VARIABILITY OF JUPITER’S SYNCHROTRON EMISSION IN MID-2009 D. Santos-Costa , R. Sault , S. Bolton ∗ , R. Thorne † , and S. Levin Abstract In the present paper, radio observations made at the wavelength of 6 cm with the VLA in 2009 are analyzed. Reconstructed images of the brightness distribution show significant intensity variations of the equatorial peak emissions on both sides of the planet. The fluctuations are characterized by asymmetrical changes in the brightness distributions. The contribution of a comet-like impact to the observed variations is discussed. During the third week of July, ground-based measurements at different radio bands confirmed that a large projectile had struck Jupiter’s atmo- sphere. The examination of the 2009 VLA data sets shows that the steep enhance- ment of the emission radiated by Jupiter’s electron belt occurred during the same period. The increase in the synchrotron radiation was observed to go on for a couple of weeks before gradually fading in August. Two-dimensional reconstructions of the equatorial brightness distribution demonstrate that the time variability of the radio emission during the middle of 2009 was driven by the longitudinal expansion of an impact-related synchrotron “hot spot” originally located at the Jupiter System III longitude of 305 degrees. 1 Introduction Jupiter’s strong magnetic field contributes to populate the innermost region of the giant planet’s magnetosphere with very energetic electrons, resulting in an observed radio sig- nal above the galactic noise level [Drake and Hvatum, 1959]. The fast gyration of highly relativistic electrons around field lines is the basic mechanism responsible for the produc- tion of Jupiter’s intense synchrotron radiation [Field, 1959]. Measured in the decimetric band since the late 1950’s [McClain and Sloanaker, 1959; Roberts and Stanley, 1959; and others], this radiation led to the discovery of the electron belt in the 1960’s [Carr et al., 1983]. With the little amount of existing particle data for the region inside Io’s orbit, Southwest Research Institute, San Antonio, TX 78238-5166, USA University of Melbourne, Australia Department of Atmospheric and Oceanic Sciences, UCLA, Los Angeles, CA, USA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA 231