Search

We present the first Chandra X-ray Observatory (CXO) catalog of "pulsar X-ray filaments," or "misaligned outflows." These are linear, synchrotron-radiating features powered by ultrarelativistic electrons and positrons that escape from bow shock pulsars. The filaments are misaligned with the (large) pulsar velocity, distinguishing them from the pulsar wind nebula (PWN) trail, which is also often visible in CXO ACIS images. Spectral fits and morphological properties are extracted for five secure filaments and three candidates using a uniform method. We present a search of archival CXO data for linear diffuse features; the known examples are recovered and a few additional weak candidates are identified. We also report on a snapshot CXO ACIS survey of pulsars with properties similar to the filament producers, finding no new filaments but some diffuse emission, including one PWN trail. Finally, we provide an updated model for the pulsar properties required to create filaments in light of these new observations. [ABSTRACT FROM AUTHOR]

In modelling a relativistic disc around a compact object, the self-gravity of the disc is often neglected while it needs to be incorporated for more accurate descriptions in several circumstances. Extending the Komatsu–Eriguchi–Hachisu self-consistent field method, we present numerical models of a rapidly rotating neutron star with a self-gravitating disc in stationary equilibrium. In particular, our approach allows us to obtain numerical solutions involving a massive disc with the rest mass |$\mathcal {O}(10^{-1})-\mathcal {O}(10^0)\, \mathrm{ M}_\odot$| closely attached to a rotating neutron star, given that the disc is mainly supported by the relativistic electron degeneracy pressure. We also assess the impact of self-gravity on the internal structure of the disc and the neutron star. These axisymmetric, stationary solutions can be employed for simulations involving the neutron star–disc system in the context of high-energy transients and gravitational-wave emissions. [ABSTRACT FROM AUTHOR]

In this paper, the characteristics of two-dimensional magnetosonic (MS) shock waves have been studied in a nonplanar relativistic degenerate collisional magnetoplasma whose constituents are non-degenerate warm ions and relativistic degenerated electrons. Employing fluid model equations for such plasma along with Maxwell equations, a set of magnetohydrodynamic (MHD) model equations is obtained. Based on the newly obtained MHD equations, a Burgers–Kadomstev–Petviashvili (Burger–KP) equation (which describes shock wave structures) is derived in cylindrical geometry using the reductive perturbation technique. The considered plasma system was investigated under the impacts of spin-magnetization, relativistic degeneracy, cylindrical geometry, and dissipation. Numerical results revealed that the relativistic degeneracy, dissipation, and electron spin-magnetization as well as nonplanar geometry significantly altered the MS shock wave properties. Interestingly, it is found that there is a change in the shock nature and emergence of new structures due to the influences of both transverse perturbation and cylindrical geometry. The implications of our investigation may be applicable to dense astrophysical environments, particularly neutron stars, and white dwarfs at which the relativistic degenerated electrons are existed. [ABSTRACT FROM AUTHOR]

Nonlinear gyroresonant acceleration is an essential mechanism for the formation of relativistic electrons in Earth's radiation belts, yet we still have only limited knowledge of how electron motion interacts nonlinearly with plasma waves. Here we demonstrate the compelling suppression of whistler‐mode chorus emissions as a resonant wave with energetic electrons by tuning an external transmitter signal in space plasmas. Through forcible modification of electron motions via phase trapping by the external waves, the nonlinear gyroresonant current is suppressed along with the electron hole deformation. Moreover, this can decrease the population of relativistic electrons due to the lack of coherent resonant waves seen by the phase‐organized electrons. Understanding such nonlinear wave suppression effects is crucial for a potential active control of Earth's plasma environment. Plain Language Summary: Since the strong radiation in space can cause health risks for astronauts and satellite anomalies of electronics, understanding toward the active control of the particle populations is in increasing demand for sustainable space industry growth. The gyroresonant interactions between energetic electrons and whistler‐mode waves play important roles in the acceleration mechanism of the Earth's radiation belt electrons. We find that transmitter signals as an external wave source in space plasmas can utilize the changing of nonlinear electron motion by stimulating electrons in a phase space. We show that nonlinear whistler mode chorus wave excitation, which is the resonant wave source for energetic electrons, can effectively be suppressed by deforming the shape of electron holes to control resonant currents in the phase space. This mechanism is crucial for understanding the active control of Earth's plasma environments. Key Points: Chorus waves can be suppressed by inhibition of coherent interactions with resonant electrons due to adding external transmitter wavesThe external waves having a different frequency from the maximum linear growth can become incoherent from the point of view of electronsThe external waves can precipitate the seed electrons, then this reduces the anisotropy of particles and hence inhibits wave growth [ABSTRACT FROM AUTHOR]

