The presence of cryovolcanic activity in the form of geyser-like plumes at Jupiter’s moon Europa is a much-debated topic. As an active plume could allow direct sampling by a passing spacecraft of a potentially habitable interior environment, the detection and analysis of ongoing plume activity would be of the highest scientific value. In the past decade, several studies have interpreted different remote and in situ observations as providing evidence for large gaseous plumes at different locations on Europa. However, definitive proof is elusive, and visible imaging data taken during spacecraft flybys do not reveal clear indications of ongoing activity. After arrival at Jupiter in 2030, the NASA Europa Clipper spacecraft will systematically search for and constrain plume activity at Europa utilizing a variety of investigations and methods during, before, and after close flybys. Given the lack of a confirmed plume detection to date, the Europa Clipper science team has adopted a global plume search strategy, not focusing on any specific geographical area or any specific type of observation. This global search strategy assigns enhanced value to data obtained early in the mission, which allows time for further observations and characterization of any observed plume at later times. Here we describe the current state of knowledge on plume activity, the Europa Clipper search strategy, and the role of various instruments on the Europa Clipper payload in this search.

Near Ganymede, the magnetic field is a superposition of Jupiter's magnetospheric magnetic field, the field arising from sources within the moon, the field generated by plasma currents driven by the interaction of flowing magnetospheric plasma with the conducting moon, and the field arising from ionospheric currents. Previous fits to Ganymede's internal field have not identified the contributions of plasma and ionospheric currents, although their contributions can obscure the signature of sources internal to the moon. Fortunately, using magnetohydrodynamic simulations whose output agrees well with the measurements acquired on close passes by Galileo and Juno, we can estimate the moon-scale contributions of plasma sources. By subtracting the magnetic signatures of plasma and ionospheric currents from the measured field, we approximate measurements made in a current-free region. We fit the corrected data from different sets of flybys either as a sum of low order spherical harmonics or as a permanent dipole moment plus an induced dipole with approximately the same root-mean-square errors. For the induced dipole model, data from multiple flybys occurring at different phases of Jupiter's rotation are used to represent the time-variation of the external field at Ganymede. Compared with earlier estimates, the magnitude of the permanent dipole moment did not significantly change in either analysis. However, for the permanent plus induced dipole model, the induction efficiency decreases from 0.84 to similar to 0.72. The reduced efficiency places new constraints on the thickness of the ice shell above the ocean and the ocean's depth and conductivity.

The interiors of the icy moons of Jupiter hold a key to understanding habitability in the Solar System and beyond. They could serve as prototypes to comprehend similar bodies that might have the potential to sustain life. The magnetic field observations from the Galileo mission between 1996 and 2003 suggest large oceans below the icy crusts of Europa and Callisto and a probable subsurface ocean at Ganymede. It also discovered that Ganymede has an intrinsic magnetic field and a dynamic magnetosphere. NASA’s Europa Clipper and ESA’s Jupiter ICy moons Explorer (JUICE) missions have been designed to better understand and characterise these icy moons. They aim to confirm the existence of a subsurface ocean at the three moons, in particular, Ganymede and Europa, and constrain their internal structure. The missions would also explore the magnetosphere of Jupiter and its interactions within the Jovian system. Ganymede is the largest moon of our Solar System, capable of producing its own dynamo field and possibly possessing an ocean beneath its surface. However, separating the intrinsic field from the induced field is a difficult problem. Galileo measurements provided two models for Ganymede’s overall internal field- a dipole and quadrupole model or a dipole and induction model. Both the quadrupole and induction signals are quite small and well represent the observations together with the dipole field. Latest trajectory information for Europa Clipper assuming an October 2024 launch, and the predicted JUICE trajectory following the successful launch in April 2023 show initial close Ganymede flybys. In this study, we use them to understand their trajectories and highlight their importance in confirming the induced signal and thereby the ocean as well as for modelling the dynamo field. The first 2 Clipper and the first 3 JUICE flybys occur within an altitude of 500 km from Ganymede’s surface and are hence useful for overall internal field modelling. We predict the measurements that would be observed from the two internal sources as well as the external magnetospheric source to better understand the signals and decipher their differences. For the intrinsic and external fields, we use dynamo and magnetohydrodynamic models respectively while for the induced field, we use Jupiter’s background field along with the induction equation at the spacecraft locations. The 5 flybys independently as well as together with the data from the Galileo flybys would enhance our understanding of the different magnetic sources at Ganymede and the fields they produce. These joint early flyby observations will enable us to be better equipped to model the magnetic field components near Ganymede in the orbital phase of the JUICE mission.

