Jupiter’s South Temperate Domain, 2018-2024

John Rogers¹, Gianluigi Adamoli^1,5, Robert Bullen^1,5, Michel Jacquesson⁵, Marco Vedovato⁵, Hans-Joerg Mettig^1,5, Clyde Foster¹;  Candice Hansen², Gerald Eichstaedt³, Glenn Orton⁴, Tom Momary⁴.

(1) BAA, London, UK.  (2) Planetary Science Institute, Tucson, AZ, USA. (3) Independent scholar, Stuttgart, Germany. (4) JPL, CalTech, Pasadena, CA, USA. (5) JUPOS team.

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This is the fifth in our series of reviews of the South Temperate domain from amateur observations, now combined with JunoCam images.  It synthesises information from our previous posts as well as new analysis of JUPOS data.  Following on from 2018 when the last review ended, it covers 5 apparitions from 2019 to 2023/24.  The report is very long and has many illustrations (including beautiful JunoCam images), but the Summary below gives the most important information.  We are very grateful to all the observers whose images made this possible.  –JHR

The report comes in three PDF files plus several ZIP files of full-size figures, as follows:

The text is here:    STempR_2018-2024_Final-to-post

The figures are here (miniature copies):  Figures_final

The full-size figures are here (ZIP file):  STemp-2018-2024_Figures

The Appendices & Tables are here:   STempR_2018-2024_Appendices-w-Minifigs

The full-size maps and charts in the Appendices are in these ZIP files:    STempR_AppxB_Amateur-maps

STempR_AppxC_JunoCam-maps       STempR_AppxD_JUPOS-charts        STempR_AppxE_EPSC-Abstracts

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Summary

This report, like its predecessors, is based largely on our analysis of amateur ground-based images, but this is now complemented by high-resolution images from NASA’s Juno orbiter.

From 1998-2018 the South Temperate domain had shown a consistent cyclic pattern of behaviour.  It was whitened around most longitudes, with just one large anticyclonic oval, called BA, and a dark sector of STB following it (STB Segment A).  Other structured, cyclonic sectors of STB arose periodically shortly preceding BA, then expanded and prograded until they collided with Segment A and underwent vigorous transformation and merger with it.

Origins of structured sectors, and shift to a new regime:  In 2019 there was just one structured sector apart from Segment A: the ‘STB Spectre’ (Segment F). Unlike previous examples, it grew extremely long, so its following end was no longer approaching BA; and subsequent events changed the typical sequence.  When the preceding (p.) end of the Spectre arrived at Segment A, in early 2020, it did not transform internally. Instead, Segment A itself grew more active and longer, and initiated the usual streams of dark spots eastward and westward.  (The Spectre itself became unrecognisable.)

Meanwhile, we were expecting one or more new cyclonic spot(s) to arise some way p.  BA to initiate the next structured sector.  This eventually happened in 2020, when two small pale cyclones developed into two dark spots which we named spots 6 and 7.  Spot 7 began as ‘Clyde’s Spot’, when a sudden convective outburst transformed the cyclone into a turbulent feature which darkened and expanded from that time onwards to become the new STB Segment G. A year later, a similar outburst produced dark Spot 8, but that eventually became a pale cyclonic oval which has not expanded greatly. Spot 6, likewise, had changed from dark brown to white, and it was eventually destabilised and merged with Segment G.  Thus, the combination of hi-res ground-based and JunoCam images has given much insight into the origins and transformations of STB structured segments.

Following the unique manner of the Spectre’s collision with Segment A, the cycle of successive segments was broken, and the domain has moved into a new regime.  Oval BA no longer drifted slower than cyclonic features. Instead, Segment A and the new dark Segment G both continued expanding and emitting streams of dark spots on the STBn and STBs jets, re-creating a visibly dark STB around much of the circumference, which had not happened since the mid-1990s.

From mid-2022 through to 2025, there was some kind of dark STB around more than half the planet, comprising Segments G and A and the dusky or spotty space between them.  Segment G expanded at an average rate of ~1.75 to 2.0 deg/30d throughout 2022 and 2023.  Segment A also continued to lengthen irregularly, with an average rate of ~ 2.0 deg/30d from 2020 to 2023, and developed internal subdivisions with different textures.  Following Segment A there was a long dark extension (‘Sf. tail’) along the STBs jet and STZ.

Oval BA continued to vary in colour. It was strongly reddish in early 2018, then faded to almost white by late 2018, and remained white or almost so throughout 2019 & 2020. In 2021 & 2022 it was off-white with more ‘warm’ tint, then reddened during 2023 so as to be distinctly reddish throughout 2024.  BA’s rapid circulation was measured from JunoCam images at PJ17 (2018 Dec.21).  A small anticyclonic white oval (AWO-b) existed following Segment A from 2018 March to 2023 Nov.  A few even smaller AWOs were tracked but were short-lived.

Drift rates of major features such as BA, AWO-b, and the cyclonic circulations, were largely consistent with previous measurements in relation to their latitudes.  From 2020 to 2024, these all had similar drift rates (mean DL2 = -16.2 (±1.0) deg/30d), similar to those of cyclonic sectors in previous decades, defining the S. Temperate Current.  We conclude that in this domain, as in others, the slow current is normally determined by the cyclonic features, while anticyclonic ovals can move at the same or different speeds.

Measurements of these and smaller spots by the JUPOS team were used to produce zonal drift profiles (previously unpublished)  These showed several very fast retrograding speeds along the south edge of the STB Spectre, even after its p. end had disappeared into Segment A, consistent with the rapid wind speeds around such circulations.  Following Segments A and G, retrograding speeds on the STBs were variable; their relationship to the expansion of these segments was unclear, and they became unexpectedly fast f. Segment A in 2023.  Anomalous zonal drift profiles that we previously observed f. structured segments were reproduced in some cases but not all, and they have not developed into new large-scale circulations.

A group of spots on the N edge of Segment G in 2023 had particularly rapid speeds, among the fastest ever recorded in the STBn jet, again confirming the rapid speeds around structured sectors.  Zonal wind profiles, from Hubble images in 2019 June and from amateur images in 2023/24, were in line with previous data showing frequently faster jet speeds on the STBn and STBs alongside structured sectors, viz. the STB Spectre and Segment A.

The STBn jet displayed dark jet spots prograding p. oval BA in 2018 after the STB Ghost collision with Segment A, and again in late 2019/early 2020.  Then, despite the reinvigoration of Segment A as the STB Spectre arrived, there was little STBn activity p. BA until 2021, when the situation here was complex; jet spots were probably arising here from mid-2021 until late 2022 when there was little space left p. BA because of DS7 (Segment G) expanding there.  Meanwhile, Clyde’s Spot (DS7) produced a few STBn jet spots in 2020 and 2021, but from 2021 Sep., when (as STB Segment G) it had erased the remains of the Spectre p. it, it became a major source of STBn spot activity which was intense throughout 2022, 2023 and 2024. Many of the spots drifted north without change of speed, as we have often observed, and even travelled north of the usual STBn jet latitude, decelerating accordingly.  JunoCam images suggested a variety of wave patterns on the north sub-peak of the STBn jet, and weak or absent vorticities in the STBn jet spots.

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