Radial diffusion in planetary radiation belts is a dominant transport mechanism resulting in the energization and losses of charged particles by large-scale electromagnetic fluctuations. In this work we exploit the extensive radial diffusion coefficients (D LL) database created in the framework of the Horizon 2020 SafeSpace project, which spans 9 years of hourly D LL coefficients, to investigate the spatio-temporal distributions of the coefficients. Our results indicate that the radial distribution of the magnetic and electric component of the D LL , as well as their sum, the total D LL , can be well described by a power law function of L* in the [4.3–7.7] range. We show that the L*-dependent spectral index varies significantly and is far from constant as assumed and implemented in many semi-empirical models. We examine the quasi-periodic behavior of the radial profiles of the D LL throughout most of the 24th Solar cycle, which the data cover, and find an approximately 420 days dominant periodicity. This periodic behavior is linked (in terms of cross-wavelet analysis) with solar activity, nevertheless, its origin remains unclear. The uncovered features are important for understanding D LL behavior and drivers as well as for current and future modelling efforts. [ABSTRACT FROM AUTHOR]

• UV-microbursts were measured by a highly sensitive photometer in auroral zone. • UV-microbursts are observed as series of pulses with complex time structure. • The relativistic electron microbursts origin of UV emission is considered. In this paper we present data on the UV-microburst (300–400 nm flashes with a duration less than 1 s) measurements in the auroral zone. Measurements were performed during the period 09.2021–04.2022 by the highly sensitive imaging photometer installed at the Verkhnetulomsky observatory of the Polar Geophysical Institute. It is shown that microbursts are grouped in a series with a duration from 10 s to 10 min. They were observed in relatively quiet geomagnetic conditions (K P < 3) at the southern boundary of the auroral oval in the evening magnetic local time (MLT) sector. UV-microbursts are observed in different observational conditions (clouds, transparent clouds and clear sky) and spatially represent various patterns: uniform diffuse illumination, local spots. Joint analyses of the optical measurements and satellite data on charged particle fluxes demonstrates that an auroral oval, characterized by a plasma, is placed to the north of the observatory. At the same time increased flux of electrons with energy more than 100 keV is observed at the same L-shell and MLT sector. The possible origin of the UV-microbursts is a precipitation of energetic electrons from a poleward boundary of the outer radiation belt in a form of relativistic electron microbursts is discussed. [ABSTRACT FROM AUTHOR]

The role of the density gradient in the electron acceleration process by intense laser pulses for a plasma profile with a steep interface between vacuum and a strongly overdense plasma is investigated via particle-in-cell simulations with the Emi2d code. Laser pulses at relativistic intensities interacting with finite gradients at the laser–plasma interface favor collective electron motion in the underdense plasma provided that the pulse duration is long enough to form a standing wave structure. It is shown that the steepness of the gradient influences the evolution of the distribution of electrons that are injected into the dense plasma. Heating mechanisms of the electron bulk and a very energetic electron tail are identified. The heating of the targets evolves each time when bunches of electrons accelerate to relativistic energies and return to the laser–plasma interface. The heating dynamics have consequences on the ion front motion at the rear of the target. This is elaborated by determining the predominant hot electron populations and their temperatures that govern the expansion of the rear density profile. The role of the temporal dynamics of the hot electron populations is investigated with respect to the known analytic models for ion acceleration at the rear target, showing that those models are robust despite the important temporal increase in hot electron temperatures. [ABSTRACT FROM AUTHOR]

