Abstract. Data from the FAST spacecraft are used to study the temporal progression of the energy inputs to the dayside cusp and the nightside aurora, including Poynting flux, electron number flux and amplitude of extremely low frequency (ELF) waves, during a storm driven by CME (Coronal Mass Ejections), and the resulting H+ and O+ outflows. The results show that (1) On the dayside, Poynting flux, ELF waves activity and soft electron precipitation are all enhanced during the initial and main phases of the storm, and decrease during the recovery phases. On the nightside, the Poynting flux increases during the initial and main phase, but the enhancements are smaller than on the dayside. The variations in the ELF wave activity and electron precipitation are similar before and during the storm. (2) The energy inputs are strongly correlated with the solar wind – magnetosphere coupling functions, dΦMP/dt and p1/2dΦMP/dt, especially in the dayside cusp region where the energy inputs and the ion outflows are localized. (3) The O+ and H+ ion outflow flux, fO+ and fH+, and the flux ratio fO+/fH+ all increase during the storm. Both the fluxes and the flux ratio reach their peaks on the initial phase and are enhanced during the main phase. Nightside auroral H+ and O+ outflows have lower outflow number fluxes than that in the dayside cusp region. These observations show how the solar wind changes characteristics of CME storms and results in strong sustained ion outflow during the initial and main phases.

Recent studies reveal that ionospheric irregularities remain poorly understood due to the lack of models and observations capable of explaining the characteristics of the observed wave structures. In this paper, the effect of energetic electrons is investigated as a driver of such irregularities. Using a particle-in-cell (PIC) simulation framework, we model a two-temperature plasma representative of disturbed ionospheric conditions during energetic electron precipitation. The simulations demonstrate that a minority population of hot electrons provides sufficient free energy to excite electron-acoustic instabilities, which evolve nonlinearly to generate broadband electrostatic fluctuations and field-aligned density structures on kilometre- to decametre-scales. Among these, the decametre-scale modes emerge as the strongest, exhibiting the highest spectral power. These irregularities, in turn, strongly influence the propagation of propagating electromagnetic waves. For high-frequency (HF) signals, the interaction produces scattering, spectral broadening, and envelope distortion, while for ultra-high-frequency (UHF) signals, the effects manifest as localized amplitude growth and amplitude modulation. Since the strongest modes are at decameter scales, they strongly distort HF waves, whose wavelengths are comparable. In contrast, UHF waves, with centimeter-scale wavelengths, interact weakly with these low-power irregularities and experience only minor effects. These results establish a direct physical pathway linking energetic electron precipitation to ionospheric irregularities and to the degradation of radio and GNSS signals. The findings provide both a kinetic-level explanation of observed degradation phenomena and a framework for improving space weather models and forecasting capabilities.

Abstract Energetic electron precipitation plays a pivotal role in shaping Earth's radiation belt dynamics and drives significant physical and chemical changes in the upper atmosphere. However, the detailed mechanisms governing the loss of relativistic electrons have remained unclear, largely due to the limited energy coverage and coarse resolution of previous measurements. Here we report high‐resolution observations of bursty electron precipitation across a broad energy range (0.3–2.3 MeV), obtained by the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. REPTile‐2 employs a novel instrument design that minimizes background to enable clean spectral measurements with the highest energy resolution achieved to date in low‐Earth orbit for this energy range. During the conjunction events when CIRBE was close to the same field line with Arase satellite at higher altitudes, our analysis shows that pitch angle diffusion driven by chorus waves can fully account for the observed three bursty precipitation events over the entire energy range. These results provide the definitive evidence for a unified chorus‐driven electron loss process acting across a wide energy range and underscore the critical importance of high‐resolution measurements in resolving long‐standing uncertainties in radiation belt dynamics. Furthermore, they offer new insight into the energy‐dependent atmospheric impacts of electron precipitation, with broad implications for space weather forecasting and upper atmospheric chemistry.

