Jupiter's North Equatorial Belt and Jet, I, Cyclic expansions and planetary waves

Abstract

This article presents a synopsis of the activity in Jupiter's North Equatorial Belt (NEB) from 1986 to 2010, and of the speeds of dark formations on its south edge and bright streaks ('rifts') in its interior. In particular I discuss NEB expansion events (NEEs), which took place every 3-5 years during this time, and how the various features of the NEB are involved in them. I present evidence that the NEE affects not just the northern edge, but the whole width of the belt. It begins with an outbreak of a bright rift that is more northerly and slower-moving than usual; this is often involved with the first ejection of dark material northwards into the N. Tropical Zone, but typically the rift also expands southwards across the width of the NEB. NEBs dark formations are usually affected, as they are during individual interactions with rifts at other times; they may be disrupted, or intensified, and they usually undergo deceleration. The expansion of the dark NEB to the north occurs concurrently, and is followed by the appearance of new dark 'barges' and white ovals flanking the NEBn jet. The speed of the NEBs dark formations varies with their mean spacing, consistent with the prevailing hypothesis that they are planetary Rossby waves. In most apparitions since 2000 we have also detected smaller, faster features (120 m/s). I propose that these represent waves of the same type, but with higher frequency, and that their speed is slightly less than the true wind speed at cloud-top level under normal conditions.