Induction signals from Callisto's ionosphere and their implications on a possible subsurface ocean. Issue 11 (10th November 2017)

• Record Type: Journal Article
• Authors: Hartkorn, Oliver; Saur, Joachim
• Is Part Of: Journal of geophysical research. Volume 122:Issue 11(2017)
• Page Start: 11, 677
• Page End: 11, 697
• Publication Date: 2017-11-10
• Subjects: Callisto -- subsurface ocean -- ionosphere -- electromagnetic induction; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2017JA024269
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: We investigate whether induction within Callisto's electrically conductive ionosphere can explain observed magnetic fields which have previously been interpreted as evidence of induction in a saline, electrically conductive subsurface ocean. Callisto's ionosphere is subject to the flow of time‐periodic magnetized plasma of Jupiter's magnetosphere, which induces electric fields and electric currents in Callisto's electrically conductive ionosphere. We develop a simple analytic model for a first quantitative understanding of the effects of induction in Callisto's ionosphere caused by the interaction with a time‐variable magnetic field environment. With this model, we also investigate how the associated ionospheric currents close in the ambient magnetospheric plasma. Based on our model, we find that the anisotropic nature of Callisto's ionospheric conductivity generates an enhancement effect on ionospheric loop currents which are driven by the time‐variable magnetic field. This effect is similar to the Cowling channel effect known from Earth's ionosphere. Subsequently, we numerically calculate the expected induced magnetic fields due to Jupiter's time‐variable magnetic field in an anisotropic conductive ionosphere and compare our results with the Galileo C‐3 and C‐9 flybys. We find that induction within Callisto's ionosphere is responsible for a significant part of the observed magnetic fields. Ionospheric induction creates induced magnetic fields to some extent similar as expected from a subsurface water ocean. Depending on currently unknown properties such as Callisto's nightside ionosphere, the existence of layers of "dirty ice" and the details of the plasma interaction, a water ocean might be located much deeper than previously thought or might not exist at all. Key Points: Induction in Callisto's anisotropic electrically conductive ionosphere Induction in ionosphere explains significant parts of signals previously associated with a subsurface water ocean Subsurface water ocean could be very deep or might be absent