Abstract
We present calculations of magnetic potential associated with the perturbation of Saturn's magnetic field by a rotating, equatorially-situated disc of plasma. Such structures are central to the dynamics of the rapidly rotating magnetospheres of Saturn and Jupiter. They are `fed' internally by sources of plasma from moons such as Enceladus (Saturn) and Io (Jupiter). We use a scaled form of Euler potentials for the Jovian magnetodisc field (Caudal, 1986). In this formalism, the magnetic field is assumed to be azimuthally symmetric about the planet's axis of rotation, and plasma temperature is constant along a field line. We perturb the dipole potential by using simplified distributions of plasma pressure and angular velocity for both planets, based on observations by Cassini (Saturn) and Voyager (Jupiter). Our results quantify the degree of radial `stretching' exerted on the dipolar field lines through the plasma's rotational motion and pressure. A simplified version of the field model, the `homogeneous disc', can be used to easily estimate the distance of transition in the outer magnetosphere between pressure-dominated and centrifugally-dominated disc structure. We comment on the degree of equatorial confinement as represented by the scale height associated with disc ions of varying mass and temperature. For Saturn, we identify the principal forces which contribute to the magnetodisc current and make comparisons between the field structure predicted by the model and magnetic field measurements from Cassini. For Jupiter, we reproduce Caudal's original calculation in order to validate our model implementation. We also show that compared to Saturn, where plasma pressure gradient is, on average, weaker than centrifugal force, the outer plasmadisc of Jupiter is clearly a pressure-dominated structure.
Highlights
Jupiter and Saturn are the largest planets in our Solar system, they are the most rapid rotators. Gledhill (1967) first pointed out the important consequences of these properties for Jupiter’s magnetosphere
The study by Bunce et al (2007) emphasized that the current which flows in the magnetodisc current sheet is a macroscopic manifestation of the microscopic drift motions of charged particles in the plasma. These authors examined the contribution of two types of azimuthal particle drift to the magnetic moment of the ring current: (i) the magnetic gradient drift exhibited by particles of finite thermal energy whose guiding centre moves in response to changes in field strength experienced during individual gyrations and (ii) the inertial drift associated with the centrifugal force in a frame which corotates with the local plasma flow
The current profiles due to hot and cold plasma pressure gradients show generally comparable values at Saturn, while at Jupiter the cold plasma current is an order of magnitude or more weaker compared to that of the hot plasma. These results indicate that the much more expanded magnetosphere of Jupiter develops an outer region beyond ∼40RJ, where the cold plasma’s angular velocity and density decline at a rate sufficiently rapid to produce a plasma whose main energy content arises from the thermal motions of the hot particle population
Summary
Jupiter and Saturn are the largest planets in our Solar system, they are the most rapid rotators. Gledhill (1967) first pointed out the important consequences of these properties for Jupiter’s magnetosphere. These authors examined the contribution of two types of azimuthal particle drift to the magnetic moment of the ring current: (i) the magnetic gradient drift exhibited by particles of finite thermal energy whose guiding centre moves in response to changes in field strength experienced during individual gyrations and (ii) the inertial drift associated with the centrifugal force in a frame which corotates with the local plasma flow They showed that, for typical magnetospheric conditions at Saturn, the heavier (water-group) ions may generate a much stronger inertial current at distances beyond ∼10RS due to their rotational kinetic energy exceeding typical thermal energy. Caudal (1986) developed a formalism in which Jupiter’s magnetic field structure was modelled by solving a magnetostatic equation representing dynamical equilibrium, i.e. a uniformly zero vector sum for all of the aforementioned forces throughout a specified region This solution was used to infer the global distribution of current which was consistent with the equatorial distribution of plasma properties such as angular velocity, temperature, density and composition.
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