PROBING THE PROPERTIES OF PLANETARY RING DUST BY THE OBSERVATION OF MACH CONES

Compressive dust acoustic waves can be excited in dusty plasmas. Big b oulders in planetary rings move at the Keplerian velocity, while small er dust particles move at a Slightly different velocity due to the act ion of the Lorentz force. If the difference in velocity Delta upsilon is larger than the dust acoustic wave velocity, alpha(d), a wake will be formed with an opening angle of 2 theta where sin theta = \alpha(d) /Delta upsilon\. The discovery of wakes and the measurement of their o pening angles by the space experiment Cassini to Saturn will yield add ed information on the dusty plasma conditions in regions through which Cassini will not pass. We find that in some regions the waves that ar e excited by the boulders may be weak because a large fraction of the interacting dust is absorbed rather than deflected by the boulder. For a given dust size the most favourable conditions for the observations of wakes exist in two fairly narrow regions, one inside and one outsi de the corotation radius. The favorable regions are closest to the cor otation radius for the smallest dust particles and progressively furth er away for larger dust particles.