With its extended mission, JUNO is getting close to Io and performs moon flybys. It is therefore time to (re)analyse the already available magnetometer data from close flybys of Io by Galileo. Earlier studies of the J0 flyby showed the presence of ion cyclotron waves generated by the pick-up of SO2+ and determined the density of the picked-up ions. In this presentation we study all five Io flybys by Galileo and investigate the presence of ion cyclotron waves for three different species. SO2+, SO+ and S+. Through Fourier analysis and calculation of the cross-spectral matrix and strong criteria on power, polarization and ellipticity, we determine intervals of significant wave activity. Under the assumption of bi-spherical scattering of the pick-up ions in the velocity ring-distribution, an estimation of the pick-up ion density can be obtained. Through an assumption of the ionization frequency, this can be converted into a neutral density to obtain a value of the total neutral gas emitted per second and compare it to the usually assumed 1000 kg/s. Naturally, the five flybys will also give information about differences generated by local time, longitude and latitude.

The goal of NASA’s Europa Clipper mission is to assess the habitability of Jupiter’s moon Europa. After entering Jupiter orbit in 2030, the flight system will collect science data while flying past Europa 49 times at typical closest approach distances of 25–100 km. The mission’s objectives are to investigate Europa’s interior (ice shell and ocean), composition, and geology; the mission will also search for and characterize any current activity including possible plumes. The science objectives will be accomplished with a payload consisting of remote sensing and in-situ instruments. Remote sensing investigations cover the ultraviolet, visible, near infrared, and thermal infrared wavelength ranges of the electromagnetic spectrum, as well as an ice-penetrating radar. In-situ investigations measure the magnetic field, dust grains, neutral gas, and plasma surrounding Europa. Gravity science will be achieved using the telecommunication system, and a radiation monitoring engineering subsystem will provide complementary science data. The flight system is designed to enable all science instruments to operate and gather data simultaneously. Mission planning and operations are guided by scientific requirements and observation strategies, while appropriate updates to the plan will be made tactically as the instruments and Europa are characterized and discoveries emerge. Following collection and validation, all science data will be archived in NASA’s Planetary Data System. Communication, data sharing, and publication policies promote visibility, collaboration, and mutual interdependence across the full Europa Clipper science team, to best achieve the interdisciplinary science necessary to understand Europa.

Characterizing Europa’s subsurface ocean is essential for assessing Europa’s habitability. The suite of instruments on the Europa Clipper spacecraft will, among others, magnetically sound Europa’s interior by measuring the ocean’s induced magnetic field. This magnetic field is generated in response to the Jovian time-varying magnetic environment in which Europa is immersed. However, the dynamic magnetized plasma flow of the Jovian magnetosphere creates electrical currents that give rise to magnetic perturbations near Europa. These perturbations complicate the interpretation of the induction signal, and hence the characterization and inferences on potential habitability. Thus, characterization of the ocean by magnetic sounding requires an accurate characterization of the plasma as it flows across Europa. We present the Plasma Instrument for Magnetic Sounding (PIMS), the instrument for the Europa Clipper mission that will measure the plasma contribution to the magnetic field perturbations sensed by the Europa Clipper Magnetometer. PIMS is composed of four Faraday Cup plasma spectrometers that use voltage-biased gridded apertures to dissect the space plasmas that they encounter. The instrument uses sensitive preamplifiers and processing electronics to measure the current that results when charged particles strike the instrument’s metal collector plates, thus enabling a measure of the plasma characteristics near Europa to produce a more accurate magnetic sounding of Europa’s subsurface ocean. PIMS consists of two sensors: one placed near the top of the Europa Clipper spacecraft and one near the bottom. Each sensor contains two Faraday Cups with a 90° full-width field-of-view. The sensors were specifically designed to withstand the Europa environment, measure both ions and electrons, and have two separate voltage ranges intended to analyze the magnetospheric and ionospheric environments, respectively. In this paper, we describe the scientific motivation for this experiment, the design considerations for the PIMS instrument, the details of the ground calibration, and other details pertinent to understanding the scientific data retrieved by PIMS.

