Michiko Morooka

Context . Measurements of spacecraft potential ( V _sc ) are an important tool for plasma diagnostics. High-resolution V _sc measurements can be calibrated with low-cadence observations of electron plasma frequency to derive high-resolution electron density data. Aims . We analyze V _sc measured by the Radio and Plasma Waves (RPW) instrument suite on board Solar Orbiter. We investigate the evolution of V _sc within the framework of the heliocentric distance and mission lifetime. We aim to obtain electron density estimates that are valid across a wider range of V _sc regimes, including the cases of negative V _sc . Methods . We determined V _sc by combining measurements of the probe-to-spacecraft potential, V _ps , and the probe-to-plasma potential, V _pn . Then, we used the appropriate electric current balance to find a relation between electron density and V _sc , which we calibrated to the observed electron plasma frequency. With this method, we obtained the electron density at the resolution given by the V _ps measurements. Results . We find that V _sc reaches negative values for a larger amount of time than expected for a sunlit spacecraft immersed in a tenuous plasma such as the solar wind. We are now able to identify when these V _sc changes occur, enabling us to estimate the electron density accordingly. Conclusions . Solar Orbiter charges negatively during periods of high plasma density and low photoelectron emission. However, the measured V _sc is also influenced by the electrical disconnection of the solar arrays from the spacecraft ground during fuse-blowing events. This disconnection raises the local plasma potential near the probes, causing the measured V _sc to appear negative even when Solar Orbiter remains positively charged. By distinguishing between the genuine negative spacecraft charging and the apparent negative charging induced by solar panels, we can refine previous electron density estimation methods to include cases when V _sc < 0 V. This approach provides reliable, high-resolution electron density estimates across both positive and negative V _sc regimes.

In this study, we combine Cassini fields and particle observations to investigate Titan's induced magnetosphere from the TA to T82 flybys, including flybys from the Cassini prime, equinox, and part of the solstice mission, to investigate the average location and the shape of Titan's induced magnetosphere. Although earlier studies have provided valuable information on Titan's induced magnetosphere, they were largely based on separate analyses of fields and particle data. We provide an integrated map of electron density and temperature in Titan's near plasma environment to outline the external boundary of the induced magnetosphere. We identify a dense ionospheric region and an extended plasma wake with electron densities ranging between and c. In addition, we systematize the spatial distribution of pick‐up ions at Titan with respect to the background convective electric field. We indicate that pickup ions are found in the positive hemisphere of the Kronian plasma convective electric field. The mass of the observed pickup corresponds to methane group ions, ions as well as protons and molecular hydrogen ions. The Kronian background electric field progressively accelerates these ions, and we estimate its intensity by reconstructing the radial energy gain of this population in response to the convective electric field. We find the estimated from the pickup ions electric field values within 0.05 mV and 1.92 mV range, which is consistent with an estimate of 0.61 mV deduced from computation.

Collisionless shocks convert bulk flow energy into heat, electromagnetic fields, and non‐thermal particle populations. Recent studies suggest that downstream magnetic oscillations could play an important role in ion‐scale energy dissipation at low‐Mach‐number shocks; however, the specific shock and plasma parameters involved remain poorly understood. Interplanetary (IP) shocks, often characterized by low Mach numbers, provide an excellent opportunity for investigating these kinetic dissipation mechanisms. We demonstrate, using observations of an IP shock from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) missions, supported by test‐particle simulations, that gyrating protons generate the downstream magnetic oscillations. We found bursts of ion‐acoustic waves at the troughs and crests of the magnetic oscillations, suggesting their energy source is related to proton gyration. Comparing MMS and SolO observations, we conclude that the upstream flow speed to ion thermal speed ratio and magnetic compression ratio are key parameters controlling the ion kinetic behavior that produces downstream magnetic oscillations.

We report in situ evidence for Enceladus' Alfvén wing system and its coupling with Saturn's ionosphere, based on multi‐instrument observations from the Cassini spacecraft. Analysis of 36 events, including 13 from non‐flyby paths, confirms the existence of a Main Alfvén Wing (MAW) current system generated at Enceladus, and associated Reflected Alfvén Wings (RAWs) occurring both at Saturn's ionosphere and on the density gradient of Enceladus' plasma torus, extending longitudinally to at least (2,000 moon radii) downstream of the moon. Additionally, the observations reveal the systematic existence of a filamentation process of these large‐scale Alfvénic perturbations (MAW and RAWs) during their propagation at any distance from their source. These findings demonstrate a more extensive electrodynamic coupling than previously reported for Enceladus and more generally for any moon‐magnetosphere interaction. Moreover, the observation of energetic electron depletions and water‐group ion signatures at longitudes even further from the moon supports the interpretation of an extended and persistent interaction region. These results highlight Enceladus' role in shaping Saturn's magnetospheric environment and underscore the importance of future missions to exhaustively analyze this type of complex interaction between a moon and a planet.

Not-yet-known not-yet-known not-yet-known unknown We investigate an interplanetary shock at two different heliocentric longitudes using data from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) spacecraft. We focus on the kinetic ion behavior to understand the wave environment around the shock at these distinct locations. Solar Orbiter observed the flank of the shock, while MMS crossed closer to its nose. The shock exhibited similar properties at both locations and comparable whistler precursors are observed by the two spacecraft. Despite these similarities, the ion behavior varied significantly between locations. At MMS, we observed magnetic fluctuations associated with protons gyrating as they crossed the ramp, reflected protons and alphas, and more intense ion-acoustic waves than at SolO. Notably, the ion-acoustic activity displayed a periodicity that corresponded with the low-frequency magnetic fluctuations. These findings indicate that solar wind parameters significantly influence the characteristics of interplanetary shocks.

