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The low-electron flux variability (increase/decrease) in the Earth's radiation belts could cause low-energy Electron Precipitation (EP) to the atmosphere over auroral and South American Magnetic Anomaly (SAMA) regions. This EP into the atmosphere can cause an extra upper atmosphere's ionization, forming the auroral-type sporadic E layers (Esa) over these regions. The dynamic mechanisms responsible for developing this Esa layer over the auroral region have been established in the literature since the 1960s. In contrast, there are several open questions over the SAMA region, principally due to the absence (or contamination) of the inner radiation belt and EP parameter measurements over this region. Generally, the Esa layer is detected under the influence of geomagnetic storms during the recovery phase, associated with solar wind structures, in which the time duration over the auroral region is considerably greater than the time duration over the SAMA region. The inner radiation belt's dynamic is investigated during a High-speed Solar wind Stream (September 24-25, 2017), and the hiss wave-particle interactions are the main dynamic mechanism able to trigger the Esa layer's generation outside the auroral oval. This result is compared with the dynamic mechanisms that can cause particle precipitation in the auroral region, showing that each region presents different physical mechanisms. Additionally, the difference between the time duration of the hiss wave activities and the Esa layers is discussed, highlighting other ingredients mandatory to generate the Esa layer in the SAMA region. [ABSTRACT FROM AUTHOR]

The low-electron flux variability (increase/decrease) in the Earth’s radiation belts could cause low-energy Electron Precipitation (EP) to the atmosphere over auroral and South American Magnetic Anomaly (SAMA) regions. This EP into the atmosphere can cause an extra upper atmosphere’s ionization, forming the auroral-type sporadic E layers (Esa) over these regions. The dynamic mechanisms responsible for developing this Esa layer over the auroral region have been established in the literature since the 1960s. In contrast, there are several open questions over the SAMA region, principally due to the absence (or contamination) of the inner radiation belt and EP parameter measurements over this region. Generally, the Esa layer is detected under the influence of geomagnetic storms during the recovery phase, associated with solar wind structures, in which the time duration over the auroral region is considerably greater than the time duration over the SAMA region. The inner radiation belt’s dynamic is investigated during a High-speed Solar wind Stream (September 24-25, 2017), and the hiss wave-particle interactions are the main dynamic mechanism able to trigger the Esa layer’s generation outside the auroral oval. This result is compared with the dynamic mechanisms that can cause particle precipitation in the auroral region, showing that each region presents different physical mechanisms. Additionally, the difference between the time duration of the hiss wave activities and the Esa layers is discussed, highlighting other ingredients mandatory to generate the Esa layer in the SAMA region.

The Electron Loss and Fields Investigation with a Spatio-Temporal Ambiguity-Resolving option (ELFIN-STAR, or heretoforth simply: ELFIN) mission comprises two identical 3-Unit (3U) CubeSats on a polar (∼93∘ inclination), nearly circular, low-Earth (∼450 km altitude) orbit. Launched on September 15, 2018, ELFIN is expected to have a >2.5 year lifetime. Its primary science objective is to resolve the mechanism of storm-time relativistic electron precipitation, for which electromagnetic ion cyclotron (EMIC) waves are a prime candidate. From its ionospheric vantage point, ELFIN uses its unique pitch-angle-resolving capability to determine whether measured relativistic electron pitch-angle and energy spectra within the loss cone bear the characteristic signatures of scattering by EMIC waves or whether such scattering may be due to other processes. Pairing identical ELFIN satellites with slowly-variable along-track separation allows disambiguation of spatial and temporal evolution of the precipitation over minutes-to-tens-of-minutes timescales, faster than the orbit period of a single low-altitude satellite (Torbit ∼ 90 min). Each satellite carries an energetic particle detector for electrons (EPDE) that measures 50 keV to 5 MeV electrons with Δ E/E < 40% and a fluxgate magnetometer (FGM) on a ∼72 cm boom that measures magnetic field waves (e.g., EMIC waves) in the range from DC to 5 Hz Nyquist (nominally) with 1 MeV. This broad energy range of precipitation indicates that multiple waves are providing scattering concurrently. Many observed events show significant backscattered fluxes, which in the past were hard to resolve by equatorial spacecraft or non-pitch-angle-resolving ionospheric missions. These observations suggest that the ionosphere plays a significant role in modifying magnetospheric electron fluxes and wave-particle interactions. Routine data captures starting in February 2020 and lasting for at least another year, approximately the remainder of the mission lifetime, are expected to provide a very rich dataset to address questions even beyond the primary mission science objective.

The driver of energetic electron precipitation into Ganymede's atmosphere has been an outstanding open problem. During the Juno flyby of Ganymede on 7 June 2021, Juno observed significant downward‐going electron fluxes inside the bounce loss cone of Ganymede's polar magnetosphere. Concurrently, Juno detected intense whistler‐mode waves, both in the quasi‐parallel and highly oblique directions with respect to the magnetic field line. We use quasi‐linear model to quantify energetic electron precipitation driven by quasi‐parallel and very oblique whistler‐mode waves, respectively, in the vicinity of Ganymede. The data‐model comparison indicates that in Ganymede's lower‐latitude (higher‐latitude) polar region, quasi‐parallel whistler‐mode waves play a dominant role in precipitating higher‐energy electrons above ∼100s eV (∼1 keV), whereas highly oblique waves are important for precipitating lower‐energy electrons below 100s eV (∼1 keV). Our result provides new evidence of whistler‐mode waves as a potential primary driver of precipitating energetic electrons into Ganymede's atmosphere. Plain Language Summary: During the Juno flyby of Ganymede on 7 June 2021, the Juno spacecraft detected energetic electrons precipitating into Ganymede's atmosphere. Simultaneously, Juno detected intense electromagnetic whistler‐mode waves in the vicinity of Ganymede. We use a physics‐based model to quantify the role of the observed whistler‐mode waves in energetic electron precipitation. The comparison between the Juno observation and modeling results reveals that whistler‐mode waves potentially play a dominant role in precipitating energetic electrons into Ganymede's atmosphere over a broad energy range from tens of eV to several hundred keV. Our findings are potentially important for understanding the loss process of energetic electrons in Ganymede's magnetosphere, as well as the generation of Ganymede's diffuse aurora. Key Points: We provide new evidence of whistler‐mode waves as a potential primary driver of precipitating energetic electrons into Ganymede's atmosphereThis finding is potentially important for the generation of aurora and the loss of energetic electrons in the vicinity of GanymedeJuno observations and quasi‐linear modeling are used to quantify energetic electron precipitation driven by whistler‐mode waves [ABSTRACT FROM AUTHOR]

