Joan Jordi Boldu-O´fArrill Treviño

Context . On February 18, 2025, Solar Orbiter (SO) completed its fourth gravity assist maneuver of Venus (VGAM4) and reached an unprecedented proximity coming within 378 km of the planet. This flyby was necessary to steer the spacecraft into an orbit outside the plane of the ecliptic. Near the closest approach, only the Radio and Plasma Wave (RPW) and Magnetometer (MAG) instruments were operational; this enabled high-cadence measurements to be taken to investigate the plasma properties of the Venusian ionosphere. Aims . The main goal of this study is to derive the electron density and temperature in the ionosphere of Venus using electric potential measurements from RPW, and to characterize them. Methods . During approximately five minutes around the closest approach, the High Frequency Receiver of RPW detected radio emissions of a type naturally generated by planetary ionospheres whose frequency can be related to the electron density. Using quasithermal noise spectroscopy, we inferred the electron temperature at discrete altitudes and solar zenith angles. Results . Solar Orbiter measured an average density and electron temperature in the ionosphere of Venus of 12 385 ± 148 cm ⁻³ and 0.43 ± 0.05 eV, respectively. These values are in agreement with in-situ measurements by Pioneer Venus Orbiter (PVO) obtained at the solar maximum. Binned magnetic fields and temperatures are anticorrelated, which suggests that the magnetic flux ropes, observed in the Venus ionosphere, are more likely non-force-free structures. Conclusions . The findings presented in this paper, together with the measurement from the Parker Solar Probe (PSP) during the third gravity assist, support the conclusion that the plasma density in the Venusian ionosphere above 350 km varies with solar activity, whereas the electron temperature shows a much weaker dependence. Notably, the electron temperature remains consistent across the three missions (SO, PSP, and PVO), despite varying levels of solar activity. This suggests that, over the altitude and solar zenith regions probed, the thermal structure of the Venusian ionosphere is not driven by solar extreme ultraviolet (EUV) heating alone, but is also shaped by external heat sources near the ionopause. Processes such as the damping of whistler mode waves, solar wind ion heating, and thermal conduction from the hot ionosheath appear to play a major role.

Aims. The objective of this study is to characterise and understand the magnetic structure of an extremely high Mach number interplanetary shock. Methods. We analysed magnetic field and plasma data from Solar Orbiter during the shock crossing and compared them with high-resolution MMS observations and results from a hybrid particle-in-cell (PIC) shock simulation. Shock profiles, magnetic overshoots, and upstream wave activity were examined across all datasets. Results. Solar Orbiter measured an exceptionally large magnetic amplification ( B _max / B _u ∼ 10), though the magnitude was highly sensitive to the instrument sampling rate. No clear evidence of ion reflection or whistler precursors (expected for such high Mach numbers) was observed by Solar Orbiter. In contrast, MMS and simulation data revealed complex spatial and temporal structures that were not resolvable by Solar Orbiter. The hybrid PIC model reproduces several global shock features, but its agreement with observations depends strongly on the chosen spatial cut and simulation time step. Upstream Langmuir waves were observed without signs of a sustained electron foreshock, implying intermittent magnetic connectivity and a distorted shock surface. Conclusions. The results suggest that high Mach number interplanetary shocks possess substantial non-stationary and fine-scale structure, but their rapid motion and limited in situ sampling make these features difficult to resolve. Accurate interpretation therefore requires coordinated analyses using multi-mission observations and numerical simulations.

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.

Solar Orbiter entered the topside ionosphere of Venus for the first time when performing its 4th flyby of the planet, reaching an altitude of 378 km on 18 Feb 2025. High‐cadence electron density measurements showed previously unresolved fine‐structuring within plasma regions and boundaries, particularly at the ionopause. During the rapid flyby, a rare snapshot of the entire induced magnetosphere was captured during calm solar wind conditions around solar maximum. A well‐structured and a relatively steady plasma environment was observed. Assuming an electron temperature of 0.5 eV, pressure balance was found across the ionopause, while at the same time quasi‐periodic density and magnetic field variations suggest boundary oscillations. Near closest approach, non‐force free magnetic flux ropes were observed. Small‐scale perturbations in both magnetic field strength and density across them indicate them being dynamically evolving, rather than in a stationary state.

