by  John Rogers¹, Gianluigi Adamoli^1,2, Robert Bullen^1,2, Grischa Hahn², Michel Jacquesson², Marco Vedovato², Hans-Joerg Mettig^1,2, Gerald Eichstaedt³, Candice Hansen^4,5, Glenn Orton^5,6, Tom Momary^5,6.

(1) British Astronomical Association, London, UK.  (2) JUPOS team.  (3) Independent scholar, Stuttgart, Germany.  (4) Planetary Science Institute, Tucson, AZ, USA. (5) JPL, CalTech, Pasadena, CA, USA.  (6) NASA JunoCam team.

Summary

The S2 domain lies between the prograde jets at 36ºS (S2 jet) and 43ºS (S3 jet), encompassing the traditionally named South South Temperate Belt (SSTB) and Zone (SSTZ).  The most distinctive features here are 6 to 9 long-lived anticyclonic white ovals (AWOs), while cyclonic features include white oblongs, dark oblongs, and folded filamentary regions (FFRs). Here we review the features and phenomena of this domain, following our previous review up to 2012 which was based mainly on amateur observations.  Since 2012, amateur images have improved further, and since 2016, they have been complemented by hi-res imagery from JunoCam on almost every orbit of NASA’s Juno orbiter.  Combining these two data streams gives us a more complete and detailed understanding of this domain, particularly of cyclonic features which were not well resolved in earlier ground-based images.

The general conclusions of our previous review are confirmed and updated by the subsequent 12 years of observations.  The present work also presents some new findings, as follows.

Jet streams and currents:

–The S2 jet has accelerated after 2000, while the S3 jet has slightly decelerated.

–In the middle of the domain, a modest retrograde jet can usually be detected, with a peak in one or both of two preferred latitudes (~39.3ºS, 39.8ºS).

–There is a much faster retrograde jet along the wavy south edge of some cyclonic circulations and FFRs.

–The S.S. Temperate Current (SSTC: mean drift of the larger features such as AWOs) has speeded up in recent decades. (‘Fast’ and ‘slow’ refer to prograde flow unless otherwise specified.)

–Cyclonic features are mostly trapped between AWOs and share their drift, but there is limited evidence that untrapped FFRs drift more slowly. Small cyclonic dark spots sometimes also move slowly, but this may often be due to interaction with larger structures.

Anticyclonic white ovals:

–In these twelve years, no new long-lived AWOs have appeared, and two have disappeared by merging with others, confirming a nominal mean lifetime of ~56 years. Large AWOs have never been observed to disappear except by merging, so there may be no intrinsic limit to their lifetime.

–Mergers of AWOs usually occur when they are close to or passing oval BA.

–There is evidence that growth of a smaller AWO was assisted by mergers with smaller anticyclonic vortices that emerged westward from the turbulence of a large FFR.

Cyclonic features [summary adapted from EPSC 2024 abstract]:

By combining data from JunoCam and ground-based images, as well as occasional Hubble data, we present a history of the large-scale cyclonic structures in Jupiter’s S2 domain from 2015 to 2023, thus establishing the range of lifetimes of FFRs for the first time.  In addition to the three well-defined types (white oblongs, dark oblongs, and FFRs), JunoCam images reveal long, pale fawn-coloured sectors of the SSTB that resemble white oblongs in structure and can develop into them or from them.

Any of these types can last for as little as ~4 months.  White oblongs often last longer, ranging up to 2.8 years in this survey, and nearly 6 years in earlier records.  FFRs also have a large range of lifetimes, although they may sometimes be temporarily weak; two have existed for at least 8 years. Some sectors (delimited by AWOs) have shown repeated changes between the cyclonic types: any of three types can convert into any other over a matter of months, sometimes via a pale ochre oblong as intermediate.

White oblongs sometimes brighten rapidly in their early stages; two examples were very bright white and also bright in the methane band, which is very unusual for a cyclonic oval on Jupiter.  Their longitudinal expansion rates are confirmed.  Their termination is usually gradual, becoming less white and sometimes visibly disturbed.

Dark oblongs may form when a FFR becomes inactive.  Dark spots or oblongs often end their lives by becoming redder and then lighter in colour, sometimes becoming white.

FFRs appear and disappear about once a year.  A young FFR is usually small and expands, though they do not grow indefinitely.  In the few examples where we have seen a likely convective outbreak initiating a FFR, they have been very small and only occasionally methane-bright, thus much less energetic than examples recorded in the S. Temperate domain.

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The full report (29 pages) is in this PDF:      Long-term-report_S2_2012-2023_final 

Small copies of the figures (45 figures) are in this PDF:  Long-term-report_S2_FIGURES

The full-size figures are in this  ZIP file (30 MB):   S2-report_Figures

Four Appendices are in separate files:  

A. JUPOS charts, 2012-2023 (ZIP):  Appendix-A_S2_JUPOS-charts

B. JunoCam maps, 2016-202 (ZIP):  Appendix-B_S2_JunoCam-maps

C. Our principal report texts, 2016-2023 (PDF):  Appendix-C_S2_Archive-of-BAA-reports

D. EPSC abstract 2024 (PDF):  Appendix-D_S2_EPSC2024_JHR-abstract

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Our previous report is here:

Rogers J, Adamoli G, Hahn G, Jacquesson M, Vedovato M, & Mettig H-J (2014).   ‘Jupiter’s southern high-latitude domains: long-lived features and dynamics, 2001-2012.’   http://www.britastro.org/jupiter/sstemp2014.htm

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