Discovery and Infrared Characterisation of Jupiter's Equatorial Disturbance Cycle

In this work, we use a ground-based infrared dataset spanning almost four decades, between June 1984 and August 2017, to report five rare events that completely changed the appearance of Jupiter's equatorial zone at 5 µm (1-4 bar region), leaving a cloud-free band at the equator, in addition to large, narrow, and bright festoons within the northern equatorial zone. Three of these five events were previously observed in ground-based images captured in June-November 1973, in March 1979 and in January-April 1992, but were not discussed at all. Here we characterize for the first time two new episodes of these equatorial zone disturbances at 5 µm, observed between August 1999 and August 2000, and between April 2006 and September 2007. These events exhibited large grey/bluish festoons and white plume-like features at the northern edge of the equatorial zone at visible wavelengths, and a brownish coloration of the southern equatorial zone. We find that these events can form within a month, have a typical lifetime of 12-18 months, and dissipation time-scales of 4 months or less. Comparing 5 µm data of these two events with visible wavelength images, we find that aerosols change first in the upper troposphere at the cloud tops ( 700 mbar), before the disturbance propagates downwards and removes, months later, the aerosols in the cloud forming region at 1-4 bar. Analysing the brightness variability of the equatorial zone at 1.58 µm (ammonia cloud tops) and 2.12 µm (stratospheric hazes) during the 2006-2007 event, we see that the disturbances are confined to the cloud deck and do not appear to influence the stratospheric hazes. Finally, after analysing five equatorial zone disturbances, we conclude that they are periodic/stochastic with typical time intervals of 6-8 years or 13-14 years. This periodicity implies that a new event should occur in 2019-2020, permitting detailed characterization of an equatorial disturbance by the Juno spacecraft.