The Juno Waves instrument can be used to accurately determine the electron density inside Io’s orbit, the inner Io torus. These observations have revealed a local peak in the electron density just inside M=5 and at centrifugal latitudes above about 10º that is likely the ’cold torus’ as identified in Earth-based observations of S+ emissions. This peak or ’finger’ is separated from the more dense Io torus by a local minimum or ’trough’ at M ≥ 5. The electron densities are inferred by identifying characteristic frequencies of the plasma such as the low-frequency cutoff of Z-mode radiation at fL=0 and the low-frequency cutoff of ordinary mode radiation at fpe that depend on the electron density. The ’finger’ density ranges from about 0.2 to 65 cm-3 and decreases with increasing centrifugal latitude. The ’trough’ densities range from 0.05 to ~10 cm-3. This pattern of a density ’trough’ followed by the ’finger’ closer to Jupiter is found on repeated passes through the inner Io torus over a range of centrifugal latitudes. Using a simple model for the electron densities measured above about 10º centrifugal latitude, we’ve estimated the scale height of the ’finger’ densities as about 1.17 RJ with respect to the centrifugal equator, which is somewhat surprising given the expected cold temperature of the cold torus. The larger scale height suggests a population of light ions, such as protons, are elevated off the centrifugal equator. This is confirmed by a multi-species diffusive equilibrium model.

A document by Ali H. Sulaiman. Click on the document to view its contents.

Juno’s highly eccentric polar orbit takes it to perijove distances of ∼ 1.06 RJ on each orbit. For the first perijove, this occurred just north of the jovigraphic equator, but has precessed north by about a degree per orbit over the mission. Minimum altitudes vary from ∼3200 to 8000 km through the mission. The Waves instrument observes a number of plasma wave modes in and near the non-auroral ionosphere that provide information on the local electron number density, including electron plasma oscillations that occur at the electron plasma frequency fpe and whistler-mode hiss which has an upper frequency limit of fpe in Jupiter’s strongly magnetized inner magnetosphere. The electron plasma frequency provides the electron number density. Over the ∼59 perijoves analyzed to date, peak densities range from ∼100 to 80,000 cm-3. More recent perijoves reveal topside ionospheric peaks at latitudes greater than about 40°. The density profiles can be highly variable from one perijove to the next. And, there can be deviations from simple smooth increases and decreases with altitude within individual ionospheric passes. Spatial variations may be responsible for some of the variability, perhaps related to Jupiter’s complex, higher order magnetic field. We show the variation in ionospheric density profiles and the distribution of peak densities as a function of latitude and System III longitude as well as other geometric parameters. In addition to the complex magnetic field, possible factors affecting ionospheric density variations investigated here are ionospheric dynamos analogous to those at Earth and precipitation of energetic particles.

Magnetic reconnection has been commonly reported between the solar wind IMF and the magnetic field of Earth and other planets. This article is the first report of a reconnection event in the tail region of Ganymede, the only known moon with an intrinsic magnetic field. We show strong evidence of Juno passing very close to an X-line at Ganymede's tail. We report the observation of distinctive electron Bernstein mode waves with unique characteristics particular to a reconnection site. We detect a clear reversal of a magnetic field component. Electron densities and pitch angle distributions also support the findings. Finally, from the time sequence of the observations by the different instruments on Juno, we reconstruct a likely trajectory of Juno around the reconnection site.

A document by William S Kurth. Click on the document to view its contents.

The interaction of Io with the co-rotating magnetosphere of Jupiter is known to produce Alfven wings that couple the moon to Jupiter's ionosphere. We present first results from a new numerical model to describe the propagation of these Alfven waves in this system. The model is cast in magnetic dipole coordinates and includes a dense plasma torus that is centered around the centrifugal equator. Results are presented for two density models, showing the dependence of the interaction on the magnetospheric density. Model results are presented for the case when Io is near the centrifugal and magnetic equators as well as when Io is at its northernmost magnetic latitude. The effect of the conductance of Jupiter's ionosphere is considered, showing that a long auroral footprint tail is favored by high Pedersen conductance in the ionosphere. The current patterns in these cases show a U-shaped footprint due to the generation of field-aligned current on the Jupiter-facing and Jupiter-opposed sides of Io, which may be related to the structure in the auroral footprint seen in the infrared by Juno. A model for the development of parallel electric fields is introduced, indicating that the main auroral footprints of Io can generate parallel potentials of up to 100 kV.

On June 7th, 2021 the Juno spacecraft visited Ganymede and provided the first in situ observations since Galileo’s last flyby in 2000. The measurements obtained along a one-dimensional trajectory can be brought into global context with the help of three-dimensional magnetospheric models. Here we apply the magnetohydrodynamic model of Duling et al. (2014) to conditions during the Juno flyby. In addition to the global distribution of plasma variables we provide mapping of Juno’s position along magnetic field lines, Juno’s distance from closed field lines and detailed information about the magnetic field’s topology. We find that Juno did not enter the closed field line region and that the boundary between open and closed field lines on the surface matches the poleward edges of the observed auroral ovals. To estimate the sensitivity of the model results, we carry out a parameter study with different upstream plasma conditions and other model parameters.