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The icy surface of Jupiter’s moon, Europa, is thought to lie on top of a global ocean 1 – 4 . Signatures in some Hubble Space Telescope images have been associated with putative water plumes rising above Europa’s surface 5 , 6 , providing support for the ocean theory. However, all telescopic detections reported were made at the limit of sensitivity of the data 5 – 7 , thereby calling for a search for plume signatures in in-situ measurements. Here, we report in-situ evidence of a plume on Europa from the magnetic field and plasma wave observations acquired on Galileo’s closest encounter with the moon. During this flyby, which dropped below 400 km altitude, the magnetometer 8 recorded an approximately 1,000-kilometre-scale field rotation and a decrease of over 200 nT in field magnitude, and the Plasma Wave Spectrometer 9 registered intense localized wave emissions indicative of a brief but substantial increase in plasma density. We show that the location, duration and variations of the magnetic field and plasma wave measurements are consistent with the interaction of Jupiter’s corotating plasma with Europa if a plume with characteristics inferred from Hubble images were erupting from the region of Europa’s thermal anomalies. These results provide strong independent evidence of the presence of plumes at Europa. The hypothesis of an ocean under the icy surface of Jupiter’s moon Europa is strengthened by in-situ evidence of a plume, inferred by Galileo’s magnetic field and plasma density measurements obtained during the spacecraft’s closest flyby to the moon.

The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere. Science , this issue p. eaat5434 , p. eaat1962 , p. eaat2027 , p. eaat3185 , p. eaat2236 , p. eaat2382 During 2017, the Cassini fluxgate magnetometer made in situ measurements of Saturn’s magnetic field at distances ~2550 ± 1290 kilometers above the 1-bar surface during 22 highly inclined Grand Finale orbits. These observations refine the extreme axisymmetry of Saturn’s internal magnetic field and show displacement of the magnetic equator northward from the planet’s physical equator. Persistent small-scale magnetic structures, corresponding to high-degree (>3) axisymmetric magnetic moments, were observed. This suggests secondary shallow dynamo action in the semiconducting region of Saturn’s interior. Some high-degree magnetic moments could arise from strong high-latitude concentrations of magnetic flux within the planet’s deep dynamo. A strong field-aligned current (FAC) system is located between Saturn and the inner edge of its D-ring, with strength comparable to the high-latitude auroral FACs.

The magnetospheric mapping of Jupiter's polar auroral emissions is highly uncertain because global Jovian field models are known to be inaccurate beyond ∼30 RJ. Furthermore, the boundary between open and closed flux in the ionosphere is not well defined because, unlike the Earth, the main auroral oval emissions at Jupiter are likely associated with the breakdown of plasma corotation and not the open/closed flux boundary in the polar cap. We have mapped contours of constant radial distance from the magnetic equator to the ionosphere in order to understand how auroral features relate to magnetospheric sources. Instead of following model field lines, we map equatorial regions to the ionosphere by requiring that the magnetic flux in some specified region at the equator equals the magnetic flux in the area to which it maps in the ionosphere. Equating the fluxes in this way allows us to link a given position in the magnetosphere to a position in the ionosphere. We find that the polar auroral active region maps to field lines beyond the dayside magnetopause that can be interpreted as Jupiter's polar cusp; the swirl region maps to lobe field lines on the night side and can be interpreted as Jupiter's polar cap; the dark region spans both open and closed field lines and must be explained by multiple processes. Additionally, we conclude that the flux through most of the area inside the main oval matches the magnetic flux contained in the magnetotail lobes and is probably open to the solar wind.

Magnetic field measurements made near Jupiter’s moon Io strengthen the evidence for a magma ocean in its interior. Extensive volcanism and high-temperature lavas hint at a global magma reservoir in Io, but no direct evidence has been available. We exploited Jupiter’s rotating magnetic field as a sounding signal and show that the magnetometer data collected by the Galileo spacecraft near Io provide evidence of electromagnetic induction from a global conducting layer. We demonstrate that a completely solid mantle provides insufficient response to explain the magnetometer observations, but a global subsurface magma layer with a thickness of over 50 kilometers and a rock melt fraction of 20% or more is fully consistent with the observations. We also place a stronger upper limit of about 110 nanoteslas (surface equatorial field) on the dynamo dipolar field generated inside Io.

Global-scale properties of Europa’s putative ocean, including its depth, thickness, and conductivity, can be established from measurements of the magnetic field on multiple close flybys of the moon at different phases of the synodic and orbital periods such as those planned for the Europa Clipper mission. The Europa Clipper Magnetometer (ECM) has been designed and constructed to provide the required high precision, temporally stable measurements over the range of temperatures and other environmental conditions that will be encountered in the solar wind and at Jupiter. Three low-noise, tri-axial fluxgate sensors provided by the University of California, Los Angeles are controlled by an electronics unit developed at NASA’s Jet Propulsion Laboratory. Each fluxgate sensor measures the vector magnetic field over a wide dynamic range (±4000 nT per axis) with a resolution of 8 pT. A rigorous magnetic cleanliness program has been adopted for the spacecraft and its payload. The sensors are mounted far out on an 8.5 m boom to form a configuration that makes it possible to measure the remaining spacecraft field and remove its contribution to data from the outboard sensor. This paper provides details of the magnetometer design, implementation and testing, the ground calibrations and planned calibrations in cruise and in orbit at Jupiter, and the methods to be used to extract Europa’s inductive response from the data. Data will be collected at nominal rates of 1 or 16 samples/s and will be processed at UCLA and delivered to the Planetary Data System in a timely manner.