Effects of the May 10–13, 2024 extreme magnetic storm in the Asian region of Russia have been studied using experimental data from vertical and oblique sounding of the ionosphere with a continuous chirp signal. Features of ionospheric disturbances induced by the magnetic storm have been revealed: the long-lasting negative ionospheric disturbance that was manifested as a significant decrease in F2-layer critical frequencies and maximum observed frequencies of radio paths; the absence of HF signal reflections from F-region due to sporadic Es layer and increased absorption of HF signals; recording of auroral and oblique Es layers; the long-lasting G-effect during local daytime during which the F1-layer critical frequency exceeded the F2-layer critical frequency; the dusk enhancement of electron density and F2-layer peak height. We have found a correlation of variations in ionospheric parameters and the maximum observed frequencies of HF radio wave propagation modes with spatial location of the main ionospheric trough and the equatorial boundary of the diffuse electron precipitation zone.

Effects of the May 10–13, 2024 extreme magnetic storm in the Asian region of Russia have been studied using experimental data from vertical and oblique sounding of the ionosphere with a continuous chirp signal. Features of ionospheric disturbances induced by the magnetic storm have been revealed: the long-lasting negative ionospheric disturbance that was manifested as a significant decrease in F2-layer critical frequencies and maximum observed frequencies of radio paths; the absence of HF signal reflections from F-region due to sporadic Es layer and increased absorption of HF signals; recording of auroral and oblique Es layers; the long-lasting G-effect during local daytime during which the F1-layer critical frequency exceeded the F2-layer critical frequency; the dusk enhancement of electron density and F2-layer peak height. We have found a correlation of variations in ionospheric parameters and the maximum observed frequencies of HF radio wave propagation modes with spatial location of the main ionospheric trough and the equatorial boundary of the diffuse electron precipitation zone.

The paper analyzes experimental data on the position of the polar wall of the main ionospheric trough under low geomagnetic activity at Kr=0–1 from measurements made at the Yakutsk chain of vertical and oblique sounding ionosondes. The northern boundary of the trough under these conditions shifts to high magnetic latitudes 67–70°. This corresponds to the position of the geophysical structure “contracted oval” or compressed oval. Critical frequencies at the polar wall of the trough have high values ​​of about 6–8 MHz. At this time, the DMSP satellite records intense 200–300 eV electron precipitation that can create the observed ionization in the F-region of the ionosphere.

The paper analyzes experimental data on the position of the polar wall of the main ionospheric trough under low geomagnetic activity at Kr=0–1 from measurements made at the Yakutsk chain of vertical and oblique sounding ionosondes. The northern boundary of the trough under these conditions shifts to high magnetic latitudes 67–70°. This corresponds to the position of the geophysical structure “contracted oval” or compressed oval. Critical frequencies at the polar wall of the trough have high values ​​of about 6–8 MHz. At this time, the DMSP satellite records intense 200–300 eV electron precipitation that can create the observed ionization in the F-region of the ionosphere.

The impact of energetic electron precipitation on nighttime atmospheric ozone during geomagnetic storms

Abstract We analyze the solar wind (SW) properties associated with relativistic electron precipitation (REP) observed at low‐Earth orbit. A statistical analysis is performed on the SW associated with ∼7,000 REP events likely driven by wave‐particle interactions. We analyze OMNI data to quantify the SW properties prior to REP and reveal temporal patterns that are favorable for electron precipitation. Compared to typical values, SW associated with REP typically exhibits a stronger North‐South interplanetary magnetic field B z component and higher plasma density, indicating dayside reconnection and compression. REP events observed at low L shell ( L ≲ 4), particularly from noon to post‐midnight, are triggered by enhanced B z and density. Dayside REP is associated with slightly faster SW, whereas dawnside REP coincides with high plasma density without strong B z . A typical SW trend leading to REP exhibits enhanced density, magnetic field increase, and a B z minimum before the REP observation. Through k ‐means clustering, we further identify three distinct SW temporal profiles. Dayside REP at high L shells is associated with ∼500 km/s speeds, duskside REP is associated with enhanced SW density prior to REP, and REP over noon‐to‐dusk at low L shells is triggered by strong dayside reconnection. Furthermore, we found that REP events are more frequent during the declining phase of the solar cycle, with a 6‐month periodicity in occurrence rate. The findings in this study are critical to establishing a relationship between SW and REP, enabling future modeling efforts to predict REP from SW observations.

