Long‐Term Variability of Jupiter's Magnetodisk and Implications for the Aurora

Marissa F Vogt,Jonathan D Nichols,John T Clarke,William S Kurth,Emma J Bunce

doi:10.1002/2017ja024066

Marissa F Vogt, Jonathan D Nichols + Show 3 more

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https://doi.org/10.1002/2017ja024066

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Abstract

AbstractObservations of Jupiter's UV auroral emissions collected over several years show that the ionospheric positions of the main emission and the Ganymede footprint can vary by as much as 3° in latitude. One explanation for this shift is a change of Jupiter's current sheet current density, which would alter the amount of field line stretching and displace the ionospheric mapping of field lines from a given radial distance in the magnetosphere. In this study we measure the long‐term variability of Jupiter's magnetodisk using Galileo magnetometer data collected from 1996 to 2003. Using the Connerney et al. (1981) current sheet model, we calculate the current sheet density parameter that gives the best fit to the data from each orbit and find that the current density parameter varies by about 15% of its average value during the Galileo era. We investigate possible relationships between the observed current sheet variability and quantities such as Io's plasma torus production rate inferred from volcanic activity and external solar wind conditions extrapolated from data at 1 AU but find only a weak correlation. Finally, we trace Khurana (1997) model field lines to show that the observed changes in Jupiter's current sheet are sufficient to shift the ionospheric footprint of Ganymede and main auroral emission by a few degrees of latitude, consistent with the magnitude of auroral variability observed by Hubble Space Telescope (HST). However, we find that the measured auroral shifts in HST images are not consistent with concurrent changes in the current density parameter measured by Galileo.

Highlights

The first in situ measurements of Jupiter’s magnetosphere were obtained during the Pioneer 10 flyby in 1973
Following Connerney et al (1981), we considered data only at radial distances from 10 to 30 RJ, where the magnetic field is dominated by the planetary magnetic field and the field produced by the azimuthal current sheet, and the effects of the nightside magnetotail stretching and magnetopause surface currents are negligible
For each Galileo orbit we fit a current sheet model to the local time corrected perturbation field, which was given by the internal planetary field subtracted from the observed field

Summary

Introduction

The first in situ measurements of Jupiter’s magnetosphere were obtained during the Pioneer 10 flyby in 1973. The ionospheric position of these corotation enforcement currents, and the main auroral emission, depends on factors like the ionospheric conductivity (e.g., Nichols & Cowley, 2004) and the radial gradient in the magnetospheric plasma azimuthal velocity profile, which, in turn, depends on the plasma mass loading rate from Io. In addition to the main emission, Jupiter’s aurora features emissions at the ionospheric footprints of the moons Io (5.9 RJ), Europa (9.4 RJ), and Ganymede (15 RJ) (Clarke et al, 2002; Connerney et al, 1993). These footprints have been instrumental in constraining magnetic field models for mapping between the magnetosphere and the ionosphere (e.g., Connerney et al, 1998; Grodent, Bonfond, et al, 2008; Hess et al, 2011) because the satellites’ orbital locations at the jovigraphic equator are known

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