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The Seasonal Dependence of Saturn's Apparent Rotation Period.

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Title: The Seasonal Dependence of Saturn's Apparent Rotation Period.
Authors: Smith, C. B.1 (AUTHOR), Pontius, D. H.2 (AUTHOR) duane.pontius@gmail.com
Source: Journal of Geophysical Research. Space Physics. Mar2026, Vol. 131 Issue 3, p1-12. 12p.
Subject Terms: Magnetosphere, Magnetometers, Angular velocity, Electric admittance, Auroras
People: Brooks, Mo, 1954-
Abstract: Brooks et al. (2019), https://doi.org/10.1002/2019ja026870 developed a model to explain the observed dual‐periodicity observed in Saturn's magnetosphere. They posited that the northern and southern neutral thermospheres have different angular velocities because of unequal conductances leading to unequal coupling with the magnetosphere. In particular, currents to the hemisphere with stronger conductance should be higher than those to the other hemisphere. To do so, they applied a model developed previously to explain the relative insensitivity of the Io plasma torus at Jupiter to short‐period temporal fluctuations in the magnetosphere. In response, Cowley et al. (2020), https://doi.org/10.1029/2020ja028247 provided a valuable analysis of Cassini magnetometer data that calculated the torques exerted in each hemisphere as a function of latitude. In contrast to the predictions of Brooks et al., Cowley et al. found that the torque imbalance in the auroral zone was quite modest. In this paper we resolve the matter by solving time‐dependent versions of the original equations and arrive at results that are quite similar to the data presented by Cowley et al. A key step is removing the influence of currents that cross the auroral zone without diverging. Plain Language Summary: The rate at which Saturn's magnetosphere rotates appears to have two different values that differ from each other by up to a few percent. Brooks and his coauthors proposed an explanation for this phenomenon that depended on the electrical conductances of the planet's northern and southern hemispheres being different. Because of that, electric currents would flow primarily to the more conductive hemisphere. Subsequently, Cowley and his coauthors published data that did not agree with Brooks' prediction and showed that the difference was not very large. The present paper revisits this question and shows that it can be resolved if electric currents that cross the auroral zone without changing are subtracted from the total. The reason is that those currents are actually about five times larger than the currents that are involved in maintaining the different rotation rates, so they are difficult to discern relative to the larger background. Key Points: Cowley et al. (2020)'s, https://doi.org/10.1029/2020ja028247 analysis of magnetometer data reached conclusions contrary to predictions of Brooks et al. (2019), https://doi.org/10.1002/2019ja026870We revisit the paper by Brooks et al. and solve the governing equations for time‐dependent torques by adding a time‐dependent conductanceOur results compare favorably with torques calculated from magnetometer data once currents that cross the auroral zone without diverging are removed [ABSTRACT FROM AUTHOR]
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Abstract:Brooks et al. (2019), https://doi.org/10.1002/2019ja026870 developed a model to explain the observed dual‐periodicity observed in Saturn's magnetosphere. They posited that the northern and southern neutral thermospheres have different angular velocities because of unequal conductances leading to unequal coupling with the magnetosphere. In particular, currents to the hemisphere with stronger conductance should be higher than those to the other hemisphere. To do so, they applied a model developed previously to explain the relative insensitivity of the Io plasma torus at Jupiter to short‐period temporal fluctuations in the magnetosphere. In response, Cowley et al. (2020), https://doi.org/10.1029/2020ja028247 provided a valuable analysis of Cassini magnetometer data that calculated the torques exerted in each hemisphere as a function of latitude. In contrast to the predictions of Brooks et al., Cowley et al. found that the torque imbalance in the auroral zone was quite modest. In this paper we resolve the matter by solving time‐dependent versions of the original equations and arrive at results that are quite similar to the data presented by Cowley et al. A key step is removing the influence of currents that cross the auroral zone without diverging. Plain Language Summary: The rate at which Saturn's magnetosphere rotates appears to have two different values that differ from each other by up to a few percent. Brooks and his coauthors proposed an explanation for this phenomenon that depended on the electrical conductances of the planet's northern and southern hemispheres being different. Because of that, electric currents would flow primarily to the more conductive hemisphere. Subsequently, Cowley and his coauthors published data that did not agree with Brooks' prediction and showed that the difference was not very large. The present paper revisits this question and shows that it can be resolved if electric currents that cross the auroral zone without changing are subtracted from the total. The reason is that those currents are actually about five times larger than the currents that are involved in maintaining the different rotation rates, so they are difficult to discern relative to the larger background. Key Points: Cowley et al. (2020)'s, https://doi.org/10.1029/2020ja028247 analysis of magnetometer data reached conclusions contrary to predictions of Brooks et al. (2019), https://doi.org/10.1002/2019ja026870We revisit the paper by Brooks et al. and solve the governing equations for time‐dependent torques by adding a time‐dependent conductanceOur results compare favorably with torques calculated from magnetometer data once currents that cross the auroral zone without diverging are removed [ABSTRACT FROM AUTHOR]
ISSN:21699380
DOI:10.1029/2025JA034420