Abstract Field‐line curvature scattering (FLCS) within the plasma sheet–to–outer radiation belt transition region (hereafter PS2ORB ) serves as a key driver of energy‐latitude dispersion in energetic electron precipitation observed at low latitudes. This precipitation forms the isotropy boundary of electrons ( IBe pattern ), located between the isotropic keV electron fluxes of the plasma sheet and the anisotropic relativistic fluxes of the outer radiation belt. During geomagnetically active periods, the PS2ORB region becomes populated with plasma sheet injections that introduce various transient electron precipitation mechanisms, significantly complicating the structure of the IBe pattern . In this study, we show that the timescales of these precipitations can reach subsecond levels, allowing them to be interpreted as microbursts. Observations of such microbursts substantially enhance the spatial and temporal variability of the IBe pattern . By combining low‐altitude ELFIN satellite measurements with high‐temporal‐resolution (40 ms) near‐UV imaging photometer data from the Pulsating Aurora Imaging Photometers System project, we separate between FLCS‐driven precipitation patterns that form the IBe pattern and electron scattering by whistler‐mode waves, which generates microbursts. We identify, for the first time, the near‐colocation of these two precipitation mechanisms within the PS2ORB region–an important feature not previously reported.

Abstract The center of the South American Magnetic Anomaly (SAMA), located in southern Brazil, is characterized by enhanced energetic particle precipitation (EPP) at low energies (<40 keV), which can significantly impact the ionosphere during intense geomagnetic storms. Typically confined to high latitudes, sporadic auroral E layers (Es a ) have been observed near the SAMA center, particularly during storm recovery phases. However, during the intense geomagnetic storm on 10 May 2024, the Es a layer was detected for the first time over the equatorial Brazilian station Belém (BLM, 1.45°S, 48.49°W, dip = −2.5°). Simultaneously, blanketing Es layers were also observed during the storm's recovery phase, indicating that wind shear mechanisms were also occurring. Satellite data revealed that EPP‐induced ionization extended equatorward beyond the central region of the SAMA, reaching latitudes not previously associated with such effects. Concurrently, disturbed electric fields led to a weakening of the equatorial electrojet (EEJ), inhibiting the development of typical equatorial plasma irregularities. Numerical simulations using the E Region Ionospheric Model confirmed that low‐energy electron precipitation with energy (E) < 2 keV contributed to the observed enhancement in Es layer electron density. These findings provide the first evidence that the occurrence of the Es a layers at equatorial latitudes results from a combination of physical processes, including EPP, wind shear dynamics, and electrodynamic disturbances.

Abstract Recent studies have shown that global geomagnetic activity and particle precipitation are enhanced, when the dawn‐dusk component of the interplanetary magnetic field (IMF) and Earth's magnetic dipole tilt angle have opposite signs. This IMF effect has been demonstrated so far only in statistical studies, which do not reveal its timescales or dynamical evolution. In this paper we study the response of electron precipitation, field‐aligned currents (FACs) and auroral electrojets during events where the IMF rotates from northward or southward orientation to dawn/dusk orientation. The events are collected from years 1995–2022 separately for the winter and summer seasons, resulting into 573–730 events for each dipole tilt and rotation category. Our results show that the IMF ‐effect in the nightside FACs and in the westward electrojet takes 2 hr to develop and 3 hr to maximize, regardless of whether the dusk‐ or dawnward IMF turning occur from a predominantly northward or southward IMF sector. On the dayside, the IMF ‐effect was found to be faster with timescales less than 1 hr, especially in the summer hemisphere. The observed 2–3 hr timescales in the nightside suggests that an asymmetry build‐up in the magnetosphere by convecting open magnetic field lines is required to manifest the IMF ‐effect. We show that the amount of open flux in the magnetosphere responds rapidly to IMF rotations and that the open flux is larger for the opposite signs of the IMF and , suggesting a physical mechanism modulating the dayside reconnection rate.

Cyclotron resonance between electromagnetic waves and plasmas may be a universal acceleration phenomenon of charged particles in magnetized planets. Coherent fine structures of whistler-mode waves serve as a signature of nonlinear resonance. However, the fine wave structures at Mercury have remained unknown due to limited spacecraft observations. Here we show that plasma wave observations by the third BepiColombo mission Mercury flyby (2023) have identified discrete whistler-mode emission waves similar to those observed in Earth’s magnetosphere. The frequency sweep rates of Mercury’s wave chirping tones correspond to those at Earth, based on the scaling law for planetary magnetospheric size. Furthermore, although the spatial coverage on the dayside and dusk sectors is insufficient, the spatial characteristics of Mercury’s whistler-mode waves during all the Mercury flybys (2021 to 2025) reveal an asymmetric dawn-to-night sector, which suggests nonlinear growth characterized by the distorted magnetospheric shape. These spatiotemporal features strongly indicate that electron precipitation events occur primarily in the active wave (dawn-side) region through nonlinear resonant mechanisms similar to those in Earth’s magnetosphere. This study highlights the potential significance of nonlinear resonant processes in shaping Mercury’s unique plasma environment within its small magnetosphere.

