Results from an Improved Model of Electrodynamics in Saturn's Upper Atmosphere
Abstract
Recent observations of magnetic field perturbations in Saturn's upper atmosphere, obtained by the Cassini MAG instrument during the Grand Finale tour, strongly suggest the presence of equatorial electrodynamics. This is an important discovery because electrodynamics is associated with resistive (Joule) heating and ion drag (i.e. the Lorentz force), which could change the winds, temperatures and electrical currents in Saturn's ionosphere and thermosphere. Current models are unable to reproduce the observed temperature profiles in Saturn's upper atmosphere: they either over-predict temperatures at high latitudes or under-predict temperatures at low latitudes. However, resistive heating and ion drag could alter the circulation to allow energy in the auroral region to be transported equatorward, bringing the temperature profiles of our models in line with observations. However, no studies to date have explored this possibility via a simulation. In the present study, we explore electrodynamics in Saturn's upper atmosphere at low and middle latitudes using the Saturn Thermosphere-Ionosphere Model (STIM), a general circulation model of Saturn's upper atmosphere. We have improved the way STIM calculates electrodynamics by coupling it with an axisymmetric model of a wind-driven dynamo and updating its calculation of electric currents to use a more general formulation. To the best of our knowledge, this is the first time such a coupled model has been used to simulate electrodynamics in solar system giant planet atmospheres. We use the updated STIM to simulate the global impact of both auroral and dynamo-driven electrodynamics in Saturn's upper atmosphere. We further investigate what impact the presence of strong eastward winds near the equator, hinted at by the Cassini MAG data, might have on the resulting electrodynamics. To do this, we impose the winds in STIM as a lower boundary condition. We will discuss how the improved electrodynamics alters the winds, temperatures and electrodynamics in Saturn's upper atmosphere. Without the wind-driven dynamo, the more general formulation of electrodynamics leads to small changes to the temperatures at high latitudes which better match observed temperatures. In addition to exploring the polar temperature profile, we will describe how including the wind-driven dynamo affects the global circulation and temperature profile. Finally, we will present results that more closely match the MAG data and discuss their implications for Saturn's upper atmosphere.
- Publication:
-
EPSC-DPS Joint Meeting 2019
- Pub Date:
- September 2019
- Bibcode:
- 2019EPSC...13.1019V