STATISTICS OF SATURN SPOKES

Two sequences of Voyager 2 images of Saturn's B ring have been used to analyze the spoke phenomenon: about 100 low-resolution (220 km/lp) images of the morning and evening ansae covering a time span of 34 hr (i.e., more than three Saturn revolutions) and more than 40 high-resolution (70 km/lp) images covering about 12 hr. Positions of spoke edges have been measured for about 150 different spokes on up to 5 subsequent images each. The motion of spoke edges has been found to be close to Keplerian orbital velocity. If the observed deviation from angular velocity is interpreted as being due to the interaction of Saturn's magnetic field with charged dust particles it would result in a charge-to-mass ratio of spoke particles of q/m = -3 ± 3 C/kg. However, this deviation as well as the larger deviation observed for some trailing edges of spokes can be interpreted as being due to the phase speed at which spokes become visible or at which they disappear. Therefore the derived q/m value is only an upper limit for the true electromagnetic effect on spokes particles. An enhanced spoke activity between 120° and 150° SLS and between 300° and 330° SLS seems to be indicated in the data. The radial distribution of spoke activity follows closely the optical depth profile of the outer B ring, both being highest between the distances of 104,000 and 110,000 km. The age distribution of spokes shows a deficiency of young ages (<2000 sec and <6000 sec for the high- and low-resolution data, respectively) of leading edges. This result suggests that young spokes become visible only some time after initial radial formation when the contrast level with respect to the underlying ring material has become high enough. After this initial phase spokes increase further in contrast and width until they finally become "inactive," i.e., they rotate with Keplerian speed and decay at most 20,000 sec after their initial birth. We have constructed a rough model for the spoke visibility, as a function of the spoke age, and fitted this to the observations. The results indicate an active time (in which the spoke visibility increases) with an average value of 6400 sec; that a spoke decays much faster than it grows, and that the trailing edges age very little during the active time of a spoke. The results give a weak indication that the production rate of spokes in the shadow may be higher than that outside. © 1992.