Radiometer Observations

Abstract

Juno's Microwave Radiometer (MWR) is intended to measure Jupiter's thermal emission using six channels that effectively probe different depths of Jupiter’s atmosphere [Li et al. 2017 Geophys. Res. Lett. 44, 531].  However, MWR has also observed non-thermal effects such as lightning [Brown et al. 2018 Nature 558, 7708], synchrotron emission [Levin et al AGU 2022], and reflections due to aurorae [Hodges et al EGU 2022].  Each effect diminishes with increasing channel (i.e. frequency) and is usually significant only in the two or three longest wavelength channels.  In this work we characterize the impact of the northern aurora on measured MWR intensities.  Near the poles significant reductions (up to a few hundred K) in intensities from those expected from Jupiter's thermal emission are found in every orbit in the two longest wavelength channels (~50cm and ~24cm) and for some orbits can be identified at wavelengths as short as 6cm.  They are shown to correlate well with regions poleward of the location of the mean auroral oval measured by Juno's Ultraviolet Imaging Spectrometer (UVS).  Intriguingly, several orbits demonstrate sudden changes in brightness temperatures that vary on timescales of one to two spins of the spacecraft.  These variations are suggestive of a transient phenomenon (on the order of a minute or less) that is coherent on a length scale comparable to a significant fraction of the size of MWR's polar footprints (~0.1 RJ). We will present an analysis of this effect in the MWR data and compare with measurements from UVS and with field and particle measurements of temporal and spatial variations to assess the physical connections between the changes in microwave emissions and auroral processes.