In Jupiter's magnetosphere, events such as flow bursts and changes to the magnetic field are thought to be driven predominantly by internal processes. Analysis of energetic particle data has established that flow bursts are associated with magnetic reconfiguration in the Jovian magnetotail. Here we use magnetometer data throughout the Jovian magnetotail to identify events that we relate to reconnection and flow. Using quantitative criteria, we have identified 249 reconnection events that are characterized by reversals or significant increases in Bθ, the north‐south component of the magnetic field, over background levels. We discuss the distribution of the events, their occurrence rate, and location inside or outside of a putative neutral line, as functions of radial distance and local time. Using the sign of Bθ as a proxy for the flow direction, we establish the location of a statistical separatrix and find that its radial distance varies with local time. Where particle signatures of events in our data set have also been analyzed, they generally show increases of anisotropy. However, we have identified scores of additional events that have not been previously identified in the particle data; many of these new events occur in the premidnight local time sector. Finally, we examine our events for the 2–3 day periodicity that has been reported for flow bursts and auroral polar dawn spots and find that this periodicity is present only intermittently and is not statistically significant.

We describe a three‐dimensional single‐fluid MHD simulation of Ganymede's magnetosphere that accords extremely well with the Galileo particles and fields measurements. Major improvements to our previously published model involve the modification of the inner boundary condition and the implementation of an anomalous resistivity model. The improved model couples the moon's ionosphere (with finite Pedersen conductance) with the magnetosphere self‐consistently. The previous model applied only in the limit of unreasonably high ionospheric conductivity. We illustrate in detail the global convection pattern inferred from the new model and demonstrate some features of the convection that differ from that of the Earth's magnetosphere because Ganymede lacks a corotation electric field. Our new model does a better job of reproducing magnetic field and plasma observations from multiple Galileo passes, which sampled different external conditions and different regions of the magnetosphere. In particular, for a critical upstream pass (G8) during which the Galileo spacecraft entered onto closed field lines, the simulated magnetosphere provides an excellent fit to the measurements without the need for tuning the spacecraft trajectory. In comparison with the plasma measurements of the G2 flyby, our model also yields good agreement with the Galileo PLS observations and supports the conclusion reached by Vasyliūnas and Eviatar (2000) that the observed ionospheric outflow consists of oxygen ions. For constant external conditions, dynamic variations associated with magnetic reconnection on timescales of the order of tens of seconds are found over a large region near the magnetopause in the simulations. Future applications of our model, such as test particle tracing and investigating the behavior of the cross polar cap potential under different external and ionospheric conditions, will provide a more comprehensive understanding of Ganymede's magnetospheric environment.

On 3 January 2000, the Galileo spacecraft passed close to Europa when it was located far south of Jupiter's magnetic equator in a region where the radial component of the magnetospheric magnetic field points inward toward Jupiter. This pass with a previously unexamined orientation of the external forcing field distinguished between an induced and a permanent magnetic dipole moment model of Europa's internal field. The Galileo magnetometer measured changes in the magnetic field predicted if a current-carrying outer shell, such as a planet-scale liquid ocean, is present beneath the icy surface. The evidence that Europa's field varies temporally strengthens the argument that a liquid ocean exists beneath the present-day surface.

Magnetic reconnection at the terrestrial magnetopause is frequently intermittent, leading to the formation of localized reconnected flux bundles referred to as flux transfer events (FTEs). It remains unclear whether the intermittency of the process is intrinsic or arises because of fluctuations of solar wind properties. Here we use Ganymede's magnetosphere, which is embedded in a background of field and plasma whose properties vary imperceptibly over time scales pertinent to plasma flow across the moon's magnetosphere, to demonstrate that reconnection is intrinsically intermittent. We run time‐dependent global magnetohydrodynamic (MHD) simulations that describe Ganymede's magnetospheric environment realistically and reproduce plasma and field measurements made on multiple Galileo passes by the moon with considerable fidelity. The simulations reveal that even under steady external conditions, dynamic variations associated with magnetic reconnection on time scales of the order of tens of seconds occur over a large region near the upstream magnetopause. The MHD simulations give direct evidence of magnetic reconnection at the magnetopause and reproduce the amplitude and spatial distribution of observed fluctuations of the magnetic field near boundary crossings. The consistency of data and simulations leads us to conclude that even under steady upstream conditions, upstream reconnection is intermittent. The bursty magnetopause structures at Ganymede and the FTEs identified at planetary magnetospheres (Mercury, Earth, and Jupiter) extend the parameter regime for analysis of intermittent magnetopause reconnection. We find that FTE recurrence times decrease with the scale length of the system.

This paper reviews the present state of knowledge about the magnetic fields and the plasma interactions associated with the major satellites of Jupiter and Saturn. As revealed by the data from a number of spacecraft in the two planetary systems, the magnetic properties of the Jovian and Saturnian satellites are extremely diverse. As the only case of a strongly magnetized moon, Ganymede possesses an intrinsic magnetic field that forms a mini-magnetosphere surrounding the moon. Moons that contain interior regions of high electrical conductivity, such as Europa and Callisto, generate induced magnetic fields through electromagnetic induction in response to time-varying external fields. Moons that are non-magnetized also can generate magnetic field perturbations through plasma interactions if they possess substantial neutral sources. Unmagnetized moons that lack significant sources of neutrals act as absorbing obstacles to the ambient plasma flow and appear to generate field perturbations mainly in their wake regions. Because the magnetic field in the vicinity of the moons contains contributions from the inevitable electromagnetic interactions between these satellites and the ubiquitous plasma that flows onto them, our knowledge of the magnetic fields intrinsic to these satellites relies heavily on our understanding of the plasma interactions with them.