The month of May 2024 was characterized by solar energetic particles events directed towards the Earth, especially the big event causing a strong terrestrial geomagnetic storm during the night from 10 to 11 May 2024, with auroras observed everywhere in Europe. This was the strongest storm for the last 20 years with a Disturbed Storm Time index Dst < −400 nT. In the present work, we show with observations of GOES, PROBA-V/EPT and MetOP/MEPED that this exceptional event was associated with the injection of energetic protons in the proton radiation belt, with important consequences for the South part of the South Atlantic Anomaly (SAA). In addition, the geomagnetic storm caused by the solar eruption has had tremendous impacts on the electron radiation belts. Indeed, we show that for 0.3 to 1 MeV electrons, the storm led to a long lasting four belts configuration which was not observed before with EPT launched in 2013, until a smaller geomagnetic storm took place at the end of June 2024. Moreover, for the first time since its launch, observations of the EPT show that ultra-relativistic electrons with E > 2 MeV have been injected into the inner belt down to McIlwain parameter L = 2.4, violating the impenetrable barrier previously estimated to be located at L = 2.8. [ABSTRACT FROM AUTHOR]

The Relativistic Electron Proton Telescope (REPT) instrument on the Van Allen Probes observed a double‐peaked inner zone proton population throughout the 7 year lifetime of the mission. M. Hudson et al. (2023) showed that a strong SEP event accompanied by a CME‐shock in early March 2012 provided the Solar Energetic Proton (SEP) source for the higher L trapped proton population, which then diffused radially inward to be observed by REPT at L=2 $L=2$. The study followed trajectories of SEP protons launched isotropically from a sphere at 7 Re for 2.5 hr in fields calculated by the LFM‐RCM global MHD model, which includes electric fields needed to model the transport and trapping of the protons by the shock, and then a radial diffusion simulation was run for 2 years using the result from the test‐particle simulation as the initial condition. The simulation result was compared with REPT measurement in November 2013 and showed reasonable agreement. However, the simulation overestimated the Phase Space Density by a factor of four due to lack of field line curvature scattering during the storm in the model. In this study, a test‐particle simulation is performed for 2 days following the injection and trapping of protons in March 2012 using TS05 fields to simulate the field line curvature scattering of the trapped SEP due to the buildup of the ring current during the geomagnetic storm. The resulting sample distribution was then weighted using the flux at the end of the two‐hour MHD‐test particle simulation. A radial diffusion simulation is then run using the initial profile that included the loss effect, with improved comparison with REPT measurements after 2 years. Key Points: Solar Energetic Proton trapping and radial diffusion explain the double‐peaked proton inner zone observed by Van Allen ProbesExtending test particle simulations through the storm using TS05 fields improves data comparison including FLC scattering lossThree simulation timescales are required to model trapping, FLC loss and radial diffusion [ABSTRACT FROM AUTHOR]

Although the effects of electromagnetic ion cyclotron (EMIC) waves on the dynamics of the Earth's outer radiation belt have been a topic of intense research for more than 20 years, their influence on rapid dropouts of electron flux has not yet been fully assessed. Here, we make use of contemporaneous measurements on the same L $L$‐shell of trapped electron fluxes at 20,000 km altitude by Global Positioning System (GPS) spacecraft and of trapped and precipitating electron fluxes at 450 km altitude by Electron Losses and Fields Investigation (ELFIN) CubeSats in 2020–2022, to investigate the impact of EMIC wave‐driven electron precipitation on the dynamics of the outer radiation belt below the last closed drift shell of trapped electrons. During six of the seven selected events, the strong 1–2 MeV electron precipitation measured at ELFIN, likely driven by EMIC waves, occurs within 1–2 hr from a dropout of relativistic electron flux at GPS spacecraft. Using quasi‐linear diffusion theory, EMIC wave‐driven pitch angle diffusion rates are inferred from ELFIN measurements, allowing us to quantitatively estimate the corresponding flux drop based on typical spatial and temporal extents of EMIC waves. We find that EMIC wave‐driven electron precipitation alone can account for the observed dropout magnitude at 1.5–3 MeV during all events and that, when dropouts extend down to 0.5 MeV, a fraction of electron loss may sometimes be due to EMIC waves. This suggests that EMIC wave‐driven electron precipitation could modulate dropout magnitude above 1 MeV in the heart of the outer radiation belt. Key Points: Electron precipitation likely driven by electromagnetic ion cyclotron (EMIC) waves is observed within 1–2 hr of electron flux dropoutsPitch angle diffusion rates inferred from low altitude measurements allow estimating the contribution of EMIC wave‐driven losses to dropoutsEMIC wave‐driven electron precipitation could significantly modulate electron flux dropouts above 1.5 MeV and sometimes down to 0.5 MeV [ABSTRACT FROM AUTHOR]