The resonant interaction between a runaway electron (RE) beam and a reactor-grade background plasma is investigated through two-dimensional particle-in-cell simulations, employing a simplified model of the system. The temporal evolutions of the electron momentum distribution function for two separate initial beam energies (1 and 10 MeV) are tracked, revealing the occurrence of plasma wave growth concomitant with pitch angle scattering or momentum distribution diffusion within the RE beam. Notably, we identify and confirm the dependence of the dominant resonance condition on the initial kinetic energy of the RE beam. Furthermore, we quantify the effect of particle-wave interactions on the RE momentum distribution diffusion by assessing the average kinetic energy flux from the RE distribution function.

Abstract The driver of energetic electron precipitation into Ganymede's atmosphere has been an outstanding open problem. During the Juno flyby of Ganymede on 7 June 2021, Juno observed significant downward‐going electron fluxes inside the bounce loss cone of Ganymede's polar magnetosphere. Concurrently, Juno detected intense whistler‐mode waves, both in the quasi‐parallel and highly oblique directions with respect to the magnetic field line. We use quasi‐linear model to quantify energetic electron precipitation driven by quasi‐parallel and very oblique whistler‐mode waves, respectively, in the vicinity of Ganymede. The data‐model comparison indicates that in Ganymede's lower‐latitude (higher‐latitude) polar region, quasi‐parallel whistler‐mode waves play a dominant role in precipitating higher‐energy electrons above ∼100s eV (∼1 keV), whereas highly oblique waves are important for precipitating lower‐energy electrons below 100s eV (∼1 keV). Our result provides new evidence of whistler‐mode waves as a potential primary driver of precipitating energetic electrons into Ganymede's atmosphere.

Plasmaspheric hiss can cause energetic electron precipitation from the magnetosphere to the Earth's upper atmosphere and affect the ionospheric electron density profiles. In this study, we use Arase satellite measurements in the dayside plasmasphere to model the electron precipitation and the resultant ionospheric response, and compare the results to the electron density measured by the Poker Flat Incoherent Scatter Radar (PFISR). We analyzed two close conjunction events between Arase and PFISR at L ∼ 6 in the afternoon sector, when Arase was in the outer plasmasphere and traveled into the plasmaspheric plumes. Modest or strong hiss waves were observed with amplitudes higher than 50 pT during both events. Quasilinear modeling suggests that the hiss waves could cause intense electron precipitation ranging from several keV to several hundred keV energies. The electron density profiles at 60–90 km modeled by the Boulder Electron Radiation to Ionization (BERI) model suggest significant electron density enhancements due to the precipitating electrons. PFISR simultaneously observed electron density enhancements during both events, and provided evidence for the electron precipitation at altitudes down to <70 km. The temporal modulation of hiss caused the modulated density profiles in BERI modeling, but was not evident in PFISR observations. The modeled altitude profiles of the perturbed electron density overall agree with PFISR observation. At altitudes below 75 km, the modeled electron densities are lower than the observation, suggesting additional high energy electron precipitation possibly due to low frequency (<50 Hz) waves or hiss wave powers ducted to high latitudes. Key Points: Two Poker Flat Incoherent Scatter Radar (PFISR)‐Arase conjunction events are identified at the dayside plasmasphere when hiss wave amplitudes reach 50–100 pT at L ∼ 6Hiss‐driven electron precipitation obtained from quasilinear simulation is used to model ionospheric density profiles at 60–90 km altitudeModeled electron densities overall agree with PFISR observations with differences in the temporal modulation and at altitudes below 75 km [ABSTRACT FROM AUTHOR]

Resonant scattering by electromagnetic ion cyclotron (EMIC) waves is one of the most effective mechanisms of relativistic electron losses in Earth's inner magnetosphere. Low‐altitude spacecraft measurements, however, often show that the energy range of precipitating electrons is wider than theoretical predictions based on the cold plasma dispersion of EMIC waves. To explain this discrepancy, we examine the diffusion rates of EMIC waves by including hot plasma effects in their dispersion relation. Using the observed ion distribution functions, we investigate the hot plasma effects on the EMIC wave dispersion for a wide frequency range. We develop analytical equations for hot plasma effects on EMIC dispersion, and apply this model to diffusion rate evaluations. We show that hot ion effects tend to increase the minimum resonant energy for the frequency range around wave intensity maxima, but can decrease the minimum resonant energy for the higher‐frequency part of wave spectra. Plain Language Summary: Quantification of dynamics of relativistic electron fluxes in the Earth's radiation belts is one of the main problems of space plasma physics. Resonance of such relativistic electrons with electromagnetic ion cyclotron (EMIC) waves is considered to provide a very effective electron scattering into the Earth's atmosphere. Energies of electrons resonating with EMIC waves for typical wave characteristics and background plasma conditions rarely fall below 1 MeV, and most of the observed EMIC waves are believed to scatter and precipitate ultra‐relativistic electrons. Conjugate observations from near‐equatorial and low‐altitude spacecraft, however, often show near‐equatorial EMIC waves correlated with precipitation of sub‐MeV electrons below the minimum resonant energy. Such a decrease in resonant energy can be attributed to hot ion contribution to EMIC wave properties. This paper provides a generic hot plasma model which agrees well with the exact numerical solution and highlights the importance of hot plasma effects on the relativistic electron scattering by EMIC waves for a wide range of plasma parameters. Key Points: Hot plasma effects are analyzed and parameterized using analytical equations for observed electromagnetic ion cyclotron (EMIC) wavesHot plasma parametric model agrees very well with numerical results and can be implemented to evaluate diffusion coefficients more preciselyHot plasma effects may decrease the minimum resonant energy of electrons for high‐frequency EMIC waves [ABSTRACT FROM AUTHOR]