Measurements from the Cassini Radio and Plasma Wave Science (RPWS) experiment obtained during the entire orbital phase of the Cassini mission around Saturn (13.2 years) are processed into a meridional map of plasma densities, comprising the innermost region of the ring ionosphere, the Enceladus plasma torus, and the outer magnetosphere, up to a dipole L‐shell of 30. We combine data from RPWS wave observations, such as whistler‐mode waves and upper hybrid electrostatic emissions, and from the RPWS Langmuir probe when operated in the proxy mode, providing an estimate for the spacecraft potential. In the region between dipole L‐shells of 2.4 and 30, observed electron densities are described by an analytic model that fits two functions, one for the water group ions and one for the protons, to observed densities across latitude on each magnetic field line. The derived electron density profiles are then augmented by a model for the cold core electron temperature as a function of L‐shell to obtain a meridional map of the electrostatic potential of the ambipolar electric field. The potential is extrapolated to the inner region of the rings, i.e., to below , to solve for the distribution of electron density in the ring ionosphere. A solution is based on a diffusive equilibrium model for the electrons and two ion species, and on observations from Cassini along the Saturn Orbit Insertion trajectory. A combination of analytic and diffusive equilibrium results finally yields an average global picture for the distribution of electron density in Saturn's magnetosphere.

Interplanetary (IP) shocks ripple through the heliosphere, altering the solar wind. These shocks are characterized by abrupt changes in the solar wind properties. Due to their size, different sections of an IP shock sweep through solar wind regions with different properties. Thus, the structure of an IP shock can differ across various longitudes. We investigate an IP shock at two different locations using the Solar Orbiter and Magnetospheric Multiscale (MMS) spacecraft. We found that the local shock profile is strongly influenced by the local solar wind conditions. Notably, downstream magnetic oscillations arise from gyrating protons in the region of the shock where faster incoming ions are present. Furthermore, we observe bursts of ion-acoustic waves occurring at the troughs and crests of the magnetic oscillations. This suggests a potential relationship between the ion-acoustic waves and the downstream magnetic oscillations, which are associated with the ion kinetic behavior.

The Cassini observations of Saturn’s ionosphere during the proximal orbits 288-293 in the altitude range 1450 – 4000 km (above 1-bar level) are revisited. A thorough re-analysis is made of all 159 available Langmuir probe sweeps of the Radio & Plasma Wave Science (RPWS) measurements. We relate them to the RPWS plasma wave inferred electron number densities and compare them with the available Ion Neutral Mass Spectrometer (INMS) measurements of the H+and H3+ number densities. Different analysis methods are used by RPWS to provide consistent electron number density values for the whole measured altitude interval. Consistent RPWS electron number density (ne) and INMS positively charged ion number density (ni+) profiles are derived for altitudes above ~2200 km. Below this altitude the inability of INMS to measure ions above 8 amu at the 34 km/s flyby speed lead us to infer the presence of heavy ions (> 8 amu) and a negatively charged ion component, presumably related to infalling material from the D-ring of Saturn with its associated local ion-molecule-aerosol chemistry. This lower altitude region shows a highly time variable layered structure. The Langmuir probe data in this region are strongly affected by secondaries emitted from the spacecraft and sensor surfaces when traversing a molecule-rich atmosphere at 34 km/s. There are clear signatures of secondary electron and ion emissions from the spacecraft and sensor surfaces in the data. In the Langmuir probe sweep analysis, we correct for the effect of such impact-generated products. This gives corrected total ion number densities that can be compared to the INMS ion number densities and the electron number densities. From this analysis the number of negative ions and/or nm-sized aerosol/dust particles can be constrained. A clear ionospheric peak is not identified, not even at the lowest observed altitude of approximately 1450 km. There are clear latitudinal variations and temporal evolving structures, which we infer are representative of the difference in infalling material from different regions of the D-ring. In addition, there are indications of a strong heating source for the ambient electrons that are well above expected thermal equilibrium levels (up to 4000 K). The cause of this heating is unknown but may be linked to collisional deacceleration of infalling ring material. The observational profiles presented here can be used for ionosphere theory/model comparisons in the future.

The interaction of Titan's ionosphere with Saturn's magnetosphere leads to a mix of perturbed electromagnetic fields and accelerated and thermalized plasma in the induced magnetosphere. The complexity of this region has been noted in previous studies. However, many local structures and processes have not been studied and addressed in detail before. In this case study, we examine the origin of quasi‐periodic plasma structures in Titan's induced magnetosphere observed during the T36 flyby. We use data from the electron and ion spectrometers CAPS/ELS and IMS, the RPWS Langmuir probe and electric antenna, and the fluxgate magnetometer (MAG) to analyze plasma parameters, for example, density and temperature and magnetic field fluctuations, to characterize the processes involved. The observed plasma structures are quasi‐periodic on a scale of about 20 s (or local ion gyroperiod) and possess acceleration signatures from a few eV up to 700 eV. A burst of low‐frequency (around the ion‐cyclotron and lower‐hybrid frequency) and low‐amplitude ( nT, 0.14) waves are observed in the proximity of the plasma structures. We discuss possible mechanisms leading to the development of the observed plasma structures, for example, magnetohydrodynamics instabilities and the contribution of the local electric fields.