100 to , so the dust must be continuously replenished in the collisions between large bodies with sizes from 1 cm to 0.5 km and between the same large bodies and high velocity particles coming from outside the Jovian system. This parent body population is confined to the narrow—about —and bright outer part of the main ring, and includes Metis and Adrastea. The largest parent bodies must be less than 0.5 km in size. The upper limit on their size was obtained by New Horizons spacecraft. The previous upper limit, obtained from HST and Cassini observations, was near 4 km. The dust produced in collisions retains approximately the same orbital elements as the parent bodies and slowly spirals in the direction of Jupiter forming the faint (in back-scattered light) innermost part of the main ring and halo ring. The age of the main ring is currently unknown, but it may be the last remnant of a past population of small bodies near Jupiter. Vertical corrugations Images from the Galileo and New Horizons space probes show the presence of two sets of spiraling vertical corrugations in the main ring. These waves became more tightly wound over time at the rate expected for differential nodal regression in Jupiter's gravity field. Extrapolating backwards, the more prominent of the two sets of waves appears to have been excited in 1995, around the time of the impact of Comet Shoemaker-Levy 9 with Jupiter, while the smaller set appears to date to the first half of 1990. Galileo'''s November 1996 observations are consistent with wavelengths of and , and vertical amplitudes of and , for the larger and smaller sets of waves, respectively. The formation of the larger set of waves can be explained if the ring was impacted by a cloud of particles released by the comet with a total mass on the order of 2–5 × 1012 kg, which would have tilted the ring out of the equatorial plane by 2 km. A similar spiraling wave pattern that tightens over time has been observed by Cassini in Saturns's C and D rings. Halo ring Appearance and structure The halo ring is the innermost and the vertically thickest Jovian ring. Its outer edge coincides with the inner boundary of the main ring approximately at the radius (). From this radius the ring becomes rapidly thicker towards Jupiter. The true vertical extent of the halo is