Whistlers are electromagnetic modes commonly observed in Earth’s magnetosphere, where their resonant interaction with electrons over a broad frequency range plays a key role in regulating radiation belt dynamics. In this case study, we analyze whistler waves propagating through regions characterized by different electron densities, namely the plasmatrough, a plasmaspheric plume, and the plasmasphere. Using high-resolution spectral measurements, wave parameters, and energy-flux data from Van Allen Probes EMFISIS suite, we characterize the evolution of the emissions as they propagate through these regions. The whistler waves exhibit higher amplitudes near the plume boundaries, where the presence of rising-tone elements indicates nonlinear wave growth. Chorus-like whistler emissions originating in the plasmatrough are reflected at the plume boundary with oblique wave-normal angles. Evidence of energy conversion between different whistler waves is also observed near the plume boundaries. These results provide new insight into the behavior of whistler waves across density gradients in the inner magnetosphere.

How to cite: Medeiros da Nóbrega, G., Alves, L., Da Silva, L., Ferreira, K., and Deggeroni, V.: Propagation of Whistler Waves Through Density Gradients in Earth’s Inner Magnetosphere, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-852, https://doi.org/10.5194/egusphere-egu26-852, 2026.

As one of the typical electrostatic waves in the terrestrial magnetosphere, electron cyclotron harmonic (ECH) waves are capable of scattering hundreds of eV to several keV electrons and precipitating them into the atmosphere. In this study, using combined observations from the Van Allen Probes, Arase, and MMS missions spanning 2012–2023, we present a comprehensive statistical survey of electrostatic electron cyclotron harmonic (ECH) waves in Earth’s magnetosphere. ECH waves are observed over a broad region covering L = 3–15, MLAT < 40°, and nearly all magnetic local time sectors, exhibiting pronounced spatial and regional variations. In the inner magnetosphere (L < ~6), wave power preferentially peaks from premidnight to noon, whereas in the outer magnetosphere (L > 6), ECH waves occur most frequently on the dayside. Moreover, ECH waves are predominantly confined near the magnetic equator (MLAT < 5°) at L < ~8, while showing a much broader latitudinal extent (up to MLAT < 35°) at higher L. Furthermore, we investigate the dependence of ECH waves on solar wind conditions and geomagnetic activity indices, revealing pronounced day–night differences in the wave responses to solar wind driving and geomagnetic disturbances. These results suggest different generation and modulation processes of ECH waves in the dayside and nightside magnetosphere. In addition, based on the multi-satellite statistical results, we construct a global empirical model of ECH wave distribution, providing a quantitative framework for incorporating ECH waves into radiation belt and space weather studies.

How to cite: Lou, Y., Ni, B., Cao, X., Ma, X., Chen, S., and Li, J.: Responses of Electrostatic Electron Cyclotron Harmonic Waves to Solar Wind Parameters and Geomagnetic Activity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9114, https://doi.org/10.5194/egusphere-egu26-9114, 2026.

Electromagnetic ion cyclotron (EMIC) waves are one of the most frequently observed plasma wave modes in Earth’s magnetosphere and play an important role in particle precipitation and magnetospheric energy redistribution. In this study, we present a comprehensive statistical analysis of H⁺-band and He⁺-band EMIC waves observed by the THEMIS mission from January 2012 to June 2025. The spatial distribution and wave properties, including occurrence rate, wave amplitude, ellipticity, normal angle, mean frequency, and frequency bandwidth, are systematically examined. Consistent with earlier studies, our results show that H⁺-band EMIC waves predominantly occur in the dawn and afternoon sectors of the outer magnetosphere, while He⁺-band EMIC waves are mainly concentrated in the afternoon sector. In the dawn sector, both H⁺- and He⁺-band EMIC waves exhibit more oblique normal angles and predominantly linear polarization. In contrast, EMIC waves in the afternoon sector tend to have more parallel normal angles and left-hand polarization. In addition, both EMIC wave occurrence and wave properties display clear solar-cycle-dependence. Both H⁺- and He⁺-band EMIC waves have higher occurrence rate during solar minimum than solar maximum. H⁺-band EMIC waves tend to be left-hand polarized during solar minimum and more linearly polarized during solar maximum, whereas He⁺-band EMIC waves exhibit the opposite polarization behavior. These results provide new statistical evidence for the modulation of EMIC wave generation and propagation by the solar cycle.

How to cite: Zhou, R., Artemyev, A., Zhang, X., and Nakamura, R.: Spatial distribution and solar cycle variability of EMIC waves observed by THEMIS, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10644, https://doi.org/10.5194/egusphere-egu26-10644, 2026.