The very‐low frequency (VLF) and low frequency (LF) waves from ground transmitters propagate in the ionospheric waveguide, and a portion of their power leaks to the Earth's inner radiation belt and slot region where it can cause electron precipitation loss. Using Van Allen Probes observations, we perform a survey of the VLF and LF transmitter waves at frequencies from 14 to 200 kHz. The statistical electric and magnetic wave amplitudes and frequency spectra are obtained at 1 < L < 3. Based on a recent study on the propagation of VLF transmitter waves, we divide the total wave power into ducted and unducted portions, and model the wave normal angle of unducted waves with dependences on L shell, magnetic latitude, and wave frequency. At lower frequencies, the unducted waves are launched along the vertical direction and the wave normal angle increases during the propagation until reaching the Gendrin angle; at higher frequencies, the normal angle of unducted waves follows the variation of Gendrin angle. We calculate the bounce‐averaged pitch angle and momentum diffusion coefficients of electrons due to ducted and unducted VLF and LF waves. Unducted and ducted waves cause efficient pitch angle scattering at L = 1.5 and 2.5, respectively. Although the wave power from ground transmitters at frequencies higher than 30 kHz is low, these waves can cause the pitch angle scattering of lower energy (2–200 keV at L = 1.5) electrons, which cannot resonate with the VLF transmitter waves at frequencies below 30 kHz, lightning generated whistlers, or plasmaspheric hiss. Key Points: Statistical wave spectral density of transmitter waves at 14–200 kHz is obtained using Van Allen Probes observationDiffusion rates calculated based on wave properties from ray‐tracing indicate the major role of unducted (ducted) waves at lower (higher) LTransmitter waves at 30–200 kHz frequencies cause pitch angle scattering of 2–200 keV electrons at L = 1.5 and 0.2–20 keV electrons at L = 2 [ABSTRACT FROM AUTHOR]

We present a statistical study on the properties of equatorial electron pitch angle distributions (PADs) in the Earth's outer radiation belt, for the first time based on particle measurements from the entire Van Allen Probes mission. A detailed selection criteria is used to identify intervals when flux measurements at energies from 0.2 to 3.4 MeV are available across a wide range of pitch angles close to the geomagnetic equatorial plane. To better characterize the shape of each pitch angle distribution, the flux data is fitted to functions of the equatorial pitch angle based on Legendre polynomials. Using this technique, we show that the shape of the PADs strongly depends on the particle's energy, location and geomagnetic activity. These results are used to identify the dominant physical processes responsible for creating PADs with different shapes. The results presented here mainly focus on the occurrence statistics and properties of butterfly PADs. Significantly, we find clear evidence that butterfly PADs have a peak flux that preferentially occurs at two distinct and discrete equatorial pitch angles, either 35° ± 5° or 65° ± 5°. Energy, L‐shell and magnetic local time variations indicate that the flux peaks at equatorial pitch angles ∼35° appear consistent with magnetopause shadowing, whilst those peaking at ∼65° may be associated with wave‐particle interactions. We also show that the flux at low and high pitch angles, for both butterfly and nonbutterfly PADs, remains remarkably well correlated even as the flux intensity varies by four orders of magnitude. Plain Language Summary: We present a statistical study on the properties and occurrence of equatorial electron pitch angle distributions (PADs) in the Earth's outer radiation belt based on particle measurements from the entire Van Allen Probes mission. The flux data is fitted to functions of the equatorial pitch angle. Using the PAD fits, the characteristics of PADs that have a local flux minimum near equatorial pitch angles of 90°, known as butterfly PADs, are determined. We find clear evidence that these butterfly PADs have a peak flux that preferentially occurs at two distinct and discrete equatorial pitch angles, either 35° ± 5° or 65° ± 5°. Energy, L‐shell and magnetic local time variations indicate that the butterfly PADs with flux peaks at equatorial pitch angles of ∼35° appear consistent with outer radiation belt electron loss through the magnetopause, whilst those peaking at ∼65° may be associated with wave‐particle interactions. We also show that the flux at low and high pitch angles, for both butterfly and nonbutterfly PADs, remains remarkably well correlated even as the flux intensity varies by four orders of magnitude. Key Points: Equatorial butterfly pitch angle distributions have preferred flux peaks at ∼35° or ∼65°, each occurring across different L and MLT rangesThe depth of butterfly distribution dips tends to get smaller as the flux increases, inconsistent with growing off‐equatorial peaksFrom L*∼3 to ∼5, the flux at 90° is highly correlated with the flux at lower pitch angles, even as the flux varies by four orders of magnitude [ABSTRACT FROM AUTHOR]

In this study, we present simultaneous multi‐point observations of whistler‐mode waves detected by RBSP‐B, associated with conjugate electron precipitation observed through enhanced BARREL X‐rays at L ∼ 6 from noon to dusk. Both long period modulation at periods of several to tens of minutes and short period modulation at about tens of seconds are observed in X‐ray measurements. Similar periodicities are also observed for whistler‐mode wave amplitude. We show that the correlation coefficient between whistler‐mode waves and electron precipitation is high in several regions, including plumes and plasma trough. Ultra‐low‐frequency waves (8–30 mHz), which have been suggested to play a potential role in precipitating electrons by modulating whistler‐mode wave intensity or loss cone size, show a weak correlation with whistler‐mode wave amplitudes and the X‐ray counts during the conjunction. We further evaluate whistler‐mode wave driven electron precipitation using a physics‐based technique. The time evolution of the modeled electron precipitation is found to be remarkably consistent with the modulation in the BARREL X‐ray counts both in plumes and plasma trough. By taking advantage of the high‐resolution wave data and close conjunction, we provide strong evidence that whistler‐mode waves are not only directly responsible for the longer modulation (several to tens of minutes), but also the shorter modulation (tens of seconds) of the electron precipitation. Plain Language Summary: Whistler‐mode waves, which are right‐handed polarized electromagnetic plasma waves commonly observed in the Earth's inner magnetosphere, have been known to be efficient in precipitating electrons. Extensive studies have revealed the pitch angle scattering of energetic electrons caused by whistler‐mode waves in the plasmasphere (near‐Earth regions where cold and dense plasmas are corotating with the Earth) or in the plasma trough (regions further away from the Earth where cold plasmas are less dense). However, the role of whistler‐mode waves in the plasmaspheric plumes (the extension of the plasmasphere at further distances from the Earth), especially the temporal and spatial evolution of plume whistler‐mode waves and associated energetic electron precipitation, requires further investigation. In the present study, by taking advantage of multi‐point measurements, we evaluate the time evolution of whistler‐mode wave driven energetic electron precipitation in both the plumes and the plasma trough regions, with waves observed by RBSP‐B near the equator and electron precipitation detected through enhanced X‐rays observed by BARREL. We show that whistler‐mode waves in both the plumes and the plasma trough account for the energetic electron precipitation, which is essential to understand the radiation belt electron dynamics. Key Points: Modulated energetic electron precipitation is observed by BARREL at L ∼ 6 from noon to duskWhistler‐mode waves observed by RBSP‐B are well correlated with electron precipitation both in the plumes and plasma troughModulation of the modeled electron precipitation driven by whistler‐mode waves is consistent with the enhancement in BARREL X‐rays [ABSTRACT FROM AUTHOR]