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.

The Galileo mission to Jupiter revealed that Europa is an ocean world. The Galileo magnetometer experiment in particular provided strong evidence for a salty subsurface ocean beneath the ice shell, likely in contact with the rocky core. Within the ice shell and ocean, a number of tectonic and geodynamic processes may operate today or have operated at some point in the past, including solid ice convection, diapirism, subsumption, and interstitial lake formation. The science objectives of the Europa Clipper mission include the characterization of Europa’s interior; confirmation of the presence of a subsurface ocean; identification of constraints on the depth to this ocean, and on its salinity and thickness; and determination of processes of material exchange between the surface, ice shell, and ocean. Three broad categories of investigation are planned to interrogate different aspects of the subsurface structure and properties of the ice shell and ocean: magnetic induction, subsurface radar sounding, and tidal deformation. These investigations are supplemented by several auxiliary measurements. Alone, each of these investigations will reveal unique information. Together, the synergy between these investigations will expose the secrets of the Europan interior in unprecedented detail, an essential step in evaluating the habitability of this ocean world.

The last 22.5 orbits of the Cassini mission brought the spacecraft to less than 3000 km from Saturn's 1-bar surface. These close encounters offered an unprecedented view of Saturn's magnetic field, including contributions from the internal dynamo, the ionosphere, and the magnetosphere. In this chapter, we highlight the new picture of Saturn's magnetic field from the Cassini mission including the persistent yet time-varying low-latitude field-aligned currents, Alfv\'en waves planet-ward of the D-ring, extreme axisymmetry, and high-degree magnetic moments. We then discuss the implications and new questions raised for Saturn's innermost magnetosphere, equatorial ionosphere, and interior. We conclude this chapter with an outlook for the future exploration of Saturn and other giant planets.

