Juno Characterisation of Cyclonic “Folded Filamentary Regions” within Jupiter’s Polar Domains

Jupiter’s cyclonic features are known to undergo transitions between quiescent states with smooth edges (often appearing as dark brown ‘barges’) to states with convective outbursts of billowing white clouds, chaotically churned into filamentary structures.  Cyclones in the latter state are known as ‘Folded Filamentary Regions’ (FFRs), and Voyager images (Ingersoll+1979, doi: 10.1038/280773a0) revealed them to be rapidly-varying turbulent regions, occurring in cyclonic domains on the poleward side of Jupiter’s prograde jets.  JunoCam visible-light observations (Orton+2017, doi:10.1002/2016GL072443, Rogers+2021, doi:10.1016/j.icarus.2021.114742), reveal the increasing prevalence of FFRs at mid-to-high latitudes.  They dominate the polar domain alongside smaller anticyclonic white ovals, drifting westward in latitude bands between the narrow prograde jets, and rapidly evolving over timescales of days. 

The present study makes use of Juno’s ever-improving microwave observations of the north polar domain, as the latitude of closest-approach (“perijove”) moves northward.  We therefore focus on FFRs in the northern hemisphere, where we find them to occur in zonally-organised latitude bands even at high latitudes.  Statistics of the FFRs suggest that they occur on the poleward sides of the N4 (43.3^oN, centric), N5 (52.3^oN), and N7 (66.1^oN) prograde jets (N6 at 61.2^oN coincides with a ‘bland zone’ lacking notable FFRs), and scattered in the polar domain up to the octagon of circumpolar cyclones at 85^oN.  JIRAM 5-µm imaging of both poles reveal FFRs as generally dark structures with elevated aerosol opacity blocking thermal infrared emission from the 4-6 bar level, coinciding with the white stratiform clouds observed by JunoCam. Clusters of small cumulus-like clouds, as well as curvilinear cloud streaks, provide texture to the flat stratiform clouds to give the appearance of a network of filaments.  Visibly-dark lanes border the brighter filaments, creating an intricate network of narrow, aerosol-free, and 5-µm-bright striations within each FFR.  This is in contrast to cyclonic features such as barges at lower latitudes, where an absence of overlying aerosols generally renders them 5-µm bright. 

A survey of 1.4-50 cm observations acquired by Juno’s Microwave Radiometer (MWR) between PJ20 (May 2019) to PJ37 (October 2021) reveals that FFRs share a key characteristic with their low-latitude counterparts:  they are microwave-bright in channels sounding the 0.6-2.0 bar range (1.4-3.0 cm), become hard to distinguish from their surroundings near 5 bars (5.75 cm), but are then microwave-dark in the channel sounding 10-15 bar (11.5 cm). This suggests FFRs are depleted in ammonia gas and/or locally warmer at levels above the putative location of Jupiter’s water cloud, the latter implying a decay of cyclonic winds with altitude.  This shallow ammonia depletion is surprising, given the apparent convective nature of the FFRs - maybe NH₃-rich plumes occupy a sufficiently small area of the cyclonic structure, so that they have negligible impact on the warm emission observed by MWR.  This shallow depletion is balanced by a local NH₃ enrichment (or local cooling) at depth, below the water cloud, like a cyclonic lens.  However, the extension of FFR signatures to deeper levels (p>20 bars) is currently unclear due to insufficient spatial coverage and resolution at the longest-wave channels, and confusion arising from auroral contributions to the MWR dataset at 50 cm.  

An inversion in microwave brightness with depth was previously identified for Jupiter’s larger-scale belts and zones (Fletcher+2021, doi:10.1029/2021JE006858) and mid-latitude discrete features (Bolton+2021, doi:10.1126/science.abf1015), and now appears to be common at high latitudes as well.  Geostrophy implies that cyclonic circulations (low-pressure centers) cause a rise in potential-temperature surfaces in deeper layers, potentially triggering moist convection in the water cloud (e.g., Dowling & Gierasch, 1989 Bull. Amer. Astron. Soc 21, 946; Fletcher+2017, doi:10.1016/j.icarus.2017.01.001), producing the distinct convective cloud structure observed by visible and infrared imaging: juxtaposed tall convective towers, deep water clouds, and narrow clear lanes (Imai+2020, doi:10.1029/2020GL088397). Lightning sferics measured by MWR at 50 cm/600 MHz are more frequent in the N4, N5, and N7 domains where FFRs are common (Brown+2018, doi:10.1038/s41586-018-0156-5), and are further clustered in FFRs themselves (Wong+2020, doi:10.3847/1538-4365/ab775f).  Thus the evolution of the cyclonic FFRs, and their penetration to the moist depths below the water condensation level, may hold the key to understanding the distribution of lightning activity on Jupiter.