A newly-developed Imaging Neutral Particle Analyzer (INPA) in the DIII-D tokamak interrogates phase space occupied by fast ions on multiple different orbit topologies, including passing, stagnation, trapped and potato orbits. Depending on plasma parameters and beam injection geometries, this new INPA system is capable of visualizing distributions of fast ions on the selected orbit topology and its associated orbit topological boundaries. More importantly, the system is able to directly visualize the effective pitch angle scattering ν _eff in phase space by measuring fast ions that are scattered across the trapped-passing orbit topological boundaries and from counter-passing orbits to co-passing orbits. It also enables visualization of fast ion confined-loss boundaries and resolves the change of the boundary in phase space, as plasma equilibrium evolves. The key goal of this new INPA system is to directly measure ν _eff across phase space induced by drift waves and its interaction with Alfvén eigenmodes, i.e. a key issue towards a future fusion power plant.

While previous studies suggested whistler mode waves as a potential driver of Jupiter's diffuse aurora, their quantitative contribution to generate diffuse aurora remains unclear. We perform an in‐depth analysis of an intriguing diffuse auroral electron precipitation event using coordinated observations of precipitating electrons and whistler mode waves from the Juno satellite. A physics‐based technique is used to quantify energetic electron precipitation driven by whistler mode waves. We find that the modeled electron precipitation features are consistent with the electron measurements from several keV to several hundred keV over M‐shells of 8–18, while additional mechanisms are needed to explain the observed electron precipitation at lower energies (

A new 3-D diffusion code is used to investigate the inward intrusion and slow decay of energetic radiation belt electrons (>0.5MeV) observed by the Van Allen Probes during a 10day quiet period on March 2013. During the inward transport, the peak differential electron fluxes decreased by approximately an order of magnitude at various energies. Our 3-D radiation belt simulation including radial diffusion and pitch angle and energy diffusion by plasmaspheric hiss and electromagnetic ion cyclotron (EMIC) waves reproduces the essential features of the observed electron flux evolution. The decay time scales and the pitch angle distributions in our simulation are consistent with the Van Allen Probe observations over multiple energy channels. Our study suggests that the quiet time energetic electron dynamics are effectively controlled by inward radial diffusion and pitch angle scattering due to a combination of plasmaspheric hiss and EMIC waves in the Earth's radiation belts.

Due to its importance for global energy dissipation in the ionosphere, the diffuse aurora has been intensively studied in the past 40 years. Its origin (precipitation of 0.5-10 keV electrons from the plasma sheet without potential acceleration) has been generally attributed to whistler-mode chorus wave scattering in the inner magnetosphere (R < ∼8 RE), while the scattering mechanism beyond that distance remains unresolved. By modeling the quasi-linear diffusion of electrons with realistic parameters for the magnetic field, loss cone size, and wave intensity (obtained from Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations as a function of magnetospheric location), we estimate the loss cone filling ratio and electron cyclotron harmonic (ECH) wave-induced electron precipitation systematically throughout the entire data set, from 6 RE out to 31 RE (the THEMIS apogee). By comparing the wave-induced precipitation directly with the equatorially mapped energy flux distribution of the diffuse aurora from ionospheric observations (OVATION Prime model) at low altitudes, we quantify the contribution of auroral energy flux precipitated due to ECH wave scattering. Although the wave amplitudes decrease, as expected, with distance from the Earth, due to the smaller loss cone size and stretched magnetic field topology, ECH waves are still capable of causing sufficient scattering of plasma sheet electrons to account for the observed diffuse auroral dissipation. Our results demonstrate that ECH waves are the dominant driver of the diffuse aurora in the outer magnetosphere, beyond ∼8 RE.

Abstract In this study, we analyze an electromagnetic ion cyclotron (EMIC) wave event of rising tone elements recorded by the Van Allen Probes. The pitch angle distributions of relativistic electrons exhibit a direct response to the two elements of EMIC waves: at the intermediate pitch angle, the fluxes are lower; and at the low pitch angle, the fluxes are higher than those when no EMIC was observed. In particular, the observed changes in the pitch angle distributions are most likely to be caused by nonlinear wave‐particle interaction. The calculation of the minimum resonant energy and a test‐particle simulation based on the observed EMIC waves support the role of the nonlinear wave‐particle interaction in the pitch angle scattering. This study provides direct evidence for the nonlinear pitch angle scattering of electrons by EMIC waves.

Utilizing the DEMETER observations at 670 km, we examined the day‐night difference of energetic electrons (100–800 keV) in the South Atlantic Anomaly (SAA) region and their dependence on geomagnetic activities in different seasons. Under geomagnetically quiet conditions, the fluxes of higher‐energy electrons (>200 keV) in the dusk and midnight (MLT ~ 19–24 hr) are usually larger than those in the morning (MLT ~ 8–12 hr) in the core region of the SAA (−50 ≤ λ ≤ − 20deg) during the northern and southern summers (21 March 2007 to 23 September 2007 and 23 September 2007 to 21 March 2008). The day‐night difference of energetic electrons in SAA region depends not only on the electron energy but also on the geomagnetic activity levels. Enhanced geomagnetic activities increase the energetic electrons in morning and hence weaken their day‐night difference in the SAA region. Key Points: Fluxes of energetic electrons (>200 keV) in SAA region are larger in the nighttime than the morning during geomagnetically quiet timesThe day‐night difference of energetic electrons in SAA region depends not only on the electron energy but also on the geomagnetic conditionsEnhanced geomagnetic activities increase the energetic electrons in morning and hence weaken their day‐night difference in the SAA region [ABSTRACT FROM AUTHOR]

