Local time variations in Jupiter's magnetosphere‐ionosphere coupling system

Abstract

The ionization of neutral material ejected by Jupiter's volcanically active moon, Io, results in a plasma disc that extends from Io's orbit out through the Jovian magnetosphere. This magnetospheric plasma is coupled to the planetary ionosphere via currents which flow along the magnetic field. Inside of ∼40 R J , these currents transfer angular momentum from the planet to the magnetospheric plasma, in an attempt to keep the plasma rigidly corotating with the planet. Jupiter's main auroral emission is a signature of this current system. To date, one-dimensional models of Jupiter's magnetosphere-ionosphere (M-I) coupling have either assumed a dipole field or used a field description appropriate to the postmidnight region of the Jovian magnetosphere. Vogt et al. (2011) described the variation of the N-S component of the magnetic field in the center of the current sheet, B N , with local time and radius. We apply a 1-D model of Jupiter's M-I current system every hour in local time using a modified description of the Vogt et al. (2011) magnetic field to investigate how local time variations in the magnetosphere affect the auroral currents and plasma angular velocity. Our model predicts the strongest aurora at dawn, with a minimum in the auroral currents existing from noon through dusk. This is a few hours duskward of the discontinuity predicted by Radioti et al. (2008). While our model predictions are consistent with some of the observations, future MI coupling models must account for the azimuthal bendback in the magnetic field.

The far-ultraviolet main auroral emission at Jupiter – Part 1: Dawn–dusk brightness asymmetries

“…These results are consistent with both the results from MHD simulations and the estimates of the field-aligned currents derived from equatorial magnetic field measurements (Khurana, 2001). However, they are not consistent with the corotation enforcement currents 1-D model of Ray et al (2014), which predicts stronger currents on the dawn side. According to Tao et al (2010), one partial explanation could be the difference in Pedersen conductivity in the dusk sector compared to the dawn sector due to the different history of the solar EUV illumination.…”

“…Furthermore, the fact that the northern dusk sector, which is usually located in a region with a strong magnetic field on the ACS images, appears weaker than the southern one suggests that the emitted power is inversely proportional to the magnetic field strength. If we consider the probably more representative southern hemisphere alone, the brighter dusk side is consistent with the electric currents derived from the Galileo magnetic field measurements (Khurana, 2001) but contradicts the model results from Ray et al (2014), which predict a brighter dawn side. Following Khurana (2001), we suggest that our results, and possibly their discrepancy with the results from the 1-D model of Ray et al (2014), could be caused by a partial ring current on the nightside (with dawn and dusk side field-aligned current legs) that is not accounted for in their model.…”

“…If we consider the probably more representative southern hemisphere alone, the brighter dusk side is consistent with the electric currents derived from the Galileo magnetic field measurements (Khurana, 2001) but contradicts the model results from Ray et al (2014), which predict a brighter dawn side. Following Khurana (2001), we suggest that our results, and possibly their discrepancy with the results from the 1-D model of Ray et al (2014), could be caused by a partial ring current on the nightside (with dawn and dusk side field-aligned current legs) that is not accounted for in their model. Because the Juno spacecraft will fly over the polar regions along a dawn-dusk orbital plane, offering a bird's eye view of the aurorae without any CML selection bias, the combination of both its remote-sensing and in situ observations should resolve the observational ambiguities discussed here, especially in the north, and thus confirm or disprove our interpretation of the dawn-dusk brightness asymmetry.…”

“…The resulting quasi-static electric field accelerates electrons downward into the atmosphere, which ultimately generates the main auroral emission. Ray et al (2014) provide an excellent review of the successive evolutions of the analytical models describing these currents, and Chané et al (2013) do the same for the magneto-hydrodynamic (MHD) numerical simulations. The size of the main emission contour changes from one day to another, but it also experiences significant variations on a timescale of months Bonfond et al, 2012).…”

“…Two different strategies have been used so far to account for such local time variations: 3-D MHD simulations of the magnetosphere (Walker and Ogino, 2003;Chané et al, 2013) or 1-D models of the magnetosphere-ionosphere coupling adapted for every hour in local time . The analytical model of the auroral currents by Ray et al (2014) predicts that they shall be strongest in the dawn sector with values at least 1 order of magnitude larger than in the noon-todusk sector. These authors however note that their 1-D model only accounts for the variations in the normal component of the magnetic field in the equatorial plane but does not account for the azimuthal bendback of the magnetic field lines nor the presence of azimuthal currents.…”