Context. HESS J1702-420 is a peculiar TeV complex with a morphology changing from a diffuse source (HESS J1702-420B) at ≲2 TeV to a point-like one (HESS J1702-420A) at ≳10 TeV energies. The morphology and spectral properties of HESS J1702-420 could be understood in terms of (diffusive) hadronic or leptonic models in which the observed TeV emission arises correpondingly from proton-proton or IC-radiation of relativistic particles present in the region. Aims. In this work, we perform searches of the X-ray counterpart of the HESS J1702-420B source originating from the synchrotron emission of the primary or secondary relativistic electrons produced within leptonic or hadronic models. Such an emission can be extended and remain beyond the detection capabilities of narrow field-of-view instruments such as XMM-Newton. Methods. We utilised the publicly available first-6-months eROSITA dataset (DR1) fully covering the selected analysis region of > 5°-radius around HESS J1702-420. We discuss biases connected to variable plasma temperature or neutral hydrogen column density in the region and present results based on the background modelling approach. Results. The performed analysis does not allow us to detect the extended X-ray counterpart of HESS J1702-420 of 0.07° −3°-radii sizes. The derived upper limits are significantly higher than the expected hadronic model flux of the X-ray counterpart. For the leptonic model, the derived limits indicate the magnetic field in the region B ≲ 2 μG. We argue that further advances in the diffuse X-ray counterpart searches could be achieved with either next-generation missions or Msec-long observational campaigns using currently operating instruments. [ABSTRACT FROM AUTHOR]

The Shabansky orbit effect can exert a significant influence on the dynamics of particle motion. However, the north‐south asymmetry of the Shabansky orbits has received relatively little attention in previous research. This study investigates this asymmetry to elucidate the recently observed phenomenon of flux separation in the pitch angle distribution during drift echo events. The asymmetry in Shabansky orbits between the northern and southern hemispheres leads to two distinct drift periods for particles with specific equatorial pitch angles, resulting in the flux separation. To quantify this phenomenon, we develop a mathematical model for the magnetic field, enabling the generation of asymmetrical Shabansky orbits. The calculated drift periods for particles with different equatorial pitch angles corresponds with the characteristics of flux separation, providing evidence to support our hypothesis that the north‐south asymmetry of Shabansky orbits is responsible for flux separation. Plain Language Summary: The drift motion represents a typical behavior of particles within the Earth's magnetosphere. Observing particle flux during drift echo events provides us insights into magnetosphere dynamics. In our recent investigation, we discovered a rarely observed phenomenon during a relativistic electron drift echo event: the unexpected separation of flux stripes in the pitch angle distribution. We propose that this phenomenon can be explained by the asymmetry of electrons' Shabansky orbits between the northern and the southern hemispheres. The asymmetry of Shabansky orbits causes some particles to have two different drift periods and lead to the emergence of two different parts in the pitch angle spectrogram. Moreover, we developed a magnetic field model that produces the asymmetrical Shabansky orbits. We calculate the drift periods of particles with different pitch angles, which align with our observation. This suggests that our model can effectively reflect the characteristics of the magnetic field in this case. Our findings indicate that the north‐south asymmetry of electron Shabansky orbits can cause the flux separation in pitch angle distribution. Key Points: Separation of electron flux in pitch angle distribution is observed in drift echo eventsThe north‐south asymmetry effect of Shabansky orbits results in the separation of pitch angle distributionA modified image dipole magnetic model is proposed to study the effect of Shabansky orbits [ABSTRACT FROM AUTHOR]