On 22 December 2015, the two Van Allen Probes observed two sets of electromagnetic ion cyclotron (EMIC) wave bursts during a close conjunction when both Probe A and Probe B were separated by 0.57 to 0.68 RE. The EMIC waves occurred during an active period in the recovery phase of a coronal mass ejection‐driven geomagnetic storm. Both spacecraft observed EMIC wave bursts that had similar spatial structure within a 1–2 min time delay. The EMIC waves occurred outside the plasmasphere, within ΔL ≈ 1–2 of the plasmapause and within a few degrees in magnetic latitude of the equatorial plane. The spatial structure of the EMIC wave bursts may have been related to the proton drift paths outside the plasmasphere and influenced by total magnetic field strength variations associated with solar wind pressure enhancements. The EMIC waves were observed in a narrow L shell region from L ≈ 4.55–5.32 between 10 and 11 magnetic local time (MLT) on the outbound halves of the spacecraft orbits and from L ≈ 4.82–5.51 between 13 and 14 MLT on the inbound halves of the spacecraft orbits. However, Pc1 pulsations were observed on the ground over a broad range of local times. The anisotropy of the proton pitch angle distributions was enhanced when the EMIC waves were observed. Although the overall radiation belt response during this storm was dominated by acceleration and transport processes, the EMIC waves produced local pitch angle scattering of 13–15 keV protons and 2.1–2.6 MeV electrons, consistent with calculations of the expected resonant energies. Plain Language Summary: In 1958, James Van Allen discovered Earth was surrounded by rings of electrons and ions, now called the radiation belts. In 2012, NASA launched the twin Van Allen Probes to study the radiation belts. Radiation belt electrons traveling near the speed of light can damage communication and weather satellites and pose a risk to astronauts. Electromagnetic waves from 1–2 cycles every 10 min up to radio frequencies (several thousand cycles per second) are critical to radiation belt physics. Some waves energize the radiation belts, while others decrease their strength. We studied electromagnetic ion cyclotron (EMIC) waves (1–4 cycles per second) during a geomagnetic storm. The Van Allen Probes encountered two EMIC wave activity regions 25,000 km above Earth's surface, one just before local noon and one slightly after local noon. The two satellites were close together and observed the waves in each region simultaneously. The wave activity regions were 2,000–4,000 km wide and 8,000 km across (twice the distance from Washington, DC, to San Francisco). Although this geomagnetic storm increased the global radiation belt strength, EMIC waves ejected electrons and protons from their paths around Earth, temporarily reducing the local radiation belt strength measured by the Van Allen Probes. Key Points: Electromagnetic ion cyclotron waves generated by enhanced solar wind dynamic pressure were observed during the recovery phase of a stormBoth of the Van Allen Probes observed electromagnetic ion cyclotron waves with similar spatial structure outside the dayside plasmapauseElectromagnetic ion cyclotron waves with amplitudes up to 9 nT produced pitch angle scattering of >10 keV protons and ~2 MeV electrons [ABSTRACT FROM AUTHOR]

In this study, we analyze an electromagnetic ion cyclotron (EMIC) wave event of rising tone elements recorded by the Van Allen Probes. The pitch angle distributions of relativistic electrons exhibit a direct response to the two elements of EMIC waves: at the intermediate pitch angle, the fluxes are lower; and at the low pitch angle, the fluxes are higher than those when no EMIC was observed. In particular, the observed changes in the pitch angle distributions are most likely to be caused by nonlinear wave‐particle interaction. The calculation of the minimum resonant energy and a test‐particle simulation based on the observed EMIC waves support the role of the nonlinear wave‐particle interaction in the pitch angle scattering. This study provides direct evidence for the nonlinear pitch angle scattering of electrons by EMIC waves. Key Points: Direct evidence of EMIC‐induced pitch angle scattering of relativistic electrons is observedThe nondiffusive pitch angle distributions of electrons indicate the importance of nonlinear wave‐particle interactionThe calculation of minimum resonance energy and a test‐particle simulation support the observations [ABSTRACT FROM AUTHOR]

Electrons with energies in the keV range play an important role in the dynamics of the inner magnetosphere. Therefore, accurately modeling electron fluxes in this region is of great interest. However, these calculations constitute a challenging task since the lifetimes of electrons that are available have limitations. In this study, we simulate electron fluxes in the energy range of 20 eV to 100 keV to assess how well different electron loss models can account for the observed electron fluxes during the Geospace Environment Modelling Challenge Event of the 2013 St. Patrick's Day storm. Three models (Case 1, Case 2, and Case 3) of electron lifetimes due to wave‐induced pitch angle scattering are used to compute the fluxes, which are compared with measurements from the Van Allen Probes. The three models consider electron losses due to interactions with whistler mode hiss waves inside the plasmasphere and with whistler mode chorus waves outside the plasmasphere. The Case 1 (historical) model produces excessive loss at low L shells before and after the storm, suggesting that it overestimates losses due to hiss during quiet times. During the storm main phase and early recovery all three models show good agreement with the observations, indicating that losses due to chorus during disturbed times are, in general, well accounted for by the models. Furthermore, the more recent Case 2 and Case 3 models show overall better agreement with the observed fluxes. Plain Language Summary: Energetic electrons play a key role in the dynamics of the inner magnetosphere. Consequently, it is of great interest to estimate the electron fluxes in this region. Since the electron lifetimes available to date have limitations, an accurate estimation has been a challenging task. This study aims to assess how well different electron loss models can account for the observed electron fluxes during a geomagnetic storm. Three models of electron losses due to interactions with magnetospheric waves are used in the flux calculations, and the results are then compared with fluxes observed by the Van Allen Probes mission. Our results show that the historical model overestimates the losses at low altitudes before the storm and in the late recovery phase of the storm, indicating that the losses due to interactions with hiss waves in the model are too strong. Furthermore, we find that the more recent models used, based on empirical wave models, are overall in better agreement with the observations over the entire storm. Key Points: Three models of electron lifetimes due to wave‐induced pitch angle scattering in the magnetosphere are comparedElectron flux computed with the lifetimes based on recent empirical wave models show the best agreement with Van Allen Probes observationsThe historical model produces fluxes that are lower than the observed at low L shells, suggesting that it overestimates hiss losses [ABSTRACT FROM AUTHOR]