1. Introduction With a launch readiness date of late 2024, NASA’s Europa Clipper will set out on a journey to explore the habitability of Jupiter’s moon Europa. Among others, observations by Europa Clipper’s in-situ instruments will provide an understanding of the satellite’s interior structure and compositional makeup. The variability of Jupiter’s magnetic field at Europa induces electric currents within the moon’s conducting ocean layer, the magnitude of which depends on the ocean’s location, extent, and conductivity. Europa is also embedded in a flow of corotating plasma, which continuously impacts and sputters the surface to produce the moon’s atmosphere. In addition, micrometeorite impacts eject particles of the surface to wrap Europa in a cloud of dust. The neutral atmosphere is readily ionized by energetic particles to produce an ionosphere, which gives rise to current systems electromagnetically connecting Europa to Jupiter. The in-situ instruments comprise the Europa Clipper Magnetometer (ECM), the Plasma Instrument for Magnetic Sounding (PIMS), the SUrface Dust Analyzer (SUDA), and the MAss Spectrometer for Planetary Exploration (MASPEX). We present the highlights of these instruments’ development on their path to delivery to assembly, test, and launch operations (ATLO), which have begun in March 2022. 2. Relevant Investigations 2.1. Europa Clipper Magnetometer The Europa Clipper Magnetometer (ECM) will measure magnetic fields generated by currents induced in Europa’s subsurface ocean and the electromagnetic coupling of the moon to Jupiter. Jupiter’s tilted dipole magnetic field and Europa’s eccentric orbit expose the moon to time-varying magnetic fields inducing eddy currents in the ocean. By measuring the induced field response at multiple frequencies, the ice shell thickness and the ocean layer thickness and conductivity can be uniquely determined. The ECM consists of three fluxgate sensors mounted on an 8.5-m-long boom and a control electronics hosted in a vault shielding it from radiation damage. The use of three sensors allows for dynamic removal of higher-order spacecraft-generated magnetic fields. The flight-model (FM) sensors (Figure 1) have been delivered to Instrument Integration and Testing, and the instrument is awaiting calibration in the fall of 2022. Figure 1. ECM FM fluxgate sensor. 2.2. Plasma Instrument for Magnetic Sounding The Plasma Instrument for Magnetic Sounding (PIMS) will measure ions and electrons in Europa’s ionosphere to quantify magnetic fields associated with electric currents driven by dynamic pressure gradients, which are produced by the interaction of corotating Jovian plasma with Europa and its neutral atmosphere. PIMS will further measure the particle population precipitating onto Europa to model surface sputtering rates and constrain the effects of space weathering. In addition, PIMS will characterize the distribution of electrons carrying field-aligned currents coupling Europa to Jupiter’s ionosphere. The PIMS instrument consists of two sensors each hosting two Faraday cups with a 90° field of view. The instrument measures electrons and ions with energies of up to 2 keV and 7 keV/q, respectively, with an energy resolution of <15% and a temporal resolution of 1−4 s. The instrument, including sensor assemblies (Figure 2), is scheduled for delivery to ATLO in June 2022. Figure 2. PIMS FM Fully Integrated Sensor Assemblies 2.3. Mass Spectrometer for Planetary Exploration The MAss Spectrometer for Planetary Exploration (MASPEX) measures trace neutral species to determine the composition of Europa’s atmosphere. The atmosphere is produced by particles sputtering off the surface, is augmented by possible contributions from plumes, and is altered by radiolysis. MASPEX will determine the distribution of major volatiles and key organic compounds with respect to latitude and longitude, altitude, solar local time, and will distinguish endogenic and exogenic sources. MASPEX is a multi-bounce time-of-flight mass spectrometer for species with atomic mass ranging from 2 to 500 u. The instrument’s mass resolution m/Dm of 4275 at mass 50 enables measurement of individual fragment ions generated by electron impact ionization of parent molecule. The FM instrument is presently undergoing testing (Figure 3) and is scheduled for delivery to ATLO in August 2022. Figure 3. Testing of FM MASPEX instrument. 2.4. Surface Dust Analyzer The SUrface Dust Analyzer (SUDA) will map the chemical composition of particles ejected from Europa’s surface by hypervelocity impacts of micrometeoroids. The dust particles have a typical size of 0.5−1 μm and exhibit a near isotropic distribution and only weak temporal variations. Small fluctuations are expected from variability of exogenous sources, resulting, e.g., from the orbital motion of sources on Io and the stochastic nature of volcanic activity. SUDA is a time-of-flight mass spectrometer for species with atomic mass ranging of up to 200 u and is capable of detecting up to 40 ejecta per second. Owing to the deterministic nature of the ballistic trajectories of the ejected particles, the composition measurements at spacecraft altitude can be correlated with geologic features on the surface. It has been demonstrated that trace amounts of complex organic species embedded in ice grains with abundances <0.1 ppm can be detected. In addition to micrometeorite ejecta, SUDA will measure the makeup of potential plumes by directly sampling microscopic particles. The FM sensor assembly (Figure 4) will be delivered to ATLO in August 2022 as part of the instrument. Figure 4. SUDA FM sensor assembly. 3. Mission Concept The current mission design consists of 49 flybys of Europa executed over a ~3.5 year period while the spacecraft is in orbit about Jupiter. The altitudes of closest approach typically range from 25 km to 100 km. The tour is divided into two principal campaigns, visiting first the anti-Jovian hemisphere followed by observations of the sub-Jovian hemisphere. The flybys occur over a wide range of latitudes and longitudes (Figure 5), and they are widely distributed in true anomaly and solar local time. The tour is thus well suited for characterization of the ice shell and ocean and of the atmosphere and ionosphere created through Europa’s interaction with Jupiter’s magnetosphere. Figure 5. Planned Europa Clipper close approach ground tracks below 1000 km altitude (black), 400 km (green), 100 km yellow, and 50 km (red).

The Galileo spacecraft passed close to Europa on 11 encounters, two of which (E12 and E26) came within 400 km of the surface. In E12 data, there are perturbations in field and plasma data consistent with effects of a nearby plume (Jia et al., 2018). Huybrighs et al. (2020, https://doi.org/10.1029/2020GL087806 ) report depletions of proton flux in one channel of the Galileo Energetic Particle Detector (EPD) as Galileo passed close to Europa on E26. They trace particle trajectories in the magnetic field provided by a magnetohydrodynamic simulation and conclude that the spacecraft probably also passed through or close to a vapor plume on E26. However, the absence of a related signature in the measured magnetic field led us to question this conclusion. Examination of the EPD data remote from Europa on the E26 flyby reveals that the putative plume signature in the EPD data is an artifact.

