The propagation characteristics and Landau damping of Jovian whistlers in the Io torus

The propagation characteristics and path-integrated gain of Jovian lightning-generated whistlers are explored using the HOTRAY code. All waves are launched into the magnetosphere from just above the density peak in the ionosphere and followed using a realistic analytical density model based on Voyager and Pioneer data. Over a broader range of input L shells (3.5 ≤ L ≤ 6.5) these unducted waves are strongly guided into the equatorial region near 5.3 ≤ L ≤ 6 by the density maxima in the Io plasma torus. This is consistent with the limited spatial extent of the region where whistlers were detected by Voyager. The path-integrated attenuation of whistlers is relatively weak in the cool inner torus but becomes strong for waves that propagate into the outer warm torus region due to Landau damping by suprathermal electrons (E ≥ 200 eV). This can explain the absence of whistlers observations by Voyager at L > 6. To account for the observed upper frequency cutoff (ƒ ≤ 9 kHz ), our calculations indicate that the lightning source locations should be confined to lower L shells (L ≤ 4). The alternative explanation based on the plasma frequency cutoff requires an unreasonable plasma density model