[1] We observed negative spectra of quenched background L burst emissions with a negative drift rate of À5 MHz s À1 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. (2007bHess et al. ( , 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 Â 10 6 -2 Â 10 7 cm À3 . 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.
[1] On 4 June 2008 UT, the position of the satellite Io with respect to Jupiter was the so-called Io-A, we observed Jovian decametric (DAM) radio emissions using a waveform receiver (WFR) and detected wave modulations (WMs) in the DAM emissions. WMs appeared four times at intervals of approximately 7 min for durations of 3-10 s each. We found that the WMs had fundamental frequencies of 2.5-5 Hz, and the 1st and 2nd harmonics of these frequencies were odd resonances at the fundamental frequencies. Simulations confirmed that strong Alfvén waves arrive at the polar regions of Jupiter at 5-7 min intervals when Io is located at the center of the Io plasma torus, and Io was located at that location when WMs were detected. The 7 min intervals of WMs are consistent with the characteristic periods of Alfvén waves, suggesting the existence of the ionospheric Alfvén resonator (IAR) expected in the system of Jupiter. Thus far, few observations have suggested the existence of IAR in Jupiter. In this research, we suggest the existence of IAR in Jupiter by using a WFR and the millisecond modulations of Jovian L-burst emissions.
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