Energetic electron precipitations from the flow-breaking region—the transition between the outer radiation belt and the plasma sheet—are typically characterized by dispersive signatures consistent with scattering via field-line curvature. However, during intervals of injections and magnetic field dipolarizations, low-altitude spacecraft have observed precipitation patterns more characteristic of whistler-mode scattering. In this study, we analyze examples of such precipitation patterns collected by the ELFIN CubeSat in conjunction with equatorial observations from the THEMIS mission. We show that whistler-mode-driven precipitation can include sub-second bursts, which are usually associated with nonlinear resonant scattering of electrons by chorus waves. Based on ELFIN and THEMIS observations, we discuss the possibility of nonlinear resonant electron scattering occurring in the flow-breaking region.

How to cite: Artemyev, A., Zhang, X., and Angelopoulos, V.: Electron Precipitations from the Flow-Breaking Region: Can Whistler-Mode Waves Nonlinearly Resonate with Plasma Sheet Electrons?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-948, https://doi.org/10.5194/egusphere-egu26-948, 2026.

Diffuse electron precipitation driven by resonant interactions between particles and whistler-mode waves is an important element of magnetosphere–ionosphere coupling and represents a major source of energy input into the nightside ionosphere. Although the majority of the precipitating particle flux is contributed by plasma-sheet electrons with energies below 30 keV, a substantial fraction of the total energy flux can be carried by energetic (30–300 keV) and relativistic (>500 keV) electrons.

In this presentation, we provide a quantitative assessment of this precipitation and its impact on ionospheric ionization. By combining ELFIN and DMSP measurements with an ionization model, we show that at sub-auroral latitudes—particularly during substorm injections—the contribution of whistler-driven energetic electron precipitation can dominate the electron density enhancement in the E and D layers. We further discuss the physical mechanisms that enable whistler-mode waves to effectively scatter and precipitate energetic and relativistic electrons in the vicinity of plasma-sheet injections.

How to cite: Zhang, X.-J., Tonoian, D., Kamaletdinov, S., Artemyev, A., Shen, Y., Liang, J., and Angelopoulos, V.: On the Importance of Whistler-Driven Energetic Electron Precipitation for Magnetosphere–Ionosphere Coupling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3268, https://doi.org/10.5194/egusphere-egu26-3268, 2026.

Earth’s magnetotail is characterized by stretched magnetic field lines, which is a favorable condition for magnetic field line curvature scattering (FLCS) of energetic electrons. Low-altitude observations usually detect isotropic electron precipitations from the magnetotail, without tell-tale signatures of wave-driven precipitations. However, meso-scale transient dipolarization within fast plasma flows may locally suppress the field-aligned curvature scattering, clearing a path for electron precipitations due to wave-particle resonant interactions. In this presentation, we analyze low-altitude ELFIN observations of whistler-driven precipitations from the magnetotail, in particular their typical energy range and spatial characteristics. Such ELFIN observations are supported by in-situ observations of magnetic field dipolarization and whistler-mode waves by near-equatorial THEMIS spacecraft. We also discuss if such joint ELFIN/THEMIS measurements of whistler-mode wave activity and associated precipitations may be used for magnetic field line mapping during plasma sheet injections.

How to cite: Vainchtein, D., Artemyev, A., and Zhang, X.: Low-altitude observations of whistler-driven precipitations from plasma sheet, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8386, https://doi.org/10.5194/egusphere-egu26-8386, 2026.

Earth’s magnetosphere provides a unique natural environment in which plasma processes unfold across broad temporal, spatial, and energy scales. A particularly important class of these processes is wave–particle interaction, which governs both the acceleration and loss of energetic particles and strongly influences radiation belt dynamics. Relativistic electrons in the radiation belts can exchange energy and momentum with plasma waves through resonant interactions, leading to pitch-angle scattering and, in some cases, precipitation into the upper atmosphere. Despite decades of theoretical and observational work, quantitatively characterizing wave–particle interactions remains an outstanding challenge in magnetospheric physics, largely due to the scarcity of co-located and simultaneous measurements of both energetic particles and electromagnetic waves at interaction sites.

The Leucippus mission is designed to address this limitation by combining controlled wave generation with coordinated in-situ observations. The mission concept consists of two 6U CubeSats operating in formation to enable active experiments on wave–particle interactions. One spacecraft acts as a transmitter, generating Very-Low-Frequency (VLF) electromagnetic waves, while the second spacecraft performs targeted measurements of the resulting plasma and particle response. This architecture enables direct observation of resonant scattering signatures under representative inner magnetospheric plasma conditions, providing an experimental capability that has not previously been available.

The transmitter CubeSat carries a rotating magnetic dipole antenna optimized for VLF wave emission in the 5–20 kHz frequency range, complemented by a Langmuir probe for measuring local plasma density and improving coupling to whistler-mode propagation. The receiver CubeSat is equipped with VLF electric and magnetic field sensors to characterize wave properties, as well as an energetic particle detector capable of resolving electron energy distributions. By exploiting magnetic conjunctions between the two spacecraft, Leucippus enables measurements along shared magnetic field lines where wave–particle interactions are expected to be strongest.

