AN UNUSUAL ROTATIONALLY MODULATED ATTENUATION BAND IN THE JOVIAN HECTOMETRIC RADIO-EMISSION SPECTRUM

A well-defined attenuation band modulated by the rotation of Jupiter h as been found in the spectrum of Jovian hectometric radiation using da ta from the Galileo plasma wave instrument. The center frequency of th is band usually occurs in the frequency range from about 1 to 3 MHz an d the bandwidth is about 10 to 20 percent. The center frequency varies systematically with the rotation of Jupiter and has two peaks per rot ation, the first at a system III longitude of about 50 degrees, and th e second at about 185 degrees. It is now believed that the attenuation occurs as the ray path from a high-latitude cyclotron maser source pa sses approximately parallel to the magnetic field near the northern or southern edges of the Io L-shell. The peak at 50 degrees system III l ongitude is attributed to radiation from a southern hemisphere source and the peak at 185 degrees is from a northern hemisphere source. The attenuation is thought to be caused by coherent scattering or shallow- angle reflection from field-aligned density irregularities near the Io L-shell. The narrow bandwidth indicates that the density irregulariti es are confined to a very narrow range of L values (Delta L = 0.2 to 0 .4) near the Io L-shell.