Orals: Thu, 7 May, 10:45–12:30 | Room 0.15
Rapid injections of energetic and relativistic particles into the Earth’s radiation belts have been observed to produce multi-energy enhancements on timescales of tens of minutes, with effects that can persist for hours. Motivated by recent observations demonstrating that such injections occur nearly simultaneously across a broad energy range and exhibit sharp, step-like signatures (e.g. Kim et al., GRL, 48, 2021), we investigate the subsequent evolution of magnetically localised particle populations. We first develop an analytical description of the ballistic evolution of MLT-localised injections confined to a given L-shell, using drift-kinetic theory to quantify curvature and gradient drift trajectories as functions of pitch angle, first adiabatic invariant, energy, and time. This framework provides explicit predictions for the azimuthal phase mixing and phase-space evolution expected under purely adiabatic transport. We then examine energetic particle injections using GPS observations, cross-calibrated with Van Allen Probes measurements, to track their global evolution and assess deviations from ballistic drift behaviour. Together, the combined theoretical and observational approach constrains the extent to which rapid radiation belt enhancements can be explained by adiabatic drift physics alone and identifies signatures of non-ballistic transport processes.
How to cite: Osmane, A., Olifer, L., An, X., and Ratliff, D.: Analytical and Observational Study of MLT-Localised Energetic Particle Injections in the Radiation Belts , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5403, https://doi.org/10.5194/egusphere-egu26-5403, 2026.
The radial diffusion of radiation belt electrons due to interacting with ultra-low frequency (ULF) waves has traditionally been studied by assuming either a constant wave frequency at specific L drift-resonating with the electron or broad-band waves, resonating across a wider L range. We investigate a special case of radial diffusion caused by narrow-band ULF waves, generated by field line resonance (FLR), whose frequency varies with L in a manner that continuously satisfies the drift resonance condition throughout an electron’s radial motion. The conditions for this continuous resonance are derived for both non-relativistic and relativistic electrons in a dipolar magnetic field, which are further validated by two-dimensional test-particle simulations. The results show that, under conditions with inverse power-law relationship of -0.3~-0.5 between the wave frequency and L, relativistic electrons experience significantly enhanced radial diffusion, with the diffusion coefficient exceeding that of constant-frequency conditions by more than an order of magnitude.
How to cite: Hao, Y.: Enhanced radial diffusion of radiation belt electrons caused by field line , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5350, https://doi.org/10.5194/egusphere-egu26-5350, 2026.
Radial diffusion of energetic electrons by Ultra-low frequency (ULF) waves is a key mechanism for the acceleration and radial transport of hundreds-keV to few-MeV electrons in the radiation belts, via their drift-resonant interactions. Until recently, estimates of the radial diffusion rates have focused on the equatorial plane and have been derived for equatorially mirroring electrons. Recent statistical in situ observations based on THEMIS, Arase and Cluster have shown that the wave power of broadband magnetic and electric field ULF fluctuations is significantly enhanced away from the magnetic equator. Using 3D particle tracing under broadband ULF waves that are guided by these observations, we show that there is a significant dependence of the radial transport of relativistic electrons on their pitch angle, and that the diffusion coefficients of off-equatorial electrons can be up to an order of magnitude higher than that of equatorially-mirroring electrons. These findings point to the need for incorporating new radial diffusion coefficients in global radiation belt models that are pitch angle-dependent, together with magnetic latitude-dependent ULF wave power.
How to cite: Sarris, T., Tu, W., Zhao, H., Li, X., Riggs, G., Tourgaidis, S., Papadakis, K., and Liu, W.: Derivation of Pitch-Angle-Dependent Radial Diffusion Coefficients of off-Equatorial Relativistic Electrons in the Radiation Belts , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21579, https://doi.org/10.5194/egusphere-egu26-21579, 2026.
In this study we focus on the acceleration of the most energetic part of the radiation belt population so-called ultra-relativistic electrons. We perform simulations with coupled cold plasma and radiation belt codes. Our simulations show that the acceleration to such high energies occurs only when cold plasma density is extremely depleted. Our coupled simulations demonstrate that when realistic density variability is included, we can accurately reconstruct the dynamics of the radiation belts. We also perform statistical analysis of all storms during the Van Allen Probes era that show acceleration to 7.7 MeV and have observations on the dawn side. These observations show that the presence of 2 MeV seed population and the presence of prolonged deplitions in density are required for acceleration to 7.7MeV.
This study also reveals the intricate interplay between cold plasma and the enhancements of ultra-relativistic electrons that are millions of times more energetic than plasma particles.