The primary scientific goals of the mission are to demonstrate efficient space-based VLF wave injection into the magnetosphere, to investigate wave propagation behavior across varying plasma conditions, and to directly quantify electron energy diffusion driven by injected wave fields. The resulting observations will place new constraints on long-standing theoretical models, including questions related to nonlinear effects, ducted versus oblique wave propagation, and the effectiveness of controlled VLF transmission as a tool for radiation belt modification. Leucippus is currently in the concept development phase, with subsystem design, antenna–plasma coupling simulations, and wave–particle interaction modeling actively underway.

How to cite: Tourgaidis, S. and Sarris, T.: Probing Magnetospheric Wave–Particle Interactions Through Coordinated VLF Transmission and In-Situ Measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18340, https://doi.org/10.5194/egusphere-egu26-18340, 2026.

Resonant interactions of radiation belt electrons with intense whistler-mode waves can lead to rapid nonlinear acceleration through phase trapping. The efficiency of this process depends strongly on wave coherence. In the random-phase approximation (fully incoherent), particle transport in velocity space can be described as diffusion with coefficients given by quasilinear theory. However, intense and coherent whistler-mode chorus waves are ubiquitous in the Earth’s radiation belts during geomagnetically active periods, raising the question of whether the diffusive Fokker–Planck equation implemented in state-of-the-art radiation belt models remains applicable.

In this work, we study the phase space density evolution of electrons interacting with narrow-band whistler-mode waves using test-particle simulations and compare the results with solutions of the diffusion equation. A key element of our approach is following particles over many bounce periods between magnetic mirror points, ensuring bounce-phase mixing and a gradual transition toward stochastic behavior. Starting from step-function initial conditions in pitch-angle phase space density, we analyze the broadening and erosion of initially sharp gradients and extract effective diffusion and drift coefficients.

Focusing on regions where quasilinear theory predicts nearly homogeneous diffusion and pitch-angle transport dominates over energy transport, we represent the theoretical solution using a Legendre polynomial expansion and determine transport coefficients via least-squares fitting. We find that the inferred diffusion coefficients agree with quasilinear predictions within a factor of 1.3 over a broad range of energies and pitch angles. A small negative effective drift term is sometimes required to reproduce the observed gradient erosion. This agreement persists even at very low pitch angles, where anomalous phase trapping occurs, suggesting that such nonlinear effects do not preclude a diffusive description of phase space density evolution and do not strongly modify diffusion rates relative to quasilinear expectations. While a wider range of wave parameters needs to be explored, these preliminary results support the continued use of quasilinear diffusion models in radiation belt simulations.

How to cite: Hanzelka, M., Allanson, O., Albert, J., Haas, B., Wang, D., and Santolík, O.: Diffusive approximations of nonlinear interaction of electrons with whistler-mode waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9997, https://doi.org/10.5194/egusphere-egu26-9997, 2026.

We present observations of modulation of higher frequency waves (lower hybrid, whistler mode, magnetosonic, ion acoustic) by lower frequency waves (electromagnetic ion cyclotron, ultra-low frequency) in the Earth’s magnetosphere including the radiation belts, plasma sheet boundary layer, and magnetotail, as well as in the solar wind. This cross-scale coupling links the vastly different ion and electron temporal and spatial scales, and can have dramatic effects on wave-particle interactions. Such modulations can have a significant impact on the formation and depletion of Earth’s radiation belts, and the mechanisms that control the heat flux in the solar wind.

We have recently developed a new automated technique to identify modulations in the RBSP data using the filterbank data products, and found that such modulation is considerably more common than previously understood.  Similar modulations were observed in MMS measurements taken in the radiation belts and elsewhere in the magnetosphere, as well as PSP measurements in the solar wind.  We use these data sets to automatically detect modulated wave events. This database of modulated events and characteristics of the waves and plasma environment, including geomagnetic conditions for magnetospheric observations, allows us to determine the prevalence of this process and under which conditions it can occur.

We will present the results of applying our algorithm across the entire RBSP dataset, including the location and prevalence of modulations across the range of frequencies and geomagnetic conditions. Observations from MMS will be added to this database. We also present comparative studies between events at Earth and those observed in the solar wind by PSP to provide insights into the coupling process.

How to cite: Colpitts, C., Elliott, S., and Seebaluck, K.: Identifying and Quantifying Wave Cross-scale Coupling in the Earth’s Magnetosphere and the Solar Wind, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5973, https://doi.org/10.5194/egusphere-egu26-5973, 2026.

