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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. 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.

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.

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.