Similar acceleration may occur in planetary radiation belts, for lab plasmas, at exoplanets, and in other magnetized astrophysical objects.
How to cite: Shprits, Y.: Acceleration of electrons to ultra-relativistic energies in the Earth's radiation belts. , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16633, https://doi.org/10.5194/egusphere-egu26-16633, 2026.
In this study, we present results using observations from a conjunction of three satellites to study the outer radiation belt electron dynamics during the April 20, 2018, geomagnetic storm. Between 0900 UT and 1230 UT, Van Allen Probe B, Van Allen Probe A, and Arase were located within similar L-shell ranges (5 – 6) but separated in local time, which provided a unique opportunity to study the variation of electron fluxes along their drift trajectory. The electron fluxes exhibited different responses in three energy ranges: at <∼100 keV, the 90° fluxes remained almost constant while fluxes at lower pitch angles decreased rapidly; for ~100–300 keV, the fluxes decreased at all pitch angles, with larger decreases at larger pitch angles; and at >∼300 keV, the fluxes showed a decrease following the injection closer to local midnight, and an increase further along the drift trajectory toward dawn. To understand the observed flux variations, we calculated the pitch angle and momentum diffusion coefficients and found the results to be consistent with the observations: the pitch angle diffusion coefficients were higher at smaller pitch angles for <~100 keV electrons, and at larger pitch angles for ~100 – 300 keV electrons, while they were low for >~300 keV electrons. The momentum diffusion coefficients were significantly low at all energies. Our results showed that intense chorus waves can drive rapid precipitation of several hundreds of keV electrons on injection timescales (~tens of minutes) and that using multi-spacecraft observations can provide a higher-fidelity picture of the systemic response of the radiation belts to solar wind drivers.
How to cite: Chakraborty, S., Mann, I., Olifer, L., Black, R., Allanson, O., Rae, J., Ozeke, L., and Watt, C.: Using observations from a conjunction of three spacecraft to study the outer radiation belt electron dynamics during the April 20, 2018, geomagnetic storm, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12687, https://doi.org/10.5194/egusphere-egu26-12687, 2026.
Whistler-mode chorus waves are electromagnetic emissions in the Earths’ magnetosphere that play a central role in the dynamics of the outer radiation belt, contributing to both acceleration and loss of relativistic electrons. The efficiency of these processes strongly depend on the wave intensity and the ratio of the electron plasma frequency to the electron gyrofrequency (fpe/fce). Using approximately 24.5 years of THEMIS wave observations, we examine how chorus wave intensity and spatial distribution vary with relative frequency, geomagnetic activity and fpe/fce. The strongest chorus waves are observed during periods of high geomagnetic activity (AE > 200nT). At low relative frequencies (flhr < f < 0.1fce), equatorial chorus is strongest during periods of high fpe/fce, primarily within 5 < L* < 8 and 22-12 MLT. In contrast, at high relative frequencies (0.5fce < f < 0.7fce), equatorial chorus is strongest when fpe/fce is low, between 4 < L* < 6 and 21-09 MLT. At intermediate relative frequencies (0.3fce < f < 0.4fce), chorus intensities are observed in the same region and are largely independent of fpe/fce. We find that off-equatorial chorus emissions are also largely independent of fpe/fce. Results show that the locations of peak chorus intensity are controlled by the availability of resonant source electrons and reduced Landau damping. Our findings identify regions where chorus-driven acceleration of electrons to relativistic energies is expected to be the most significant.
How to cite: Bunting, K., Meredith, N., Bortnik, J., Ma, Q., Matsuura, R., and Shen, X.-C.: Global Morphology of Chorus waves in the Outer Radiation Belt and the Effect of Geomagnetic Activity and fpe/fce, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18931, https://doi.org/10.5194/egusphere-egu26-18931, 2026.
The ion foreshock is the region upstream of Earth’s bow shock where magnetic field lines connect to the quasi-parallel shock surface. Within this region, there exist a variety of backstreaming ion populations from the shock ramp that can generate ultra-low frequency (ULF) waves through wave-particle interactions. In this work, we use data from the Magnetospheric Multiscale (MMS) mission to study such ULF waves and backstreaming ions during a prolonged interval of above-average solar wind helium abundance, embedded in a multi-day period of strong solar activity driven by a CME. When interplanetary magnetic field (IMF) orientations positioned MMS within the ion foreshock, the spacecraft captured the complete evolutionary sequence of the backstreaming ion velocity distributions: the initial formation of a reflected ion beam, followed by phase bunching and generation of coherent ULF waves, and eventual thermalization and randomization in velocity space to form diffuse ions. Intervals with elevated energetic He++ flux exhibited broadened and frequency-downshifted wave spectra, consistent with heavy-ion cyclotron resonance effects. The unusually rapid beam-to-diffuse transitions observed near the foreshock boundary likely result from the combined effects of multi-species wave-particle interactions and higher backstreaming ion densities during this active interval. These findings underscore the need for simulations and modeling that incorporate multi-species effects.
