The Structure of the Warped Io Plasma Torus Constrained by the Io Footprint

Abstract

<title xmlns:mml="http://www.w3.org/1998/Math/MathML">Abstract</title><p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en">Standard models of force balance along Jovian field lines predict the location of the Io Plasma Torus to be the centrifugal equator of Jupiter’s magnetosphere, that is, the position along the magnetic field lines farthest away from Jupiter’s rotational axis. In many models, the centrifugal equator is assumed to lay on a plane, calculated from a (shifted) dipole magnetic field, rather than on a warped surface which incorporates Jupiter’s higher magnetic field moments. In this work, we use Hubble Space Telescope observations of the Io Main Footprint to constrain density, scale height, and lateral position of the Io Plasma Torus. Therefore, we employ the leading angle of the footprints to calculate expected travel times of Alfvén waves and carry out an inversion of the observations. For the magnetic field, we use the JRM33 magnetic field model. The inversion results show peak densities between <italic>ρ</italic>₀ = 1,830 cm⁻³ and <italic>ρ</italic>₀ = 2,032 cm⁻³ and scale heights between <italic>H</italic> = 0.92<italic>R</italic>_{<italic>J</italic>} and <italic>H</italic> = 0.97<italic>R</italic>_{<italic>J</italic>} consistent with current literature values. Using a warped multipole centrifugal equator instead of a planar dipole increases the quality of the fit by about 25%. We additionally develop two tests to confirm that the multipole centrifugal equator from the JRM33 model fits explains the applied data set better than the dipole centrifugal equator. The quadropole moments alter Io’s relative position to the torus, which changes the plasma density around Io by up to Δ<italic>ρ</italic>/<italic>ρ</italic> = 20%.</p>

Key Points: <list list-type="bullet"> <list-item> <p xml:lang="en">Based on the Io Footprint positions, we show quantitatively that the Io Plasma Torus is centered on the centrifugal equator of Jupiter’s multipole magnetic field</p></list-item> <list-item> <p xml:lang="en">Position of the Io Footprint can be used to constrain a density model of the Io Plasma Torus</p></list-item> <list-item> <p xml:lang="en">The displacement of the Io Plasma Torus due to higher magnetic field moments can change the plasma density at Io by up to 20%</p></list-item> </list> </p>