The 90-day oscillations of Jupiter’s Great Red Spot revisited

We have analyzed a large set of ground-based CCD images covering the period from 1993 to early 1999, to address the current status and behavior of the 90-day longitudinal oscillations of the Jovian Great Red Spot (GRS). The good temporal sampling of these data have been complemented with a small set of high resolution Hubble Space Telescope images. The average zonal velocity of the GRS from 1994 to 1998 was −3.8 ms−1 with extreme values of −2.4 and −4.2 ms−1 in 1993–1994 resulting from the interactions with nearby features. Our study confirms the existence of the longitudinal oscillations with a mean period of 89.8±0.15 days and an amplitude of 1.2±0.7°. The oscillations survived the strong interactions between the GRS and a variety of disturbances that took place during the observing period. No significant changes in the parameters defining the oscillations are observed when a comparison is made with a previous analysis from years 1961 to 1981.