Spectral Structures of Jovian Broadband Kilometric Radiation Revealed by Cassini and Juno

Abstract

Cassini flew past Jupiter in 2000/2001, and Juno has been orbiting the gas giant since mid-2016. Here we focus on the spectral properties of Jovian broadband kilometric radiation (bKOM), and we classified them according to their slope in the time–frequency spectrum and distinguish four categories with negative, positive, mixed, or no slope in frequency. These bKOM structures had mostly negative slopes during the inbound portion of the Cassini Jupiter flyby, whereas they were mostly positive for the outbound. The reason for this could be higher intensities of bKOM emissions from midnight to dawn local times. We interpret the semi-circles of the Jovian radio emission nicknamed “bullseyes” as negatively sloped bKOM connected with positively sloped bKOM, and we detected 40 bullseyes in Juno Waves data until the end of 2017. Cassini and Juno show similar distributions of bKOM with respect to the Jovian Central Meridian Longitude like the Voyagers and Ulysses. This means that bKOM has not changed over several decades and “rotates” with the system III period. Several features of the bKOM structures suggest emission cones with thick mantles and moderate beaming angles around $50°$. We found an enhancement of the occurrence probability of southern bKOM for certain phases of Ganymede and Callisto.