ALBERT

Description: We study the frequency and time variations of Jovian hectometric emissions (HOM) recorded by the Radio and Plasma Wave Science (RPWS) experiment onboard the Cassini spacecraft during its Jupiter flyby. The capabilities of the RPWS experiment enable us to analyze the intensity extinction of HOM radiation, the so-called attenuation band. Using about 7 weeks of RPWS data obtained around the closest approach, a statistical analysis investigates the spectral variations of this phenomenon with respect to the spacecraft magnetic latitude and central meridian longitude. We show that the ‘trace’ of the attenuation band is usually not a full sinusoid as reported in previous studies and only parts of the curve are observed. Also, the intensity extinction can occur when the spacecraft is not in the planetary magnetic equator plane. This specific feature appears or vanishes when the observer (e.g., Cassini or Galileo spacecraft) is far from or close to the planet, respectively. The regular and systematic observations of the attenuation band suggest that the plasma medium at the origin of these features is steady and stable. The Io torus may be considered to be the most probable plasma medium where HOM emission is refracted through its raypath propagation. Furthermore, intensity extinction at frequencies higher than 3 MHz and up to 5 MHz implies the presence of particular electronic density irregularities in the Io torus. Volcanic activity, particularly in the northern hemisphere of the Io satellite, may be the source of such Io torus plasma irregularities.