We have developed a new global magnetohydrodynamic (MHD) model for Jupiter's magnetosphere based on the BATSRUS code and an ionospheric electrodynamics solver. Our model includes the Io plasma torus at its appropriate location and couples the global magnetosphere with the planetary ionosphere through field‐aligned currents. Through comparisons with available particle and field observations as well as empirical models, we show that the model captures the overall configuration of the magnetosphere reasonably well. In order to understand how the magnetosphere responds to different solar wind drivers, we have carried out time‐dependent simulations using various kinds of upstream conditions, such as a forward shock and a rotation in the interplanetary magnetic field (IMF). Our model predicts that compression of the magnetosphere by a forward shock of typical strength generally weakens the corotation enforcement currents on the dayside and produces an enhancement on the nightside. However, the global response varies depending on the IMF orientation. A forward shock with a typical Parker‐spiral IMF configuration has a larger impact on the magnetospheric configuration and large‐scale current systems than with a parallel IMF configuration. Plasmoids are found to form in the simulation due to tail reconnection and have complex magnetic topology, as they evolve and propagate down tail. For a fixed mass input rate in the Io plasma torus, the frequency of plasmoid occurrence in our simulation is found to vary depending on the upstream solar wind driving.

Jupiter's inner magnetosphere plasma is primarily comprised of heavy ions in the form of O n+ or S n+ and protons (H + ) (

Magnetic reconnection in the magnetotail results in the formation of helical or loop-like magnetic structures called plasmoids, which contain strong plasma pressure gradients that maximize along the central axis and balance the magnetic forces directed inward (Hones et al., 1984; Kivelson & Khurana, 1995; Slavin et al., 1989). However, a subset of plasmoids, called "flux-ropes," lack strong pressure gradients in their interior, and the magnetic force of the outer wraps is balanced by the strong axial core field present at their center (Moldwin & Hughes, 1991; Sibeck et al., 1984). Flux ropes in which magnetic stresses are completely self-balancing are referred to as "force-free" as   J B p u 0. These force-free flux ropes correspond to the minimum energy state for a plasmoid that all such structures will evolve toward with increasing time (Priest, 2013; Taylor, 1974). Plasmoids which lack a core field and possess weak magnetic fields at their center compared to their surroundings are termed "O-lines." Decades of in situ observations in the terrestrial magnetosphere, together with kinetic simulations (Drake et al., 2006a; Drake et al., 2006b), have revealed that magnetic flux ropes in the night-side plasma sheet can range in size from order 1 to 10 Earth radii (

We expand on previous observations of magnetic reconnection in Jupiter's magnetosphere by constructing a survey of ion‐inertial scale plasmoids in the Jovian magnetotail. We developed an automated detection algorithm to identify reversals in the Bθ ${B}_{\theta }$ component and performed the minimum variance analysis for each identified plasmoid to characterize its helical structure. The magnetic field observations were complemented by data collected using the Juno Waves instrument, which is used to estimate the total electron density, and the JEDI energetic particle detectors. We identified 87 plasmoids with “peak‐to‐peak” durations between 10 and 300 s. Thirty‐one plasmoids possessed a core field and were classified as flux‐ropes. The other 56 plasmoids had minimum field strength at their centers and were termed O‐lines. Out of the 87 plasmoids, 58 had in situ signatures shorter than 60 s, despite the algorithm's upper limit being 300 s, suggesting that smaller plasmoids with shorter durations were more likely to be detected by Juno. We estimate the diameter of these plasmoids assuming a circular cross section and a travel speed equal to the Alfven speed in the surrounding lobes. Using the electron density inferred by Waves, we contend that these plasmoid diameters were within an order of the local ion‐inertial length. Our results demonstrate that magnetic reconnection in the Jovian magnetotail occurs at ion scales like in other space environments. We show that ion‐scale plasmoids would need to be released every 0.1 s or less to match the canonical 1 ton/s rate of plasma production due to Io.

<p>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.</p> <p>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<sub>J</sub>); 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.</p> <p>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<sub>J</sub> and 92 R<sub>J</sub> 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<sup>-3</sup>), we estimated the diameters of these flux ropes to be 0.14 and 0.19 R<sub>J</sub> 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<sub>J</sub>, (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.</p> <p>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.</p>