Abstract Pitch‐angle diffusion is a key mechanism driving radiation belt electron loss into the atmosphere. However, traditional bounce‐averaged models, which assume complete atmospheric absorption for loss cone electrons, cannot accurately quantify the loss cone fluxes. This shortfall arises primarily from two issues: bounce‐averaging assumptions generally breakdown within the loss cone, and atmospheric backscatter effects are omitted. In this study, we use a modified bounce‐averaged pitch‐angle diffusion model to show that the inclusion of backscatter effects inside the loss cone can adequately estimate the pitch‐angle distribution of electrons inside the loss cone for the case of Electromagnetic Ion Cyclotron (EMIC) wave‐driven precipitation. In this modified model, atmospheric backscatter effects are incorporated as an additional backscatter‐induced pitch‐angle diffusion (), and an attenuation of atmospheric absorption (). Here, we quantify the values of and using Geant4‐based Monte Carlo atmospheric backscatter simulation and applied to model electron precipitation driven by EMIC waves. Comparisons with ELFIN satellite observations during EMIC events reveal that the inclusion of atmospheric backscatter resolves previously reported discrepancies at electron energies . For electrons, where EMIC wave diffusion is dominant over backscatter‐induced diffusion, backscatter effects become negligible. Notably, we find that accounts for nearly all the observed enhancements in the loss cone flux, while plays a secondary role.

Abstract Polar cap patches and polar cap arcs are two polar phenomena that typically occur under a southward and a northward interplanetary magnetic field (IMF), respectively. However, their morphology and plasma characteristics are significantly different due to completely different generation mechanisms. Here, we present multi‐instrument observations of coexisting polar cap patches and arcs (distance ∼500 km) in an all‐sky imager field of view as IMF Bz turns northward. When IMF Bz turns northward, the resulting sunward flow will prolong the evolution time of patches from the dayside to the nightside. A polar cap arc can form 1–2 hr after the northward turning, and it is driven by electron precipitation near the nightside poleward auroral boundary. As a consequence, both slow‐moving polar cap patches and polar cap arcs may appear at close proximity in the nightside polar cap. Statistical results show that such events generally occur under specific IMF conditions, such as the IMF turning from southward to northward, the northward to southward direction constantly jumping back and forth, etc. The polar cap arc is sometimes accompanied by increased auroral brightness and GNSS total electron content, although weaker than that when the patches exist alone. Simultaneous echoes from the Hankasalmi SuperDARN radar indicate the formation of ionospheric irregularities, where polar cap arcs may be a potential source for these irregularities (like patches).

Resonant interactions between electromagnetic whistler-mode waves and energetic electrons play a key role in controlling electron flux dynamics in Earth's radiation belts and driving electron precipitation into the upper atmosphere. Although this process is well investigated and modeled under the assumption of a dipole magnetic field, the dipole approximation often breaks down in plasma injection regions, where strong currents of hot ions significantly deform the magnetic field configuration. In these regions, spacecraft often detect intense whistler-mode waves, whereas ground-based observations suggest enhanced electron precipitation. In this study, we combine quasi-linear theory, extended to account for the non-dipole magnetic field configuration, and spacecraft observations of whistler-mode waves to quantify wave–particle interactions. We demonstrate that electron scattering by these waves is largely affected by non-dipole magnetic fields. We also provide a simple empirical fit for realistic electron scattering rates, which can be readily incorporated into existing global models of electron dynamics.

Abstract A unique rippling luminous structure near the main aurora, lacking a red glow and smaller in scale, has been documented. These ripples are not caused by electron or ion precipitation along geomagnetic field lines. We have characterized these Fragmented Aurora‐like Emissions (FAEs) through statistical analysis, revealing that they appear as green, periodic structures near the poleward edge of a strong aurora arc, especially when the arc retreats from higher latitudes. Moreover, FAEs tend to appear earlier when the aurora arc retreats more rapidly from its maximum expansion. The distances between the ripples range from 4 to 5 km. We present new evidence supporting the hypothesis that FAEs are caused by plasma gradient drift instability ripples occurring near the aurora ionosphere. This research expands our understanding of aurora dynamics and the physical processes in aurora regions; FAEs can be regarded as a signal of the environment around aurora reach to the gradient drift instability favorable conditions.

