Juno Observations of Ion-Inertial Scale Flux Ropes in the Jovian Magnetotail

Magnetic flux ropes – helical magnetic structures which are produced due to simultaneous reconnection at multiple X-lines, have been observed at the magnetospheres of most magnetized planets. The size of these flux ropes, also called “plasmoids” if they contain significant plasma pressure, can vary from being a significant fraction of the system size (e.g. tens of Earth radii at the terrestrial magnetotail) to small flux ropes with diameters less than the local ion inertial length. The smallest flux ropes are expected because reconnection in the Earth’s cross-tail current sheet only occurs when it thins to or below the ion-inertial scale and tearing instabilities produce periodic X-lines with spacing of ~2 times the thickness of the current sheet. While much is still to be understood, it is hypothesized on the basis of Particle-in-Cell simulations that the smaller flux ropes soon come together and “coalesce”, via reconnection, into larger flux ropes. The coalescence process continues until the observed distribution of plasmoid diameters is produced.

For the giant magnetospheres like Jupiter, which encompass multiple moons that lose mass to the rapidly rotating inner plasma disk, the momentum in the outer layers of the disk is believed to continuously shed mass by the release of plasmoids down the tail plasma sheet. This periodic ejection of plasmoids to balance the mass being added to the magnetosphere by Jupiter’s moons is termed the Vasyliunas-cycle. Rather than being formed by multiple x-line reconnection in a highly thinned current sheet, these Vasyliunas-cycle plasmoids are thought to form when a single X-line disconnects a highly stretched closed flux tube and allows its momentum to carry it down the tail. Due to the limited single-spacecraft measurements obtained by Galileo in the dusk-side magnetosphere, relatively little is known about these Vasyliunas-type plasmoids. Signatures of most Jovian plasmoids and flux ropes lasted ~6.8 minutes on average (Vogt et al., 2014), corresponding to diameters larger than 1 Jovian radii (R_J); much larger than the ion inertial length expected in the outer magnetosphere. Potential flux ropes on the ion-inertial scale, which would typically last for less than a minute could not have been identified using the Galileo magnetometer owing to the low cadence of several seconds per vector measurement.

As part of its 53-day orbits, Juno spent a considerable amount of time in the dawn-side magnetotail. Using the high-resolution data from the Juno magnetometer, we identified two potential ion-scale flux ropes in the Jovian magnetotail by searching for bipolar variations in the magnetic field component normal to the current sheet. The two events were 22 s and 62 s in duration and were located at radial distances of roughly 74 R_J and 92 R_J between 03 and 04 local time. Assuming that the travel speed of the flux rope is limited by the Alfven speed in the surrounding magnetotail lobes, which is calculated using the plasma density inferred by the cutoff for the continuum radiation detected by the Waves instrument (0.003 to 0.012 cm^-3), we estimated the diameters of these flux ropes to be 0.14 and 0.19 R_J respectively. The flux ropes’ diameters were comparable to the ion inertial length during these intervals, which was roughly between 0.11 to 0.23 R_J, (assuming a mass of 16.6 amu for the average ion). The selected events were analyzed using the minimum variance analysis and both events were seen to possess a strong core field with relatively high eigenvalue ratios, indicating that the MVA coordinate system was well-defined. Using a force-free model which is fitted to the observations, it was found that the flux ropes are quasi-force-free.

These are the first reported observations of ion-scale flux ropes in the Jovian magnetotail. Although the large-scale dynamics of the magnetosphere may be dominated by the Vasyliunas cycle, the observations show that small-scale flux ropes, which are likely generated due to the tearing instability in a thin current sheet, also exist in the Jovian magnetotail, similar to the magnetotails of Earth and Mercury.