Magnetic field and current structures in the magnetosphere of Uranus

Abstract

We compare Voyager 2 magnetic field and plasma data with theoretical model calculations for the magnetosphere of Uranus in order to derive a global picture from the quite limited set of measurements. In two dimensions we use a tilt-dependent linear MHD equilibrium model for the entire magnetosphere to calculate the plasma parameter /• = 8r•P/B 2 and the resulting B field in the tail plasma sheet. The three-dimensional model satisfies the MHD equilibrium equations in an approximate fashion; it is used to calculate the large-scale magnetic field structure, the tilt-dependent shape and position of the Uranian magnetotail plasma sheet, and the influence of the interplanetary magnetic field (IMF). The main results of the model calculations are the following: (1) The actual solar wind pressure at Uranus was found to be about a factor of 5 higher than a calculated solar wind pressure derived from an adiabatically expanding quiet solar wind. This means that the Uranian magnetosphere must have experienced a major magnetic storm prior to the Voyager encounter. (2) The previous conjecture is supported by the fact that the asymptotic plasma beta parameter in the neutral sheet,/•o =/•(Ixl >> 1), relaxes from/•o = 37 to/•o -14 between the first and second neutral sheet crossings. (3) The excellent agreement between measured and modeled tail lobe magnetic field values suggests that during the time period of the Voyager encounter the Uranian tail plasma sheet, measured in units of planetary radii, was thicker and less stretched than the average Earth's tail plasma sheet. (4) Despite sizable temporal variations suggested by the data and the model, the Uranian magnetosphere seems to reach the state of quasi-static, i.e., slowly time-dependent, MHD equilibrium. (5) In relation to a given IMF orientation the magnetosphere changes periodically every 8.62 hours (the Uranian day is 17.24 hours) from an "open" to a "closed" configuration and back. Thus the convection process, driven by the direct influence of the IMF, is periodically interrupted by the planet's rotation, such that the plasma sheet might not experience its full stretching. Consequently, the Uranian magnetosphere might not experience convectively driven, Earth-type substorms. On the other hand, the possibility of substorms cannot be ruled out either. 1. 1986]. This scenario is sketched in Figure 1 from a fixed observer's point of view. Figure 1 also depicts the two extreme orientations of the dipole axis with respect to the solar wind direction. Magnetospheric rotation causes magnetotail flux tubes to twist. A small helical twist (5.5 ø __+ 3 ø) has been discovered by Voyager 2 [Behannon et al., this issue].The purpose of this study is to interpret these and related signatures in the light of the quasi-static MHD equilibrium theory. This theory enables us to construct large-scale mathematical models of the Uranian magnetosphere. In particular, models of this type help us answer the following question: can we understand the Uranian magnetosphere as a system that reaches a ...

Space Exploration of Planetary Magnetism

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The effect of tidal locking on the magnetospheric and atmospheric evolution of “Hot Jupiters”

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Magnetohydrodynamic simulation of an equatorial dipolar paleomagnetosphere

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