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Wave-particle resonance is a key mechanism for energy transfer in collisionless plasmas with important implications for wave excitation and particle acceleration. Cyclotron resonance occurs when a particle's gyromotion synchronizes with the wave fields, with their interactions usually describable by quasilinear diffusion or nonlinear trapping. The nonlinear wave-particle energy transfer is facilitated by background inhomogeneities such as magnetic field gradients and/or frequency sweeping. When wave amplitudes become large, the particle's gyromotion can be strongly perturbed, altering the classical cyclotron resonance condition and giving rise to a nonlinear process known as anomalous resonance or anomalous trapping. This mechanism has been explored in idealized, uniform settings; however, its behavior in realistic inhomogeneous environments remains poorly understood. Here, we present in situ evidence of simultaneous cyclotron and anomalous resonances between electromagnetic ion cyclotron (EMIC) waves and charged particles in Earth's dayside magnetosphere. The local magnetic-field gradient enables bidirectional wave-particle energy transfer: high-energy ions drive the waves through cyclotron resonance, while lower-energy ions absorb energy from the same waves via anomalous resonance. We also identify signatures consistent with anomalous resonance of sub-keV electrons, suggesting that efficient wave-particle coupling may extend far below the classical cyclotron-resonant energy. This interplay of resonances reveals an inhomogeneity-driven pathway for energy redistribution across a broader energy range than previously recognized. These findings highlight the need to incorporate anomalous resonance into models of magnetospheric dynamics, especially in regions with strong inhomogeneities, and open new avenues for understanding energy transfer in complex plasma environments.

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.

Despite heat flux's role in regulating energy conversion in collisionless plasmas, its properties and evolution in the magnetosheath downstream of the Earth's bow shock are scarcely explored. We use Magnetospheric Multiscale in situ measurements to quantify and characterize the electron heat flux in the magnetosheath. We find that the heat flux is shaped by the magnetosheath magnetic field as it drapes around the magnetosphere. While it is affected by solar wind upstream conditions and increases with magnetic field strength, it is not substantially changed by local magnetosheath processes. Also, the heat flux is limited by whistler instability thresholds.

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.

We investigate the use of a novel two-spacecraft Liouville-mapping method using data from the Magnetospheric Multiscale mission to determine local magnetic-field-aligned (parallel) electric fields in the Earth's magnetotail. The method detects the presence of local acceleration potentials by mapping phase-space density between electron velocity distribution functions from two field-aligned spacecraft upstream and downstream of acceleration regions. Applying the method to a magnetic reconnection event, we find that local parallel electric fields near the current sheet (CS) center are, on average, directed away from the center, resulting from the need to maintain quasi-neutrality across the CS. Despite significant measurement uncertainties, we find that the local acceleration potentials are smaller than the total acceleration potential, typically 1%–2% on average and up to 9% for individual measurements. This indicates that many potential drops, over distances much larger than the spacecraft separations, contribute to the net work done on electrons by parallel electric fields.

Solar radio bursts, and astrophysical radio emissions in general, are observed either in space or on the ground by measuring their fluctuating electric field. Here, we report the first measurement of the magnetic component of a solar radio wave, observed simultaneously by the Solar Orbiter and Parker Solar Probe missions. The observations were made during the type III radio burst on 2021 October 28. The analysis of the wave polarization and magnetic and electric field amplitudes allows us to estimate the refractive index and put constraints on the direction of the wave. The wave is found to be consistent with an ordinary-mode wave and with a source near the southeast limb of the Sun. These results pave the way for future observations and analyses of the magnetic field of radio waves, in particular, for solar radio bursts.

In collisionless plasmas, relaxation of the deviations of ion velocity distribution functions (VDFs) from local thermodynamic equilibrium occurs through particle interactions with electromagnetic fields. In particular, in the Earth's magnetotail, the deviations of the ion VDFs, typically consisting of multiple components, from the equilibrium must be limited to maintain stability of the current sheet. Curvature scattering is a leading candidate mechanism to limit such deviations, but its role remains insufficiently understood. We investigate the limits of ion temperature anisotropy in a magnetotail-like configuration by modeling a quasi-1D current sheet with a finite magnetic field curvature and three ion populations. We derive analytical thresholds for anisotropy based on current sheet stability and validate against spacecraft observations and numerical simulations. Our findings demonstrate that curvature scattering imposes limits on ion anisotropies, thereby maintaining the stability of the current sheet.

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.

Electric field instruments function by electrically coupling to the surrounding plasma, resulting in a response function that varies depending on local conditions. This variable coupling can complicate quantitative interpretation of wave measurements, yet is rarely considered. While this effect has recently been quantified for the Van Allen Probes, we present the first quantitative analysis for the Magnetospheric MultiScale (MMS) mission. First, MMS observations are evaluated against Faraday's Law, revealing that the angle between the measured whistler-mode electric and magnetic fields directly depends on the wave propagation direction, a feature consistent with sheath impedance effects. A novel technique for determining electric field observations along each measurement direction is introduced, addressing limitations of previous works. This reveals that, for MMS in low-density plasma, spin-plane amplitudes are similar to 60% of expected values, with small phase shifts, while spin-axis measurements are accurate within 5%-10%, with phase shifts up to -20 degrees. At intermediate densities, spin-plane amplitudes match, or slightly exceed, expected values whereas spin-axis observations can be overestimated by 70% and experience frequency-dependent phase shifts. At high-density, spin-plane measurements generally agree with expected values, but spin-axis observations are overestimated by similar to 30% and experience similar to 30 degrees phase shifts. Accurate measurements are critical, with electric field fluctuations increasingly being used to infer wave-particle energy exchange rates. If electric field observations can be under or over-measured depending on the local plasma environment, this directly impacts these computations of energy exchange rates, potentially leading to a misinterpretation of the fundamental physical mechanisms that drive particle dynamics.

Aims: Thermal electron measurements in space plasmas typically suffer at low energies from spacecraft emissions of photo-and secondary electrons and from charging of the spacecraft body. We aim to examine these effects by using numerical simulations in the context of electron measurements acquired by the Electron Analyser System (SWA-EAS) on board the Solar Orbiter mission.

Methods: We employed the Spacecraft Plasma Interaction Software to model the interaction of the Solar Orbiter spacecraft with solar wind plasma. In the model, we implemented a virtual detector to simulate the measured electron energy spectra as observed in situ by the SWA-EAS experiment. For comparison with the real SWA-EAS data, numerical simulations were set according to the measured plasma conditions at 0.3 AU. From the simulation results, we derived the electron energy spectra as detected by the virtual SWA-EAS experiment for different electron populations and compared these with both the initial plasma conditions and the corresponding real SWA-EAS data samples.

Results: We found a qualitative agreement between the simulated and real data observed in situ by the SWA-EAS detector. Contrary to other space missions, the contamination by cold electrons emitted from the spacecraft is seen well above the spacecraft potential energy threshold. A detailed analysis of the simulated electron energy spectra demonstrates that contamination above the threshold is a result of cold electron fluxes emitted from distant spacecraft surfaces. The relative position of the break in the simulated spectrum with respect to the spacecraft potential slightly deviates from that in the real observations. This may indicate that the real potential of the SWA-EAS detector with respect to ambient plasma differs from the spacecraft potential value measured on board. The overall contamination is shown to be composed of emissions from a number of different sources and their relative contribution varies with the ambient plasma conditions.