How to cite: Le, G., Blanco-Cano, X., Chen, Y., Pandya, M., Poh, G., Wei, H., Boardsen, S., Belbase, P., Russell, C., Gershman, D., Cohen, I., and Fuselier, S.: MMS Observations of Multi-Species Wave-Particle Interactions and Rapid Foreshock Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2835, https://doi.org/10.5194/egusphere-egu26-2835, 2026.
In the Earth’s magnetosheath, several processes contribute to energy dissipation and plasma heating, one of which is wave-particle interactions between whistler waves and electrons. The whistler-heat flux instability is known to scatter the strahl electrons in the solar wind. However, the heat flux properties and evolution across the Earth’s magnetosheath region have not yet been explored.
We present Magnetospheric Multiscale (MMS) observations from 18 hours of burst-mode measurements using the unbiased magnetosheath campaign. We quantify the electron heat flux in the magnetosheath and examine the role of whistler instabilities in regulating it. Our results show that the heat flux follows the magnetosheath magnetic field as it drapes around the magnetosphere. We find that the heat flux is constrained by whistler instability thresholds and is aligned with the propagation of low-frequency whistler waves. We also present a case study investigating direct evidence for the whistler-heat flux instability in the magnetosheath.
How to cite: Svenningsson, I., Yordanova, E., Larsson, M., Khotyaintsev, Y. V., André, M., Cozzani, G., Chasapis, A., and Schwartz, S. J.: Electron heat flux and whistler instability in the Earth’s magnetosheath, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6615, https://doi.org/10.5194/egusphere-egu26-6615, 2026.
We presented magnetic field and plasma measurements of three case studies analyses of 3-seconds (or 3-s) waves as observed by MMS at the Earth’s ion foreshock region. We identified intervals of 3-s waves in each of the case studies with a strict frequency selection criteria close to ~0.3 Hz (or 3.3s). Our minimum variance analysis results indicate that these waves are nearly circularly right-handed polarized in the spacecraft frame, consistent with earlier studies. We also utilized multi-spacecraft techniques to determine an anti-sunward propagation direction of the 3-s wave in the plasma rest frame. We found that, in all of the three case studies, the 3-s waves occur sequentially with steepened and compressive 30-s waves, and can modulate or scatter the backstreaming field-aligned ion beam to form complex ion distributions in the terrestrial foreshock. Both phenomena have not been previously reported. We further investigated the instability that generates the 3-s waves based on the ion distributions associated with the observations of 3-s waves using a linear dispersion solver. We concluded that the 3-s waves play an even more important role in wave-particle interactions than previously thought, with implications in the formation and growth of other wave modes in the ion foreshock.
How to cite: Poh, G., Le, G., Blanco-Cano, X., Belbase, P., Romanelli, N., Chen, Y., and Wei, H.: Multi-Case Observations of 3-second Waves at Earth's Foreshock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15462, https://doi.org/10.5194/egusphere-egu26-15462, 2026.
Understanding the balance between charged particle acceleration and loss is central to radiation belt research. Jupiter’s Galilean moons orbit within its intense radiation environment and can act both as sources and sinks of energetic particles. Using observations from the Juno spacecraft, we identify large-scale depletions of energetic electrons along Europa’s orbit. These depletions are too deep to result from direct absorption by the moon alone. Here we show that rapid electron losses, occurring within a timescale shorter than Jupiter’s rotation, are driven by pitch angle scattering via whistler-mode waves co-located with Europa’s orbit. This suggests that Europa maintains a plasma environment capable of sustaining a slot-like region, similar to the one seen in Earth’s Van Allen belts. However, this Jovian slot only partially extends along Europa’s path, implying that additional, unidentified acceleration mechanisms may act to refill the region and maintain high radiation levels close to Jupiter.
How to cite: Long, M., Roussos, E., Ni, B., Ma, Q., Kollmann, P., Zhou, R., Clark, G., Krupp, N., Cao, X., Lu, P., Hao, Y., and Wang, S.: A slot region in the magnetosphere of Jupiter, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11669, https://doi.org/10.5194/egusphere-egu26-11669, 2026.
Posters on site: Tue, 5 May, 08:30–10:15 | Hall X4
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.