Abstract. We extended the Linearized ozone scheme – LINOZ in the ICON (ICOsahedral Nonhydrostatic) – ART (the extension for Aerosols and Reactive Trace gases) model system to include NOy formed by auroral and medium-energy electrons in the upper mesosphere and lower thermosphere, and the corresponding ozone loss, as well as changes in the rate of ozone formation due to the variability of the solar radiation in the ultraviolet wavelength range. This extension allows us to realistically represent variable solar and geomagnetic forcing in the middle atmosphere using a very simple ozone scheme. The LINOZ scheme is computationally very cheap compared to a full middle atmosphere chemistry scheme, yet provides realistic ozone fields consistent with the stratospheric circulation and temperatures, and can thus be used in climate models instead of prescribed ozone climatologies. To include the reactive nitrogen (NOy) produced by auroral and radiation belt electron precipitation in the upper mesosphere and lower thermosphere during polar winter, the so-called energetic particle precipitation indirect effect, an upper boundary condition for NOy has been implemented into the simplified parameterization scheme of the N2O/NOy reactions. This parameterization, which uses the geomagnetic Ap index, is also recommended for chemistry-climate models in the CMIP6 experiments. With this extension, the model simulates realistic “tongues” of NOy propagating downward in polar witner from the model top in the upper mesosphere into the mid-stratosphere with an amplitude that is modulated by geomagnetic activity. We then expanded the simplified ozone description used in the model by applying LINOZ version 3. The additional ozone tendency from NOy is included by applying the corresponding terms of the version 3 of LINOZ. This NOy, coupled as an additional term in the linearized ozone chemistry, led to significant ozone losses in the polar upper stratosphere in both hemispheres which is qualitatively in good agreement with ozone observations and model simulations with EPP-NOy and full stratospheric chemistry. In a subsequent step, the tabulated coefficients forming the basis of the LINOZ scheme were provided separately for solar maximum and solar minimum conditions. These coefficients were then interpolated to ICON-ART using the F10.7 index as a proxy for daily solar spectra (UV) variability to account for solar UV forcing. This solar UV forcing in the model led to changes in ozone in the tropical and mid-latitude stratosphere consistent with observed solar signals in stratospheric ozone.

Abstract Electromagnetic ion cyclotron (EMIC) waves are commonly observed electromagnetic emissions in Earth's magnetosphere and are widely considered to efficiently scatter relativistic electrons into bounce loss cones. However, their precise scattering effects remain highly debated due to limited energy coverage and coarse resolution of previous measurements. Here, we present high‐energy‐resolution measurements of EMIC‐induced relativistic electron precipitation from the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. A long duration >1 MeV electron precipitation event was measured by CIRBE/REPTile‐2 in both the northern and southern hemispheres on 25 April 2023. The energy versus L dispersions of these >1 MeV precipitating electrons show good agreement with minimum resonance energies of electrons interacting with He + band EMIC waves at specific frequencies. These novel observations unveil the detailed scattering effect of EMIC waves and provide important clues regarding wave‐particle interaction processes near the equator.

Abstract We present electron measurements from the Acute Precipitating Electron Spectrometer (APES) that flew on the VISualizing Ion Outflow via Neutral atom imaging during a Substorm‐2 (VISIONS‐2) sounding rocket which launched from Svalbard, Norway into dayside cusp aurora. APES measured the precipitating electron distribution from 200 eV to 16 keV with one millisecond time resolution. These observations show that the electron precipitation associated with the cusp aurora contains highly variable and short‐lived features. These features exhibited distinct energy‐time dispersions, some of which were very short in time (lasting 50 ms or less) while others extended to high energies (10 keV), for short times (100 milliseconds). Time‐of‐flight analyses were completed for eight separate dispersed features and their source altitudes ranged from 1,715 to 6,321 km above Earth's surface. This suggests that electromagnetic wave activity is interacting with the precipitating electrons within 1 of Earth's surface.