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.

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.

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.

We analyze data from multiple flybys by the Solar Orbiter, BepiColombo, and Parker Solar Probe (PSP) missions to study the interaction between Venus' plasma environment and the solar wind forming the induced magnetosphere. Through examination of magnetic field and plasma density signatures we characterize the spatial extent and dynamics of Venus' magnetotail, focusing mainly on boundary crossings. Notably, we observe significant differences in boundary crossing location and appearance between flybys, highlighting the dynamic nature of Venus' magnetotail. In particular, during Solar Orbiter's third flyby, extreme solar wind conditions led to significant variations in the magnetosheath plasma density and magnetic field properties, but the increased dynamic pressure did not compress the magnetotail. Instead, it is possible that the increased EUV flux at this time rather caused it to expand in size. Key findings also include the identification of several far downstream bow shock (BS), or bow wave, crossings to at least 60 (1 = 6,052 km is the radius of Venus), and the induced magnetospheric boundary to at least 20 . These crossings provide insight into the extent of the induced magnetosphere. Pre‐existing models from Venus Express were only constrained to within 5 of the planet, and we provide modifications to better fit the far‐downstream crossings. The new model BS is now significantly closer to the central tail than previously suggested, by about 10 at 60 downstream.

Ion‐acoustic waves (IAWs) commonly occur near interplanetary (IP) shocks. These waves are important because of their potential role in the dissipation required for collisionless shocks to exist. We study IAW occurrence statistically at different heliocentric distances using Solar Orbiter to identify the processes responsible for IAW generation near IP shocks. We show that close to IP shocks the occurrence rate of IAW increases and peaks at the ramp. In the upstream region, the IAW activity is highly variable among different shocks and increases with decreasing distance from the Sun. We show that the observed currents near IP shocks are insufficient to reach the threshold for the current‐driven instability. We argue that two‐stream proton distributions and suprathermal electrons are likely sources of the waves.

Context.Langmuir waves (electrostatic waves near the electron plasma frequency) are often observed in the solar wind and may play a role in the energy dissipation of electrons. The largest amplitude Langmuir waves are typically associated with type II and III solar radio bursts and planetary foreshocks. In addition, Langmuir waves not related to radio bursts occur in the solar wind, but their source is not well understood. Langmuir waves have been observed inside isolated magnetic holes, suggesting that magnetic holes play an important role in the generation of Langmuir waves.Aims.We provide the statistical distribution of Langmuir waves in the solar wind at different heliocentric distances. In particular, we investigate the relationship between magnetic holes and Langmuir waves. We identify possible source regions of Langmuir waves in the solar wind, other than radio bursts, by analyzing the local plasma conditions.Methods.We analyzed data from Solar Orbiter’s Radio and Plasma Waves (RPW) and Magnetometer (MAG) instruments. We used the triggered electric field snapshots and onboard statistical data (STAT) of the Time Domain Sampler (TDS) of RPW to identify Langmuir waves and investigate their properties. The plasma densities were derived from the spacecraft potential estimated by RPW. The MAG data were used to monitor the background magnetic field and detect magnetic holes, which are defined as regions with an isolated decrease in |B| of 50% or more compared to the background level. The statistical analysis was performed on data from 2020 to 2021, comprising heliocentric distances between 0.5 AU and 1 AU.Results.We show that 78% of the Langmuir waves in the solar wind not connected to radio bursts occur in regions of local magnetic field depletions, including the regions classified as isolated magnetic holes. We also show that the Langmuir waves occur more frequently inside magnetic holes than in any other region in the solar wind, which indicates that magnetic holes are important source regions of solar wind Langmuir waves. We find that Langmuir waves associated with magnetic holes in the solar wind typically have lower amplitudes than those associated with radio bursts.