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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.

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.

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 multipoint observations of a merger between a Short Large-Amplitude Magnetic Structure (SLAMS) and 1-Hz whistler wave packets upstream of Earth's quasi-parallel bow shock. Using high-resolution data from the Magnetospheric Multiscale (MMS) mission in a string-of-pearls configuration, we track the SLAMS as it evolves, exhibiting enhanced magnetic field strength, increasing spatial scale, variable ion reflection at the ramp, and increasingly complex morphology. The whistler wave packets show distinct propagation properties, propagating at a different velocity from the SLAMS and approaching it from upstream. Eventually, the SLAMS merges with the wave packets, altering its structure and forming a new ramp characterized by an intensified magnetic field and plasma density. These observations demonstrate how SLAMS and wave packets can coexist and interact, leading to changes in local plasma conditions and revealing the complexity and dynamic nature of bow shock transitions.

Studies have long suggested that shocks can undergo cyclical self-reformation as a type of shock nonstationarity. Until now, providing solid evidence for shock reformation in spacecraft observation and identifying its generating mechanisms remain challenging. In this Letter, by analyzing magnetospheric multiscale observations, we unambiguously identified shock reformation occurring in a quasiperpendicular shock. A 2D PIC simulation reproduces and explains the observed shock reformation. The result reveals that the shock reformation is driven by the ion-scale whistler waves, manifested in two distinct types: whistler precursor and modified two-stream instability whistlers. An interesting finding is that the two mechanisms compete and dominate the reformation at early and later stages, respectively. Our results not only provide evidence of ion-scale whistlers as mechanism driving shock reformation, but also demonstrate the detailed kinetic processes how it happens, offering valuable insights into shock dynamics.

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.

Type II and III solar radio bursts involve electron beams driving Langmuir waves that then couple wave energy into radio emission. These Langmuir waves can be driven to large enough amplitudes that they undergo electrostatic (ES) decay into a backward propagating Langmuir wave and a forward propagating ion acoustic wave. The forward and backwards propagating Langmuir waves can then "beat" together to produce characteristic Langmuir waveforms and spectra, plus radio emission. Stochastic growth theory (SGT) predicts that the probability distribution of the Langmuir wave electric field strength should be lognormal, with known modifications if nonlinear processes like ES decay are occurring at high fields. Other analyses suggest the more general set of Pearson distributions may be relevant. Here, previous work on Langmuir waves in type II and III source regions is generalized and tested by analysing the probability distributions of the waveforms of Langmuir waves observed by the STEREO spacecraft. The focus is on a set of published events identified using spectral analyses to have or not have spectral evidence for ES decay. For events for which spectral analyses provide evidence of ES decay, 92 % of the probability distributions are consistent with the combination of SGT and a nonlinear process like ES decay, while 68 % of those without spectral evidence for ES decay are better fitted by pure SGT. Better fits with stronger statistical significance are obtained for pure and nonlinear SGT than for Pearson distributions in the majority of events ( 90 ). These results provide strong evidence for SGT and ES decay proceeding in type II and III source regions, particularly type III source regions. (c) 2026 The Author(s). Published by Elsevier B.V. on behalf of COSPAR. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Magnetic reconnection jets in Earth's magnetotail are regions of electron heating and acceleration. They are often associated with dipolarization fronts (DFs), which are characterized by sharp increases in the northward magnetic field component B z . Electrons can be accelerated at DFs mainly through the betatron and Fermi mechanisms. We perform a statistical study of betatron and Fermi electron rate of energy change at Earthward-propagating DFs observed by the Magnetospheric Multiscale (MMS) spacecraft. Our results show that betatron acceleration is dominant ahead of DFs, where suprathermal electron fluxes perpendicular to the magnetic field are detected. Up to ∼ 20 ion inertial lengths inside the outflow, Fermi acceleration acts on the low-density electron population, potentially driving them to high energies (∼100 k e V ). Spatially in the magnetotail, betatron acceleration is statistically larger near the most probable X-line location, while Fermi-dominated events are observed closer to the Earth.

Electrostatic waves are frequently observed at collisionless shocks and are believed to play a key role in energy dissipation. Recent spacecraft observations have identified a proton-alpha instability in the Earth's bow shock as a potential mechanism to dissipate the energy associated with a relative drift between protons and alpha particles, u_α. At plasma parameters relevant to this setting, we investigate the dependence of the evolution of this instability on u_α through particle-in-cell simulations. We find that the instability produces strong ion heating, increasing ion temperatures by factors of two to five. The electrons, in contrast, experience only minimal heating (∼1%). The heating is highly anisotropic and is most efficient when the wave vector is at a 45° angle to the alpha flow velocity. To interpret these results, we introduce a geometric model based on Landau resonance interaction bands in velocity space. It explains both the angular dependence and the magnitude of heating through the change in velocity distribution functions. At the same time, the wave properties are consistent with linear theory. Our results demonstrate that the proton-alpha streaming instability can be an important mechanism to convert the bulk kinetic energy of the alphas into ion heating at collisionless shocks.