Acceleration of electrons in vacuum directly by intense laser fields holds great promise for the generation of high-charge, ultrashort, relativistic electron bunches. While the energy gain is expected to be higher with tighter focusing, this does not account for the reduced acceleration range, which is limited by diffraction. Here, we present the results of an experimental investigation that exposed nanotips to relativistic few-cycle laser pulses. We demonstrate the vacuum laser acceleration of electron beams with 100s pC charge and 15 MeV energy. Two different focusing geometries, with normalized vector potential a0 of 9.8 and 3.8, produced comparable overall charge and electron spectra, despite a factor of almost ten difference in peak intensity. Our results are in good agreement with 3D particle-in-cell simulations, which indicate the importance of dephasing. Accelerating electrons in vacuum by intense laser fields is a promising yet experimentally challenging field. Here, the authors demonstrate acceleration of 100's of pC of 15 MeV electrons by shining few-cycle laser pulses on nanotips and further investigate the process by using different focusing geometries that leads to unexpected results. [ABSTRACT FROM AUTHOR]

Since a theory that can justify the condition of matching of the laser and plasma parameters to implement the relativistic self-trapping regime for laser light in a plasma, which is the most efficient for the acceleration of electrons, is still absent, doubts remain in the necessity of the corresponding extensive experimental studies, particularly in view of the lack of justification or completion of previous theories. The presented theoretical analytical approach, which includes the relativistic nonlinearity of the electron mass and electron cavitation in the plasma, allows one to overcome this problem and provides an analytical justification of the conditions of such matching for ultrarelativistic laser beams at the quantitative level, which is in agreement with the multidimensional numerical simulation. [ABSTRACT FROM AUTHOR]

A timing method for experiments on the interaction of a near-infrared laser and an ultra-relativistic electron beam via a semiconductor plasma switch is experimentally validated. As an intermediate medium, a thin Si plate is excited by the energetic, intense electron beam to produce a semiconductor plasma, which in turn deflects counter-colliding laser light having 1 μm wavelength. An electron beam of sub-nC charge sufficiently induces the needed electron number density gradient of 1 × 1020 cm−3 per tens of μm length at the interaction point. Demonstration during an inverse Compton scattering experiment by a counter-colliding electron beam of 300 pC and 70 MeV with an Nd: YAG laser at a wavelength of 1 μm is reported. [ABSTRACT FROM AUTHOR]

Currently, there is no clear understanding of the comprehensive set of variables that controls fluxes of relativistic electrons within the outer radiation belt. Herein, the methodology based on causal inference is applied for identification of factors that control fluxes of relativistic electrons in the outer belt. The patterns of interactions between the solar wind, geomagnetic activity and belt electrons have been investigated. We found a significant information transfer from solar wind, geomagnetic activity and fluxes of very low energy electrons (54 keV), into fluxes of relativistic (470 keV) and ultra‐relativistic (2.23 MeV) electrons. We present evidence of a direct causal relationship from relativistic into ultra‐relativistic electrons, which points to a local acceleration mechanism for electrons energization. It is demonstrated that the observed information transfer from low energy electrons at 54 keV into energetic electrons at 470 keV is due to the presence of common external drivers such as substorm activity. Plain Language Summary: Despite the fact that the discovery of the radiation belts occurred more than 60 years ago, a comprehensive understanding of the physical processes that are involved in the dynamics of the fluxes of relativistic electrons in the outer radiation belt is still lacking. Development of a thorough physical model accounting for the radiation belt dynamics has the potential to assist mitigation of spacecraft hazards caused by energetic particles. Herein, an information‐theoretical approach, based on the methodology of causal inference, is applied for identification of factors that control fluxes of relativistic electrons in the outer belt. The patterns of interactions between the solar wind, geomagnetic activity and belt electrons have been investigated. We have found that inward radial transport from an external source is a less favorable mechanism than local acceleration for the energization of outer radiation belt electrons from relativistic to ultra‐relativistic energies. Key Points: Evidence of direct causality from relativistic into ultra‐relativistic electrons, compatible with local acceleration in the outer beltDetection of information transfer unveils the mechanisms of energy transfer in radiation belts, important for space weather forecastingInformation flow formulation of causality has a great potential for space physics discoveries [ABSTRACT FROM AUTHOR]

The study presents the compilation of a comprehensive catalog of solar energetic protons (SEPs) in solar cycles (SCs) 23 and 24 (1996–2019) in 10 energy channels from about 20 to 100 MeV based on data from the Energetic and Relativistic Nuclei and Electron (ERNE) instrument aboard Solar and Heliospheric Observatory (SOHO). For comparison, we added previously reported SEP fluxes by a number of different sources. We identified the SEP-solar origin in terms of solar flares and coronal mass ejections and calculated the statistical correlations (Pearson and partial) as a function of the SEP energy. [ABSTRACT FROM AUTHOR]

