Jovian slow-drift shadow events

We observed negative spectra of quenched background L burst emissions with a negative drift rate of -5 MHz s^-1 by using a waveform receiver developed by Koshida [2009]. We named this phenomenon a slow-drift shadow event (SDS event). The leading and trailing edges of these SDS events showed a sudden change in their drift rates at similar frequencies. Wavy SDS-like phenomena were also observed with a frequency of 15 Hz. Both S bursts and the ionospheric Alfvén resonator have similar characteristic frequencies. The local potential jumps were discovered near the Jovian decametric (DAM) source regions by Hess et al. (2007b, 2009). SDS slope changes may be related to similar features. We have attempted to quantitatively estimate the background plasma densities by assuming that the background L burst emissions were generated by cyclotron maser instability (CMI) and that the SDS events were related to the Alfvén wave. The estimated background plasma densities were in the range of 5 × 10⁶-2 × 10⁷ cm^-3. Since f_p/f_c ≈ 0.87-1.7, the CMI must be quenched. This give rises to the question of what induced the SDS events.