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The Role of Magnetospheric Coupling in Saturn's Thermosphere.

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Title: The Role of Magnetospheric Coupling in Saturn's Thermosphere.
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, p1-15. 15p.
Subject Terms: Momentum transfer, Thermosphere, Plasma dynamics, Plasma interactions, Atmospheric models
Abstract: In a rotationally dominated magnetosphere, plasma dynamics exert persistent drags on the planet's thermosphere via electromagnetic coupling that exerts a torque and produces persistent, steady departures from corotation. Two categories of mechanisms have been proposed to resupply angular momentum to the thermosphere: vertical transport via eddy diffusion and horizontal advection from other latitudes. For the latter, Smith and Aylward's (2008) atmospheric circulation model applied a fixed function for the magnetospheric angular velocity profile. This fixes the convection electric field for the thermosphere, which then evolves toward a steady state governed by the magnetospheric velocity. We develop a model for the angular velocity of the neutral thermosphere that neglects all physical factors except for three factors: angular momentum transport by poleward advection; magnetospheric torque; and drag from the lower atmosphere, replacing the earlier invocation of eddy diffusion as a mechanism for transporting angular momentum upward. We follow Hill's (1979) derivation for radial plasma transport driven by a rigidly rotating atmosphere, but we reverse the logic and solve the thermosphere's angular velocity as constrained by the prescribed magnetospheric plasma velocity and the lower atmosphere. Our results reproduce the qualitative features of Smith and Aylward's results. However, the thermosphere should control the dynamics of the magnetosphere rather than the reverse. Plain Language Summary: Corotation occurs throughout much of the magnetospheres of Jupiter and Saturn, with departures caused by electromagnetic interactions arising when new ions are ejected into the magnetosphere. These forces exert a drag at the top of the planets' atmospheres that tends to slow them, so the lower atmosphere responds with forces that attempt to accelerate them by bringing angular momentum from other regions. The nature of this lower interaction is not entirely clear, with one approach treating angular momentum transfer as purely vertical and another as predominantly horizontal. We derive a model that includes only magnetosphere‐atmosphere coupling while neglecting many additional processes that were included in the numerical simulation used by Smith and Aylward (2008). Their use of fixed mathematical formulas to describe magnetospheric flow means that the magnetosphere is not allowed to respond to changes in the atmosphere. Our solutions for the velocity of the atmosphere closely reproduce Smith and Aylward's findings, which shows that magnetosphere‐thermosphere coupling is indeed the dominant factor governing their results. However, that conclusion is unphysical because the ratio of moments of inertia for the thermosphere to the magnetosphere is overwhelmingly in the thermosphere's favor. Key Points: In Smith and Aylward's 2008 paper, the velocity profile for magnetospheric plasma was fixed, which is a questionable assumptionThe nature of angular momentum transport in Saturn's thermosphere, whether it is predominantly poleward or vertical, is disputedThe present paper derives a differential equation that adopts their premise to illustrate why it is in error [ABSTRACT FROM AUTHOR]
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Abstract:In a rotationally dominated magnetosphere, plasma dynamics exert persistent drags on the planet's thermosphere via electromagnetic coupling that exerts a torque and produces persistent, steady departures from corotation. Two categories of mechanisms have been proposed to resupply angular momentum to the thermosphere: vertical transport via eddy diffusion and horizontal advection from other latitudes. For the latter, Smith and Aylward's (2008) atmospheric circulation model applied a fixed function for the magnetospheric angular velocity profile. This fixes the convection electric field for the thermosphere, which then evolves toward a steady state governed by the magnetospheric velocity. We develop a model for the angular velocity of the neutral thermosphere that neglects all physical factors except for three factors: angular momentum transport by poleward advection; magnetospheric torque; and drag from the lower atmosphere, replacing the earlier invocation of eddy diffusion as a mechanism for transporting angular momentum upward. We follow Hill's (1979) derivation for radial plasma transport driven by a rigidly rotating atmosphere, but we reverse the logic and solve the thermosphere's angular velocity as constrained by the prescribed magnetospheric plasma velocity and the lower atmosphere. Our results reproduce the qualitative features of Smith and Aylward's results. However, the thermosphere should control the dynamics of the magnetosphere rather than the reverse. Plain Language Summary: Corotation occurs throughout much of the magnetospheres of Jupiter and Saturn, with departures caused by electromagnetic interactions arising when new ions are ejected into the magnetosphere. These forces exert a drag at the top of the planets' atmospheres that tends to slow them, so the lower atmosphere responds with forces that attempt to accelerate them by bringing angular momentum from other regions. The nature of this lower interaction is not entirely clear, with one approach treating angular momentum transfer as purely vertical and another as predominantly horizontal. We derive a model that includes only magnetosphere‐atmosphere coupling while neglecting many additional processes that were included in the numerical simulation used by Smith and Aylward (2008). Their use of fixed mathematical formulas to describe magnetospheric flow means that the magnetosphere is not allowed to respond to changes in the atmosphere. Our solutions for the velocity of the atmosphere closely reproduce Smith and Aylward's findings, which shows that magnetosphere‐thermosphere coupling is indeed the dominant factor governing their results. However, that conclusion is unphysical because the ratio of moments of inertia for the thermosphere to the magnetosphere is overwhelmingly in the thermosphere's favor. Key Points: In Smith and Aylward's 2008 paper, the velocity profile for magnetospheric plasma was fixed, which is a questionable assumptionThe nature of angular momentum transport in Saturn's thermosphere, whether it is predominantly poleward or vertical, is disputedThe present paper derives a differential equation that adopts their premise to illustrate why it is in error [ABSTRACT FROM AUTHOR]
ISSN:21699380
DOI:10.1029/2025JA034427