The paper presents the results of studying the dynamics of the magnetic field and electron fluxes of the Earth's outer radiation belt with an energy of >2 MeV according to the GOES-15 geostationary satellite during a fairly long period (October 16, 2016 to February 16, 2017) of moderate and weak magnetospheric activity caused by the arrival of a sequence of high-speed solar wind streams. The main variations in the electron flux in the geostationary orbit are caused by the movement, deceleration and acceleration of particles in the outer radiation belt of the Earth under the influence of geomagnetic activity. The results of a comparative analysis of variations in electron fluxes and components of the magnetospheric field testify to the predominant influence of the magnitude and structure of the magnetospheric field on the dynamics of relativistic electron fluxes in the outer radiation belt. Changes in the components of the magnetospheric magnetic field and in electron fluxes are results of a single process that occurs together with changes in the magnetosphere as a whole. [ABSTRACT FROM AUTHOR]

The radiation pressure and strong magnetic fields are prominent in the structures of Rayleigh–Taylor (R–T) instability in the interior of white dwarfs. This paper investigates the radiation pressure-driven R–T instability in a compressible and magnetized ultra-relativistic degenerate strongly coupled plasma. The equation of state has been derived for such systems incorporating ultra-relativistic degenerate electrons with their radiation pressure and ion gas compressibility. The dispersion relation of the density gradients driven R–T instability is analyzed using the generalized hydrodynamic fluid model in the strongly coupled and weakly coupled limits. It is observed that the R–T instability criterion has been modified significantly due to radiation pressure, ion gas compressibility and degeneracy parameters. In the kinetic limit, the instability region is shorter than the hydrodynamic limit due to the dominance of plasma frequency over the viscoelastic relaxation frequency. The outcomes are explored in analyzing the development of R–T instability in the strongly magnetized carbon–oxygen white dwarfs. The radiation pressure, electron temperature and ion density strongly suppress the growth rate of the R–T instability in the interior of white dwarfs. The strong magnetic fields introduce asymmetry to the system by destabilizing the R–T unstable modes. The present results are also useful for understanding the R–T instability in the star formation and dense plasmas in inertial confinement fusion in some limiting cases. [ABSTRACT FROM AUTHOR]

Context. The long-period O-star binary system HD 168112 and the triple O-star system HD 167971 are well-known sources of non-thermal radio emission that arises from a colliding wind interaction. The wind-wind collisions in these systems should result in phase-dependent X-ray emissions. The presence of a population of relativistic electrons in the wind interaction zone could affect the properties of the X-ray emission and make it deviate from the behaviour expected for adiabatic shocks. Aims. We investigate the X-ray emission of these systems with the goals of quantifying the fraction of the X-ray flux arising from wind interactions and determining whether these emissions follow the predictions for adiabatic wind-wind collisions. Methods. Six X-ray observations were collected with XMM-Newton. Three observations were scheduled around the most recent peri-astron passage of HD 168112. Spectra and light curves were analysed and compared with simple predictions of model calculations for X-ray emission from colliding wind systems. Results. The X-ray emission of HD 168112 varies as the inverse of the orbital separation, as expected for an adiabatic wind interaction zone. The relative contribution of intrinsic X-ray emission from wind-embedded shocks varies between 38% at periastron to 81% at apastron. The wind-wind collision zone remains adiabatic even around periastron passage. The X-ray emission of HD 167971 displays variations on the orbital timescale of the inner eclipsing binary. The existing data of this system do not allow us to probe variations on the timescale of the outer orbit. Conclusions. Shock modification due to the action of relativistic electrons does not seem to be efficiently operating in the HD 168112 system. In the existing observations, a significant part of the emission of HD 167971 must arise in the inner eclipsing binary. The origin of this emission is as yet unclear. [ABSTRACT FROM AUTHOR]