The terrestrial magnetosheath is a turbulent region characterized by large amplitude electromagnetic fluctuations. The NASA Magnetospheric Multiscale (MMS) mission has allowed investigating at high time cadence electromagnetic fields and particles in the near-Earth environment. Here, we have analyzed a well documented 5 minute MMS quasi-parallel magnetosheath crossing, focusing on the kinetic plasma properties in sub-intervals where an intense electrostatic activity at high frequencies is detected. Numerical simulations have showed that an electrostatic branch of high frequency waves with an acoustic-type dispersion relation, the so-called ion-bulk (IBk) waves, can be excited by a wave-particle instability due to the formation of a plateau in the bulk of the ion velocity distribution function (VDF). Such waves can survive against the Landau damping also for small values of electron-to-proton temperature ratios, which typically induce a strong ion-acoustic wave damping. IBk waves induce large amplitude electric field fluctuations and both ion and electron phase-space trapping, giving rise to a beam in the ion VDF and a flat-top electron VDF. Motivated by numerical results we have explored the ion and electron kinetic features when time intervals of electrostatic fluctuations are detected in the magnetosheath region. Thanks to the high time cadence of the instruments on board MMS we were able to reconstruct all the energy turbulent cascade that starts at ion scales and gives rise to IBk excitation around the electron scales. The effects of the presence of IBk waves on ion and electron VDFs have also been investigated, finding good agreement with recent numerical experiments.

How to cite: Condoluci, M., Zanelli, S., Valentini, F., Perrone, D., and Perri, S.: Ion and electron distribution functions within regions of intense electrostatic fluctuations in the Earth’s magnetosheath, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14475, https://doi.org/10.5194/egusphere-egu26-14475, 2026.

Foreshock transients are mesoscale structures upstream of the Earth’s bow shock and they evolve from solar wind discontinuities. Foreshock transients are commonly observed, but their impact on the radiation belt dynamics has not been studied before. These structures have a global impact on the magnetosphere and, in particular, can launch ultra-low frequency (ULF) waves which can energize radiation belt electrons through resonant interactions. We present a case study of a foreshock transient event that is associated with prompt acceleration in electron fluxes, drift echoes and localized ULF wave activity using multi-satellite observations.

The transient is characterized by hot and tenuous plasma, strong flow deflection and is bounded by a compressed edge on its sunward side indicating that it is a foreshock bubble. Using multiple satellite missions, Van Allen Probes, Arase and GOES, we can assess the global view of the transient’s effects on the dayside inner magnetosphere. The electron fluxes from these satellites show signatures of an initial energization and subsequent drift echoes. This injection exhibits energy dispersion and boomerang stripes in the pitch angle distributions. Analysis of energy and pitch angle dependent drift speeds shows that the acceleration is consistent with the timing and geometry of the impact of the foreshock bubble. Wave measurements from these spacecraft show enhanced ULF wave activity at the time of the electron injection. This study shows, for the first time, that foreshock transients may play an important role in radiation belt dynamics.

How to cite: Kalliokoski, M., Turc, L., Dahani, S., Tao, S., Lipsanen, V., Ojuva, M., Osmane, A., Miyoshi, Y., Hori, T., Turner, D., Higashio, N., Mitani, T., Takashima, T., and Shinohara, I.: Acceleration of radiation belt electrons driven by a foreshock bubble, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18966, https://doi.org/10.5194/egusphere-egu26-18966, 2026.

Polarization of shear Alfven waves observed in the terrestrial magnetosphere is almost never exactly poloidal or toroidal. Ultra Low Frequency (ULF) waves of mixed polarization naturally appear if we assume that the background magnetic field lines are not contained in the meridional planes, i.e. if they have non-zero torsion. To illustrate the effect of torsion on the polarization of ULF waves, we developed a simple analytical model of the magnetic field with non-planar field lines which are similar to the magnetic field lines in the dawn or dusk flanks of the magnetosphere. This field is explicitly characterized by a control parameter describing the degree of deviation from the dipole magnetic field. Shear Alfven waves in this background field are described using covariant-contravariant formalism which allows a self-consistent calculation of the wave polarization. Our calculations show that even small torsion of the background magnetic field lines leads to significant deviations of the wave polarization from pure poloidal or toroidal direction. In contrast, the frequencies of the ULF remain practically unaffected by the torsion of the background magnetic field. The results of our model calculations show that the electric field of the commonly observed quasi-toroidal mode has an azimuthal component and, therefore, can effectively contribute the energization of the charged particles which undergo gradient-curvature drift in the inner magnetosphere.

How to cite: Kabin, K. and Degeling, A.: Effect of the field line torsion on the polarization of ULF waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-906, https://doi.org/10.5194/egusphere-egu26-906, 2026.