Cherenkov radiation from a pulse of charge propagating along the magnetic field in a magnetized plasma is analyzed using theory and fluid‐kinetic simulations. Besides radiation into whistler modes, the subject of many previous investigations in laboratory and space, radiation can occur through extraordinary (X) modes. Theory and simulations demonstrate that X mode radiation efficiencies can be orders of magnitude higher than those into whistler modes. Test particle simulations of the dynamics of energetic electrons in the beam‐generated wavefield show that X modes can also induce pitch angle scattering much more efficiently than whistlers. While coherence effects associated with spreading of realistic beam pulses may limit the size of the X mode source region, a simple model of beam dynamics suggests that the size of this region could be substantial (hundreds of meters for ionospheric conditions). These results have potentially important implications for many problems, including understanding losses in the near‐Earth environment and radiation belt remediation. Key Points: Pulsed electron beams can radiate X‐type waves more efficiently than whistler wavesThe total radiated power could be a significant fraction of the beam pulse energyX‐type waves can be more efficient than whistler waves in inducing pitch angle scattering of energetic electrons [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron (EMIC) waves have long been considered to be a significant loss mechanism for relativistic electrons. This has most often been attributed to resonant interactions with the highest amplitude waves. But recent observations have suggested that the dominant energy of electrons precipitated to the atmosphere may often be relatively low, less than 1 MeV, whereas the minimum resonant energy of the highest amplitude waves is often greater than 2 MeV. Here we use relativistic electron test particle simulations in the wavefields of a hybrid code simulation of EMIC waves in dipole geometry in order to show that significant pitch angle scattering can occur due to interaction with low‐amplitude short‐wavelength EMIC waves. In the case we examined, these waves are in the H band (at frequencies above the He+ gyrofrequency), even though the highest amplitude waves were in the He band frequency range (below the He+ gyrofrequency). We also present wave power distributions for 29 EMIC simulations in straight magnetic field line geometry that show that the high wave number portion of the spectrum is in every case mostly due to the H band waves. Though He band waves are often associated with relativistic electron precipitation, it is possible that the He band waves do not directly scatter the sub‐megaelectron volts (sub‐MeV) electrons, but that the presence of He band waves is associated with high plasma density which lowers the minimum resonant energy so that these electrons can more easily resonate with the H band waves. Key Points: Electromagnetic ion cyclotron (EMIC) waves can pitch angle scatter sub‐MeV relativistic electrons and precipitate them into the atmosphereThis occurs through interaction with short‐wavelength waves, which may have smaller amplitude than the dominant wavesH band EMIC waves, with frequency above the He+ gyrofrequency, likely cause the scattering even if the dominant waves are in the He band [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron (EMIC) waves can drive precipitation of tens of keV protons and relativistic electrons, and are a potential candidate for causing radiation belt flux dropouts. In this study, we quantitatively analyze three cases of EMIC‐driven precipitation, which occurred near the dusk sector observed by multiple Low‐Earth‐Orbiting (LEO) Polar Operational Environmental Satellites/Meteorological Operational satellite programme (POES/MetOp) satellites. During EMIC wave activity, the proton precipitation occurred from few tens of keV up to hundreds of keV, while the electron precipitation was mainly at relativistic energies. We compare observations of electron precipitation with calculations using quasi‐linear theory. For all cases, we consider the effects of other magnetospheric waves observed simultaneously with EMIC waves, namely, plasmaspheric hiss and magnetosonic waves, and find that the electron precipitation at MeV energies was predominantly caused by EMIC‐driven pitch angle scattering. Interestingly, each precipitation event observed by a LEO satellite extended over a limited L shell region (ΔL ~ 0.3 on average), suggesting that the pitch angle scattering caused by EMIC waves occurs only when favorable conditions are met, likely in a localized region. Furthermore, we take advantage of the LEO constellation to explore the occurrence of precipitation at different L shells and magnetic local time sectors, simultaneously with EMIC wave observations near the equator (detected by Van Allen Probes) or at the ground (measured by magnetometers). Our analysis shows that although EMIC waves drove precipitation only in a narrow ΔL, electron precipitation was triggered at various locations as identified by POES/MetOp over a rather broad region (up to ~4.4 hr MLT and ~1.4 L shells) with similar patterns between satellites. Key Points: We show three cases of proton and relativistic electron precipitation observed simultaneously with EMIC wavesEMIC‐driven precipitation was observed by POES/MetOp satellites at different locations over a broad L‐MLT regionEach precipitation event extended over ΔL ~ 0.3 on average, showing that wave‐driven pitch angle scattering is localized [ABSTRACT FROM AUTHOR]

Recent studies have found that even during quiet times, observed proton isotropic boundaries (IBs) are often projected to the region of high adiabaticity parameter (K≈30), where K=Rcrg is the ratio of magnetic field line radius of curvature to the particle gyroradius. This contradicts the accepted hypothesis that current sheet scattering (CSS) is the dominant mechanism of IB formation because K≈8 would be expected for this mechanism. We used magnetohydrodynamic simulations and empirical models to compute K for 30‐keV proton IB observations within 3 hr of local midnight. We found that neither class of model reliably estimates K unless supported by magnetic field observations in the current sheet. magnetohydrodynamic simulations produced higher K values than expected for CSS (K = 15–30), and empirical models gave lower values (K < 4). We obtained reliable estimates of K by controlling for the accuracy of the normal component and the gradient of the radial component in the neutral sheet, using observations from three Time History of Events and Macroscale Interactions during Substorms satellites. For the first time, we demonstrated that both these variables should be taken into account for the accurate estimation of the curvature radius. This greatly reduced the spread of K values, indicating that much of the previous spread was due to errors in the magnetic field but also that these errors can be controlled. Most of the corrected values fall within the expected range for CSS, supporting the hypothesis that the IB's were formed by CSS. Accounting for all model results, we obtain an average corrected value of K = 6.0. Key Points: Neither MHD nor empirical models can give reliable estimates of the adiabaticity parameter KA method is proposed to correct model estimates of K using concurrent observations in the magnetotailCorrected estimates of K are close to the K = 8–10 range expected for current sheet scattering [ABSTRACT FROM AUTHOR]