Jupiter's moon, Ganymede, is the only known satellite with a strong intrinsic magnetic field that forms its own magnetosphere within a planetary magnetosphere. Ganymede's magnetosphere exhibits a variety of phenomena similar to those found in planetary magnetospheres, including well-defined magnetic perturbations caused by an intrinsic field, a rich set of plasma wave modes, a significant population of charged particles confined within the moon's magnetosphere, and auroral emissions. On the other hand, Ganymede's magnetosphere differs from those of the planets in many ways. The global form of Ganymede's magnetosphere is primarily controlled by the external magnetic field, resulting in a cylindrical shape with a pair of Alfven wings that mediate the moon's interaction with Jupiter's magnetosphere. Because of the sub-Alfvenic flow of the ambient plasma, the absence of rotation of the central body, and the large magnetic shear across the magnetopause, reconnection is the dominant process that governs the global convection and drives magnetospheric dynamics. The energy sources and patterns of particle transport in Ganymede's mini-magnetosphere provide valuable insight into fundamental physics of planetary magnetospheres. This chapter reviews our current state of knowledge about Ganymede's magnetosphere based on analyses of in situ and remote observations combined with insights from numerical modeling.

We model the omnipresent quasiperiodic 60 min waves in Saturn's outer magnetosphere as field line resonances adopting a realistic magnetic field model and a measurement‐based plasma density distribution using Alfvén wave resonance theory for arbitrary field geometries (Singer et al., 1981). The modeled eigenfrequencies for the second and higher modes are roughly independent of invariant latitude, mapping into large regions of the magnetosphere up to at least 20 R S , and the third and fourth harmonic modes having close to 1 h eigenperiod. The model predicts the normalized amplitudes of these higher modes of the magnetic field perturbations at high latitudes to exceed the amplitudes at the plasma sheet, in agreement with the observations of more frequent occurrences at mid‐to‐high latitudes. The periods of the higher order field line resonances in the outer magnetosphere differ considerably from those in a dipole model, illustrating the importance of a realistic field model.

Regions 1 and 2 (R1 and R2) field‐aligned currents (FACs), manifestations of large‐scale convection in Earth's magnetosphere‐ionosphere (M‐I) system, often contain intense FAC layers of mesoscale latitudinal width near the R1/R2 interface. We refer to such layers as “embedded” R1 and R2 FACs. Likely resulting from enhanced magnetosphere‐ionosphere (M‐I) convection, these FACs may indicate M‐I configuration change and contribute significantly to substorm current wedges. We present several events in which embedded FACs were observed by low‐altitude spacecraft in the ionosphere. All the events occurred during active geomagnetic conditions or a substorm growth phase, and most map to an equatorial location on the nightside. When an embedded FAC is upward, it coincides with inverted‐V electron precipitation and a discrete auroral arc. If an upward embedded FAC is in the postmidnight‐to‐dawn sector, a dawnside auroral polarization stream appears immediately poleward of it, so it may be important for ionospheric heating, M‐I convection, and instabilities. Our results establish embedded FACs as a frequently appearing, fundamental phenomenon for understanding and modeling the magnetosphere‐ionosphere system.

Fifty years of collaboration between the authors are reviewed. Common themes cover magnetospheric magnetohydrodynamic phenomena: MHD waves, wave-particle interactions, circulation, global modes and field line resonances in the terrestrial context, and magnetosphere-moon interactions, transport processes, instabilities, and global structure in the magnetospheres of giant planets. Over the period reviewed, instrumentation has improved, particularly in particle detectors, and interpretations that seemed radical when first suggested are now supported by measurements and seem commonplace. Plain Language Summary The authors have worked together for almost 50 years. They met when the existence of the magnetosphere, the volume surrounding the Earth in space that is structured by its planetary magnetic field, had been established for only a decade. They have succeeded since then in showing that, in a field where new discoveries were regularly happening, there is a very useful role for people who, paying attention to the strengths and weaknesses of instrumentation, can move back and forth between theory, simple modeling, data analysis, and back to theory. Magnetohydrodynamics, a theoretical tool that simplifies the description of space plasmas by focusing on averaged responses, has been their key discipline, but some of their key contributions have elucidated the critical role of the energetic particles that depart from average behavior. Through a mix of theory, interpretation of observations, and even leading spacecraft instrument teams, the authors have contributed to understanding most aspects of the large-scale behavior of the magnetospheric systems at Earth, Jupiter, and Saturn.