Jupiter’s North Equatorial Belt & Jet: II. Acceleration of the jet & the NEB fade in 2011–’12

Paper I described the cyclic pattern of NEB expansion events (NEEs) that have occurred every 3-5 years in recent decades and also in some earlier decades. Even earlier, in 1893-1915, similar events occurred probably every three years but were even more striking; the NEB narrowed considerably (the NEBn edge typically receding to only 11-12°N) and then revived, in a process that was sometimes observed as a vigorous outbreak with many striking spots. These events, termed ‘NEB Revivals’, showed many similarities with the better-known SEB Revivals, although the latter appear to be more highly organised. One final NEB Revival occurred in 1926; thereafter only the more modest NEEs occurred.
In 2011, the NEBn began receding again and the next in the series of 3-5-yearly NEEs was expected. However the recession then continued, until the NEBn edge reached 12.5°N in early 2012, leaving a narrow, southerly NEB which became somewhat fainter than usual. The NEB had not been so narrow or faint since the 1920s. This appeared to set the scene for a NEB Revival, which indeed followed in 2012; this will be described in Paper III.
As this was the first such phenomenon in modern times, we were able to record fine details and associated phenomena that were unobservable a century ago. The recession of the belt was preceded by disappearance of the usual NEBs dark formations (NEDFs) and NEB rifts, in addition to acceleration of the NEBs jet to unprecedented super-fast speeds. I argue that there was a close association between all these phenomena, which are analogous to those accompanying SEB Fade/Revival cycles.
This paper begins by considering the structure of the NEBs jet – as inferred from ground-based and spacecraft observations – and its emerging similarity to the other two fast prograde jets on the planet. Section 2 describes the changes in its observed speed from 2008 to 2012; Section 3 discusses the implications for the jet structure/variability, and for the role of NEDFs. In Section 4, I describe the NEB Fade as it developed in 2011/12 and discuss how the rifts, NEDFs, and Fade/Revival cycle may be related, in the NEB as in the SEB. Paper III will describe the subsequent NEB Revival and concurrent NTB Revival in 2012.

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