Gas Giant Magnetosphere–Ionosphere–Thermosphere Coupling

Abstract

Magnetosphere-ionosphere-thermosphere (MIT) coupling describes the exchange of energy and angular momentum between a planet and its surrounding plasma environment.A plethora of phenomena are signatures of this interaction, from bright auroral and radio emissions across multiple wavelengths that are easily observed remotely, to radio bursts and field aligned currents best measured in situ. Gas giant MIT coupling differs from that in the terrestrial system because of rapid planetary rotation rates, dense hydrogenbased atmospheres, and outgassing moons embedded well within the magnetospheres.We discuss here the fundamental physics governing MIT coupling at Jupiter and Saturn. IntroductionMagnetosphere-ionosphere-thermosphere coupling is the process by which energy and angular momentum are transferred between a planet and its surrounding plasma environment. The magnetosphere is host to a variety of plasma populations which are connected to the planetary magnetic field. Stresses associated with changes in magnetic field configuration e.g. magnetic reconnection, magnetospheric compressions or expansions induced by the solar wind, or modifications to the local plasma population e.g. source and/or loss processes such as charge exchange, energisation, plasma injections triggered by reconnection or radial outflow, are communicated to the planet via electrical currents.Electrical currents in the magnetosphere are coupled to the planet through magnetic fieldaligned currents which close in the ionosphere. Ionospheric currents modify the coincident thermosphere by for example, heating the local atmosphere and driving winds. Collisions between ionospheric ions and thermospheric neutrals alter the electric currents and thus can affect magnetospheric plasma. Sections 3 and 4 of this series are dedicated to solar wind-magnetosphere and magnetosphere-ionosphere coupling processes, respectively. We concentrate on MIT coupling at the giant planets here.At Earth, MIT coupling is largely driven by the interaction between the magnetosphere and the solar wind. This is only a fraction of the picture at gas giant planets, where rapid rotation and internal plasma sources combine to drive a more dynamic MIT coupled system. Jupiter and Saturn rotate with periods of ∼9.9 hours and ∼10.7 hours, respectively. Deep within each magnetosphere, moons under tidal stresses release neutral material into the local space environment. Io ejects 700 -3000 kg s −1 neutral material into Jupiter's magnetosphere (Delamere, Bagenal, & Steffl, 2005). At Saturn, Enceladus emits neutrals at a rate of 150 -300 kg s −1 (Hansen et al., 2006). Approximately half of the material remains as plasma in the system following ionization (see Chapter 8.2, this volume). These plasma sources, embedded well within the magnetosphere, modify the MIT coupling throughout the system from that described in Chapter 4.1. Newly generated plasma, which orbited the planet at the Keplerian velocity as neutrals, must be accelerated to corotation with the planetary magnetic field. This accele...