Margaret Galland Kivelson

Context . The Jovian magnetopause is a highly dynamic boundary that experiences substantial deformation driven by interactions with the co-rotating internal plasma and the solar wind, leading to the development and propagation of wave activity along the boundary. Aims . This study aims to analyse in situ magnetic field measurements to detect wave activity along the magnetopause and identify the physical processes responsible for the observed perturbations. Methods . We applied five boundary analysis techniques to magnetic field data from seven different spacecraft to infer the local magnetopause normal. The resulting normals were then compared with a model of the magnetopause in order to detect surface wave activity and characterise its propagation direction. Results . We identified tailward-propagating waves along the Jovian magnetopause and linked them to solar wind interactions, with signatures probably associated with the Kelvin–Helmholtz instability. Conclusions . Magnetic field measurements provide robust statistical evidence of tailward-propagating perturbations on both flanks of Jupiter’s magnetopause regardless of the analysis method used.

Coronal mass ejections (CMEs) are enormous bursts of plasma and magnetic field from the Sun that propagate explosively through the Solar System. CMEs are the primary drivers of the most severe space-weather events, posing major radiation hazards for human exploration beyond Earth’s magnetosphere, including missions to the Moon and Mars. During Europa Clipper’s cruise between Earth and Mars, its Plasma Instrument for Magnetic Sounding (PIMS) recorded unusual solar-wind conditions later identified as the wake of a complex CME. Data from an unprecedented network of 17 spacecraft across the inner Solar System revealed a hidden Earth-directed component that propagated asymmetrically and could not have been predicted without off-Sun-Earth-line observations. This component also affected Europa Clipper and Mars, demonstrating how a missed forecast of this kind could endanger a crewed mission. These findings highlight the need for multipoint observations, including planetary missions in cruise, to improve space-weather forecasting, mission planning, and operations.

Context . Io, the innermost Galilean moon of Jupiter, is the main source of plasma in the Jovian magnetosphere. The neutral gas coming from the moon will get ionized through ultra-violet radiation and electron impacts. The newly created ions then get picked up by the Jovian magnetic field and start gyrating, thereby creating a ring-beam distribution in velocity space. This type of distribution is unstable with respect to the generation of ion cyclotron waves. Aims . The aim of this study is to characterize the escaping gas from Io’s atmosphere into the Jovian magnetosphere. Methods . The Galileo magnetometer data have been investigated for the five Io flybys that have magnetometer data available. The ion cyclotron waves can be measured with magnetometers and through spectral analysis the specific pick-up ions can be determined. Assuming that the energy of the ions in the ring-beam distribution is fully transferred to the cyclotron waves, the pick-up ion densities can be estimated for all these species. Results . We found evidence of sulfur-bearing ions SO ₃ ⁺ , SO ₂ ⁺ , SO ⁺ , and S ⁺ , as well as either H ₂ S ⁺ or ³⁴ S ⁺ (which have the same mass-to-charge ratio and cannot be discerned), and for non-sulfur-bearing ions: ³⁵ Cl ⁺ , ³⁷ Cl ⁺ , K ⁺ , and Si ⁺ . We also present a first plausible detection of Io-genic phosphorous through the detection of P ⁺ cyclotron waves. Conclusions . The main pick-up densities are related to SO ₂ ⁺ and SO ⁺ , varying with distance from Io between ∼10 ⁸ and ∼10 ⁶ m ⁻³ , with the other ions exhibiting a similar variation, but their pick-up densities are lower by an order of magnitude.

Ion cyclotron waves (ICWs) have shown to be a valuable tool to identify pick-up ion species around planets and moons, when plasma instruments are not sufficiently available. We investigate the high-resolution (3 Hz) Galileo magnetometer data for the presence of ICWs of various sulfur-bearing species and other elements heavier than oxygen (this because the IC frequency should be below the Nyquist frequency of 1.5 Hz). We find evidence for SOx (x = 0 – 3), Cl, K and H2S, however, the deduced pick-up densities vary strongly along the different flybys. Using the deduced pick-up densities for each flyby and a model for the neutral gas escaping Io, which gets ionized, we can obtain an estimate for the total mass loss of this volcanic moon.

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 ∼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.

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.

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...

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.

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.

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.