The article presents a theoretical study of Oleinik resonances in the process of scattering a gamma quantum by an ultrarelativistic electron in the field of a strong electromagnetic wave with intensities up to  10 27 Wcm − 2 . The resonant kinematics for three possible resonant reaction channels in a strong external field have been studied in detail. It is shown that under resonant conditions, the scattering channels of the reaction effectively split into two first-order processes according to the fine structure constant, such as the external field-stimulated Compton effect. The annihilation channel of the reaction effectively decays into direct and reverse the external field-stimulated Breit–Wheeler processes. In the absence of interference from the reaction channels, a resonant differential cross-section was obtained in a strong external electromagnetic field. The cases when the energy of the initial electrons significantly exceeds the energy of the initial gamma quanta have been studied. At the same time, all particles (initial and final) fly in a narrow cone away from the direction of wave propagation. The conditions under which the energy of ultrarelativistic initial electrons is converted into the energy of a finite gamma quantum are studied. It is shown that the resonant differential cross-section of such a process significantly (by several orders of magnitude) exceeds the corresponding nonresonant cross-section. This theoretical study predicts a number of new physical effects that may explain the high-energy fluxes of gamma quanta produced near neutron stars and magnetars. [ABSTRACT FROM AUTHOR]

Energetic particle injections are commonly observed in Jupiter's magnetosphere and have important impacts on the radiation belts. We evaluate the roles of electron injections in the dynamics of whistler‐mode waves and relativistic electrons using Juno measurements and wave‐particle interaction modeling. The Juno spacecraft observed injected electron flux bursts at energies up to 300 keV at M shell ∼11 near the magnetic equator during perijove‐31. The electron injections are related to chorus wave bursts at 0.05–0.5 fce frequencies, where fce is the electron gyrofrequency. The electron pitch angle distributions are anisotropic, peaking near 90° pitch angle, and the fluxes are high during injections. We calculate the whistler‐mode wave growth rates using the observed electron distributions and linear theory. The frequency spectrum of the wave growth rate is consistent with that of the observed chorus magnetic intensity, suggesting that the observed electron injections provide free energy to generate whistler‐mode chorus waves. We further use quasilinear theory to model the impacts of chorus waves on 0.1–10 MeV electrons. Our modeling shows that the chorus waves could cause the pitch angle scattering loss of electrons at <1 MeV energies and accelerate relativistic electrons at multiple MeV energies in Jupiter's outer radiation belt. The electron injections also provide an important seed population at several hundred keV energies to support the acceleration to higher energies. Our wave‐particle interaction modeling demonstrates the energy flow from the electron injections to the relativistic electron population through the medium of whistler‐mode waves in Jupiter's outer radiation belt. Key Points: Chorus wave generation due to electron injections is demonstrated by their correlative occurrence and wave instability analysisWhistler‐mode waves scatter electrons into the loss cone and cause diffuse auroral precipitation with intensities of 60–160 erg/cm2/sChorus waves cause local acceleration of MeV electrons in several days, further aided by the seed electron population from injections [ABSTRACT FROM AUTHOR]

Relativistic electron precipitation (REP) refers to the release of high‐energy electrons initially trapped in the outer radiation belt, which then precipitate into Earth's upper atmosphere, contributing significantly to the rapid depletion of radiation belt electron flux. This study presents a statistical analysis of REP observations collected by the Calorimetric Electron Telescope (CALET) experiment aboard the International Space Station from 2015 to the present day. Specifically, the analysis utilizes count rates acquired from the two top scintillators constituting the top charge detector, each sensitive to electrons with energies above 1.5 and 3.4 MeV, respectively. Analysis of CALET data reveals a previously unreported semi‐annual variation in the occurrence of REP events. REP periodicities resemble those observed for trapped electron fluxes in the outer belt. Furthermore, their amplitude follows the overall trend of solar wind high‐speed streams and the solar activity. Plain Language Summary: Relativistic electron precipitation (REP) refers to the release, toward the upper atmosphere, of high energy electrons initially trapped in a torus shaped region around Earth known as the outer Van Allen radiation belt. REP is relevant as it contributes to the fast depletion of the electrons from this region. This study presents a statistical analysis of the REP observations made by the Calorimetric Electron Telescope (CALET) experiment on board the International Space Station (2015–present). Data from CALET experiment reveals a previously unreported 6‐month periodicity similar to those observed for high energy electrons in the outer belt. Key Points: Semi‐annual variation of relativistic electron precipitation (REP) have been observed for the first timeReported periodicities have been compared with those characterizing the outer belt electron fluxesThe temporal variation in the REP and the trapped fluxes were found to be in strong correlation [ABSTRACT FROM AUTHOR]