A magnetic conjunction between Van Allen Probes spacecraft and the Balloon Array for Radiation‐belt Relativistic Electron Losses (BARREL) reveals the simultaneous occurrence of broadband Alfvénic fluctuations and multi‐timescale modulation of enhanced atmospheric X‐ray bremsstrahlung emission. The properties of the Alfvénic fluctuations are used to build a model for pitch angle scattering in the outer radiation belt on electron gyro‐radii scale field structures. It is shown that this scattering may lead to the transport of electrons into the loss cone over an energy range from hundreds of keV to multi‐MeV on diffusive timescales on the order of hours. This process may account for modulation of atmospheric X‐ray fluxes observed from balloons and constitute a significant loss process for the radiation belts. Plain Language Summary: Energetic particle populations in Earth's near‐space environment pose existential risks to spacecraft supporting our modern way of life. Understanding the enigmatic rapid variability of these populations is required to mitigate these risks. Coordinated observation from National Aeronautics and Space Administration (NASA)'s Van Allen Probes and Balloon Array for Radiation‐belt Relativistic Electron Losses has discovered a new mechanism through which energetic electrons in Earth's outer radiation belt may be depleted. This mechanism arises through the action of fine‐scale oscillations in Earth's magnetic field that are driven to large amplitudes in space weather events known as geomagnetic storms. These oscillations, known as kinetic Alfven waves, disrupt the orbit of energetic electrons around the Earth so that they precipitate into Earth's atmosphere. Estimates based on observations and theory suggest this mechanism is a significant driver of radiation belt depletion. Key Points: Broadband electromagnetic waves drive radiation belt electrons into the atmosphereFinite gyro‐radii effects and drift‐bounce resonances modify pitch angle distributions of radiation belt electronsBroadband electromagnetic waves may be significant unrecognized drivers of atmospheric X‐ray emission and radiation belt depletion [ABSTRACT FROM AUTHOR]

Abstract: Magnetospheric plasma waves play a significant role in ring current and radiation belt dynamics, leading to pitch angle scattering loss and/or stochastic acceleration of the particles. During a non‐storm time dropout event on 24 September 2013, intense electromagnetic ion cyclotron (EMIC) waves were detected by Van Allen Probe A (Radiation Belt Storm Probes‐A). We quantitatively analyze a conjunction event when Van Allen Probe A was located approximately along the same magnetic field line as MetOp‐01, which detected simultaneous precipitation of >30 keV protons and energetic electrons over an unexpectedly broad energy range (>~30 keV). Multipoint observations together with quasi‐linear theory provide direct evidence that the observed electron precipitation at higher energy (>~700 keV) is primarily driven by EMIC waves. However, the newly observed feature of the simultaneous electron precipitation extending down to ~30 keV is not supported by existing theories and raises an interesting question on whether EMIC waves can scatter such low‐energy electrons. [ABSTRACT FROM AUTHOR]

Abstract: We consider intense electrostatic solitary waves (ESW) observed in a supercritical quasi‐perpendicular Earth's bow shock crossing by the Magnetospheric Multiscale Mission. The ESW have spatial scales of a few tens of meters (a few Debye lengths) and propagate oblique to a local quasi‐static magnetic field with velocities from a few tens to a few hundred kilometers per second in the spacecraft frame. Because the ESW spatial scales are comparable to the separation between voltage‐sensitive probes, correction factors are used to compute the ESW electric fields. The ESW have electric fields with amplitudes exceeding 600 mV/m (oriented oblique to the local magnetic field) and negative electrostatic potentials with amplitudes of a few tenths of the electron temperature. The negative electrostatic potentials indicate that the ESW are not electron phase space holes, while interpretation in terms of ions phase space holes is also questionable. Whatever is their nature, we show that due to the oblique electric field orientation the ESW are capable of efficient pitch‐angle scattering and isotropization of thermal electrons. Due to the negative electrostatic potentials the ESW Fermi reflects a significant fraction of the thermal electrons streaming from upstream (downstream) back to upstream (downstream) region, thereby affecting the shock dynamics. The role of the ESW in electron heating is discussed. [ABSTRACT FROM AUTHOR]

Abstract: We study the origin of the 30 keV proton isotropic boundary (IB) in the nightside auroral zone during geomagnetic storms, particularly, to address the recent results that the adiabaticity parameter K (ratio of the magnetic field line curvature radius to the particle gyroradius at the equator) on the IB field line can be much larger comparing to its theoretical estimate K ∼ 8 for the field line curvature (FLC) scattering mechanism. During nine storms in 2011–2013, we investigate ∼2,000 IBs observed by low‐altitude Polar Operational Environmental Satellites (POES) satellites and apply the TS05 magnetospheric model to estimate the K value in the equatorial part of the IB field line. The statistical distribution of the estimated K parameter, while being rather broad, is centered on K = 9–13. For smaller subset of ∼250 IBs, the concurrent magnetic field measurements on board Time History of Events and Macroscale Interaction During Substorms probes in the equatorial magnetotail were used to correct the estimated K‐values accounting for the TS05 deviations from the real magnetic configuration. After correction, the K distribution becomes narrower, being still centered on K = 9–12. Different estimates give percentages of events with K < 13, which can be attributed to IBs formed by FLC scattering, between 60% and 80%. Finally, we have not found any dependence of the K distribution on magnetic local time and IB latitude, except for events with IB located at extremely low latitudes (<59°). These findings imply that the FLC scattering is a dominant mechanism of IB formation operating in a variety of magnetospheric conditions. [ABSTRACT FROM AUTHOR]

