Jupiter's North Equatorial Belt and Jet, II, Acceleration of the jet and the NEB Fade in 2011-12

Abstract

Paper I described the normal features of the North Equatorial Belt (NEB) in recent years, especially the large dark formations which are thought to represent waves in the prograde jet on the NEB south edge (NEBs, 7N), and the NEB expansion events (NEEs) in which the belt broadens to the north at intervals of 3 to 5 years. Here I describe an exceptional set of apparently coordinated changes which occurred in 2011-12, after more localised precursors in 2008 and 2010. (1) The large NEBs dark formations progressively disappeared until none remained. (2) In the sectors of NEBs thus vacated, smaller dark features all moved with unprecedented super-fast speeds, which were modulated by the few normal features as long as they lasted, and then accelerated further, reaching 139-151 m/s. (3) Rifts (expanding systems of convective white clouds) also disappeared from the NEB. (4) The NEB north half progressively faded (whitened) until there was only a narrow, southerly NEB, narrower and fainter than it had been for nearly a century. These changes have several profound implications for understanding the dynamics of the region. First, the NEBs took on the same appearance, dynamics, and speed, as the equivalent jet at 7S (SEBn), supporting the view that the two jets are essentially symmetrical, with an underlying jet in the range 150-170 m/s. Secondly, the manifestation of this jet at the surface is normally suppressed by the presence of large slow-moving formations, which are probably Rossby waves. Thirdly, the loss of the large dark formations and the narrowing of the belt may have been promoted by the decline of convective rift activity. Fourthly, these changes seem to represent a reversion to the situation that existed before 1912, when such appearances were common and were followed cyclically by vigorous NEB Revivals. Indeed, just such a Revival would ensue in 2012.