Local-time Variations In The Jovian System And Possible Connections To Solar Wind/Magnetosphere Interactions

Many current models of plasma transport in the Jovian equatorial plasma disk (magnetodisk) consider it to be azimuthally symmetric over radial distances extending from the outer edge of the Io torus (6 Jovian radii) to about 50 Jovian radii. But there are also many pieces of evidence pointing to a local time asymmetry in this system at such radial distances, and in the upper atmosphere to which it is coupled. In the magnetosphere, local-time asymmetries have been detected in the thickness of the magnetodisk and in high-energy particle fluxes; observations of magnetodisk radial and azimuthal current systems show that they vary with local time, in such a way that their divergence in the disk plane feeds a system of field-aligned currents similar to the so-called Region 2 currents observed at Earth. At ionospheric altitudes, local time asymmetries are systematically observed in the main auroral emissions, and have also been observed in neutral and ion winds detected in the near infrared by Earth-based telescopes. Finally, deep in the magnetosphere, local time asymmetries have been observed during ground-based surveys of the Io torus, as well as by JAXA’s Hisaki spacecraft observations which suggested they are modulated by the solar wind pressure. Many of these observed asymmetries have been interpreted as the result of a large-scale dawn-to-dusk electric field generated across the magnetospheric cavity by its interaction with the Solar Wind and which would be superimposed to the dominant corotation electric field. 

Despite this many pieces of evidence, no consistent model of this electric field, its generation and its penetration to different magnetospheric radial distances and ionospheric latitudes exists yet. In this study, we attempt to fill this gap by developing a simple semi-analytical model of electric fields, plasma convection and current flows in the Jovian ionosphere and magnetosphere, adapting models developed in the Earth case. We feed our model with the latest measurement of ionospheric resistive properties and magnetospheric magnetic field content. We calculate the latitudinal and local time distributions of the electrostatic potential and field-aligned currents for two particular cases, an abrupt onset of an external dawn-dusk electrostatic potential across the polar cap and a steady dawn-dusk potential, as well as various time variable external electric field, based on Solar Wind observation at the location of Jupiter. We compare these cases to the measurement of the electrostatic potential associated with the above mentioned observations of local time asymmetries, and find that observations fit well with a rapid forcing from the Solar Wind enabling an important penetration of the electric field inside the magnetosphere.