Abstract: Mozer et al. (2017, https://doi.org/10.1002/2017JA024223) suggested that time domain structures (TDSs) drive pulsating aurora (with additional contributions by kinetic Alfvén waves (KAWs)) and that chorus waves have negligible effects. In this comment, we point out that electrons scattered by TDS or KAW (dominantly at ~0.1–3 keV, <1 s modulation) cannot explain key features of pulsating aurora, which require precipitation above a few keV with a couple of tens of second modulation. Their study did not conduct quantitative evaluations of wave‐aurora correlation. The use of short burst mode data (~<10 s) may only cover a single pulse of pulsating aurora and is not suitable for examining connections to pulsating aurora. “Field‐aligned” electrons do not necessarily represent loss cone population, and their characteristic energy (hundreds of eV) is much lower than typical precipitation over pulsating aurora. By reexamining the events studied by Mozer et al., we quantitatively demonstrate that TDS and KAW are uncorrelated with pulsating aurora and that only chorus waves showed high correlations with pulsating aurora. Occasional simultaneous occurrence of TDS/KAW and pulsating aurora is found to be coincidental, because the correlation over a time scale of minutes is poor. Several auroral features analyzed in that paper are not pulsating aurora but other types of aurora. We also show that the chorus‐pulsating aurora correlation can last for 2 h or longer and can be used to highlight dynamic changes in magnetic field mapping. Chorus waves can resonate with electrons above a few keV and are in agreement with pulsating auroral properties. [ABSTRACT FROM AUTHOR]

Abstract: The Juno spacecraft's Jupiter Energetic‐particle Detector Instrument has observed field‐aligned, unidirectional (upgoing) electron beams throughout most of Jupiter's entire polar cap region. The Waves instrument detected intense broadband whistler mode emissions occurring in the same region. In this paper, we investigate the pitch angle scattering of the upgoing electron beams due to interactions with the whistler mode waves. Profiles of intensity versus pitch angle for electron beams ranging from 2.53 to 7.22 Jovian radii show inconsistencies with the expected adiabatic invariant motion of the electrons. It is believed that the observed whistler mode waves perturb the electron motion and scatter them away from the magnetic field line. The diffusion equation has been solved by using diffusion coefficients which depend on the magnetic intensity of the whistler mode waves. [ABSTRACT FROM AUTHOR]

Abstract: Whistler‐mode extremely low frequency hiss emissions commonly exist in the plasmasphere and the plasmaspheric plume and contribute to the precipitation loss of the radiation belt electrons. How these hiss waves are generated remains a critical unanswered question. Here we report the large‐amplitude (up to 1.5 nT) hiss waves in the plasmaspheric plumes, nearly an order of magnitude stronger than previous observations. These waves are found to propagate toward higher latitudes, and the corresponding frequency dependence of wave power can be qualitatively (but not quantitatively) explained by the modeled linear instability of hot electrons near the equator. At the high‐frequency end of hiss spectra, the discrete rising tones are shown to emerge, similar to the situation of whistler‐mode chorus in the plasmatrough. These data and modeling suggest that these large‐amplitude hiss waves were generated within the plasmaspheric plume probably through a combination of linear and nonlinear instabilities of hot electrons. [ABSTRACT FROM AUTHOR]

Juno observed the low-altitude polar region during perijove 1 on 27 August 2016 for the first time. Auroral intensity and false-color maps from the Ultraviolet Spectrograph (UVS) instrument show extensive diffuse aurora observed equatorward of the main auroral oval. Juno passed over the diffuse auroral region near the System III longitude of 120°-150° (90°-120°) in the northern (southern) hemisphere. In the region where these diffuse auroral emissions were observed, the Jupiter Energetic Particle Detector Instrument (JEDI) and Jovian Auroral Distributions Experiment (JADE) instruments measured nearly full loss cone distributions for the downward going electrons over energies of 0.1-700 keV but very few upward going electrons. The false-color maps from UVS indicate more energetic electron precipitation at lower latitudes than less energetic electron precipitation, consistent with observations of precipitating electrons measured by JEDI and JADE. The comparison between particle and aurora measurements provides first direct evidence that these precipitating energetic electrons are mainly responsible for the diffuse auroral emissions at Jupiter. [ABSTRACT FROM AUTHOR]

We investigate the gradual diffusion of energetic electrons from the inner edge of the outer radiation belt into the slot region. The Van Allen Probes observed slow inward diffusion and decay of ~200-600 keV electrons following the intense geomagnetic storm that occurred on 17 March 2013. During the 10 day nondisturbed period following the storm, the peak of electron fluxes gradually moved from L ~ 2.7 to L ~ 2.4, and the flux levels decreased by a factor of ~2-4 depending on the electron energy. We simulated the radial intrusion and decay of electrons using a three-dimensional diffusion code, which reproduced the energy-dependent transport of electrons from ~100 keV to 1 MeV in the slot region. At energies of 100-200 keV, the electrons experience fast transport across the slot region due to the dominance of radial diffusion; at energies of 200-600 keV, the electrons gradually diffuse and decay in the slot region due to the comparable rate of radial diffusion and pitch angle scattering by plasmaspheric hiss; at energies of E > 700 keV, the electrons stopped diffusing near the inner edge of outer radiation belt due to the dominant pitch angle scattering loss. In addition to plasmaspheric hiss, magnetosonic waves and VLF transmitters can cause the loss of high pitch angle electrons, relaxing the sharp 'top-hat' shaped pitch angle distributions created by plasmaspheric hiss. Our simulation indicates the importance of balance between radial diffusion and loss through pitch angle scattering in forming the diffusive intrusion of energetic electrons across the slot region. [ABSTRACT FROM AUTHOR]

How relativistic electrons are lost is an important question surrounding the complex dynamics of the Earth's outer radiation belt. Radial loss to the magnetopause and local loss to the atmosphere are two main competing paradigms. Here on the basis of the analysis of a radiation belt storm event on 27 February 2014, we present new evidence for the electromagnetic ion cyclotron (EMIC) wave-driven local precipitation loss of relativistic electrons in the heart of the outer radiation belt. During the main phase of this storm, the radial profile of relativistic electron phase space density was quasi-monotonic, qualitatively inconsistent with the prediction of radial loss theory. The local loss at low L shells was required to prevent the development of phase space density peak resulting from the radial loss process at high L shells. The rapid loss of relativistic electrons in the heart of outer radiation belt was observed as a dip structure of the electron flux temporal profile closely related to intense EMIC waves. Our simulations further confirm that the observed EMIC waves within a quite limited longitudinal region were able to reduce the off-equatorially mirroring relativistic electron fluxes by up to 2 orders of magnitude within about 1.5 h. [ABSTRACT FROM AUTHOR]