Saturn's magnetospheric planetary period oscillations, neutral atmosphere circulation, and thunderstorm activity: Implications, or otherwise, for physical links

Abstract

Suggestions that the planetary period oscillations (PPOs) observed in Saturn's magnetosphere may be driven or influenced by neutral atmospheric perturbations motivate an exploratory comparison of PPO rotation periods with available tropospheric and stratospheric determinations. Nonpolar atmospheric rotation periods occupy the range ~10.2–10.7 h associated with the latitudinal jet structure, are similar north and south, and are independent of season, while PPO periods lie in a narrower partly overlapping range ~10.6–10.8 h, are persistently shorter north than south, and undergo a seasonal cycle. In this cycle, widely separated north‐south PPO periods during southern summer converge across equinox to values lying within the atmospheric west jet band, remaining well‐separated from east jet periods. Closest convergence occurred 1 year post equinox, contemporaneously with the switch in seasonal thunderstorm activity from Southern to Northern Hemispheres. Since most large‐scale atmospheric phenomena are related to the west jets, rotating with closely similar periods, they also rotate with periods close to the PPOs under post equinoctial conditions but not otherwise. Specifically, post equinox northern PPOs rotate with a period close to the southern thunderstorms, as well as the north polar spot and hexagon features, while the post equinox southern PPOs rotate with a period close to the pre‐equinox northern “string of pearls” and the first colocated post equinox northern thunderstorm, the Great White Spot event. However, even under these conditions, no consistent correspondences in period are found at a detailed level, which taken together with the lack of correspondence at other times does not suggest a direct physical link exists between these phenomena.

Planetary period oscillations in Saturn's magnetosphere: comments on the relation between post-equinox periods determined from magnetic field and SKR emission data

“…The abrupt changes noted by Fischer et al (2014) that occur after the beginning and near the end of the GWS event represent the first two of these transitions (defining interval E2), six of which have now been reported up until the end of 2013 as shown in Fig. 1 Cowley and Provan, 2013). Noting these differences in reported PPO properties, we make the statement of principle that the SKR and magnetic field phenomena cannot fundamentally show contrary properties, since both are believed to be directly related to the same underlying phenomenon, i.e.…”

“…The magnetic data do suggest an increase in southern period in this interval, but by a factor of less than half that suggested by Fischer et al (2014) on the basis of their misidentification of the nature of the changes involved. The topic of the possible connection of these abrupt PPO changes with the GWS storm has previously been addressed by Cowley and Provan (2013), and need not be repeated here. We note, however, that while such changes certainly began during the GWS storm, many similar events have continued afterwards with ∼ 100-200-day cadence, as illustrated in Fig.…”

A possible influence of the Great White Spot on Saturn kilometric radiation periodicity

“…As already mentioned, Hurley et al (2012) found no changes in temperature and chemical composition in the stratosphere related to the 2000 km sized thunderstorms located at 35 • south. Therefore, no relation was found by Cowley and Provan (2013) between those phenomena and the planetary period oscillations in Saturn's magnetosphere. Regarding the GWS, their main argument against a relation is that the start and end of the GWS are not exactly contemporaneous with the jumps in period in the magnetic field data, i.e., they assume that there should be a prompt response of the upper atmosphere to processes in the troposphere.…”

Local time dependences of oxygen ENA periodicities at Saturn

“…Dual periods were observed during Saturn's southern summer and near the equinox, but not during the northern spring. Apparently, neither the solar cycle nor the seasonal cycle offers a consistent pattern to the changing periodicities, a conclusion corroborated by recent reviews of non‐ENA periodicities [ Kimura et al ., ; Cowley and Provan , ].…”