Juno Microwave Radiometer Results From 8 Years At Jupiter

Abstract

The Juno Microwave Radiometer (MWR) has been collecting data in the Jovian system since 2016.  MWR has 6 independent channels, at frequencies of 0.6, 1.2, 2.5, 4.8, 9.6, and 22 GHz, with bandwidths ranging from 3.2% to 3.5% (Janssen et al.  2016).  The angular resolution of the 0.6 and 1.2 GHz channels is roughly 21˚ and the other channels have about 11˚ beamwidth.  The two low-frequency channels have high- and low-gain outputs, to accommodate the dynamic range needed for observations of the Jovian synchrotron emission.  The absolute gain is calibrated to roughly 1%, but is very stable over hours-long time scales, so that the uncertainty in difference measurements is less than 0.2%, a fact which is very important for optimal use of the data (Zhang et al. 2020).  The antenna patterns are well characterized, and the trajectory, spacecraft orientation, and spin typically enable differential limb-darkening measurements of the atmosphere with brightness temperature estimates that deconvolve the antenna gain for better effective angular resolution (Oyafuso et al. 2020).  Lower frequencies penetrate to greater depths, and MWR’s 6 channels cover three orders of magnitude from deepest to shallowest contribution function, with the 600-MHz channel achieving passive measurements of the temperature and opacity at hundreds of km into Jupiter’s atmosphere, or several km into the ice shells of Ganymede and Europa.MWR has produced a remarkable series of discoveries.  Measurements of the atmosphere have yielded conclusions about Jupiter’s temperature and composition vs depth and latitude (e.g. Li et al. 2024), Ferrel-like circulation cells (Duer et al. 2021), 3-dimensional characterization of atmospheric storms (Bolton et al. 2021), multi-year variability of large-scale features at depth, nature of the circumpolar cyclones, prevalence and distribution of lightning (Brown et al. 2018), depletion of alkali metals (Bhattacharya et al. 2023), time-varying free electrons over the northern aurora, and more.  Measurements of Ganymede (Brown et al. 2023, Zhang et al. 2023) and Europa have given insight into their subsurface ice, while observations of Io tell us about its rocky surface and lava below. MWR observations of synchrotron emission give us an unprecedented view of the inner radiation belts.   We will present an overview of MWR results to date, with an emphasis on the most recent discoveries and how the MWR data are used to achieve them. ReferencesBhattacharya, A. et al. 2023.  ApJL, 952:L27.Bolton, S. et al. 2021.  Science, 374, 968–972.Brown, S. et al. 2018.  Nature, 558, 87.Brown, S. et al. 2023.  JGR Planets, 128, Issue 6.Duer, K. et al. 2021.  GRL, 48 (23), e2021GL095651Janssen, M. et al. 2017. Space Sci. Rev. 213, 139.Li, C. et al. 2024.  Icarus, 414, 116028.Oyafuso, F. et al. 2020. Earth and Space Science,7, e2020EA001254.Zhang, Z. et al. 2020.  Earth and Space Science,7, e2020EA001229.Zhang, Z. et al. 2023.  GRL,50, e2022GL101565.