Interplanetary shocks (IPS) can initiate prompt acceleration of relativistic electrons in the Earth's radiation belt, which is related to the generation and propagation of impulsive electric field (IEF). We investigate the effect of IEF on accelerating radiation belt electrons in the 6 September 2017 IPS event. A "surfing" effect of electrons with respect to the electric field, referring to electrons that drift together with the tailward‐propagating IEF in the duskside, is investigated in this study. Our results show that the maximum increase of electron differential flux is at 3.4 MeV by a factor of 2.2, corresponding to a drift velocity of 531 km/s, which is more consistent with the IPS propagating speed of 621 km/s rather than the fast‐mode speed of 1,074 km/s. We suggest that the effect of IPS propagation is important for radiation belt dynamics, and we highlight the potential importance of the parameter of IPS propagation speed. Plain Language Summary: Acceleration and injection of the relativistic electrons are frequently occurred when interplanetary shocks impinge on the Earth's magnetosphere. The impulsive electric field is the direct driver and its propagation may strongly affect the efficiency of the acceleration. In this study, we use the Van Allen Probes observation to investigate the interplanetary shock‐induced acceleration of the relativistic electrons occurred on 6 September 2017. It is found that the drift velocity of electrons in the energy ranges that experience a maximum increase in differential flux is better correlated with the propagation speed of the interplanetary shock rather than the fast‐mode speed in the inner magnetosphere. A new mechanism of the effect of interplanetary shocks' propagation on radiation belt dynamics is proposed, and the potential importance of the propagation speed of the interplanetary shock on electron acceleration is highlighted. Key Points: The effect of the propagation of the shock‐induced electric field on accelerating radiation belt electrons is investigatedThe drift velocity of electron experiencing maximum flux increase is consistent with the interplanetary shock's propagation speedA new mechanism of how interplanetary shocks' propagation influence the radiation belt dynamics is proposed in this study [ABSTRACT FROM AUTHOR]

In a recent study devoted to the influence of electromagnetic radiation reaction on the motion of radiating charged particles in magnetized black hole spacetimes the authors claim that the tail term cannot be neglected in the complete DeWitt–Brehme equation, putting into doubt the previous papers where such an approximation was used. Here, we demonstrate by using simple dimensional arguments that such a statement is misleading in many astrophysically relevant situations. In the case of relativistic electrons moving around a stellar-mass black hole, the tail term is ignorable if a magnetic field of at least a few Gauss is present.On the other hand, in different situations, the tail term can be relevant, as demonstrated in the case of orbital widening, where it can even amplify the effect. [ABSTRACT FROM AUTHOR]

Self-generated magnetic field and electron jet are observed when ultra-intense lasers (> 1 × 10 18 W / c m 2 ) interact with plasma. It is found that the self-generated magnetic field plays a significant role in the generation of electron jets. The generation mechanism of electron jets under the influence of a self-generated circular magnetic field is examined. It is revealed that magnetic modulation of self-generated magnetic fields can result in the collapse of these fields, consequently leading to the production of electron jets. Furthermore, it has been discovered that the velocity of the electron jets is associated with the maximum growth rate of the modulational instability. As the maximum growth rate of the modulational instability decreases, the velocity of the electron jets is reduced. The present work aids in getting a deeper understanding of the generation of electron jets in relativistic laser plasmas. [ABSTRACT FROM AUTHOR]

Time-of-flight estimates of the kinetic energy of magnetically insulated bunches of moderately relativistic runaway electrons (RAEs) were obtained with the use of an air diode equipped with a drift section. The measurements have demonstrated that the bunch energy can be controlled by changing the conditions for RAE emission in an air diode with a sharply inhomogeneous electric field, which can be done by varying the cathode geometry and material and the length of the cathode-anode gap. It has been demonstrated that if the measured electron current peaks are resolved to within 10 ps, and the bunch duration and spread in RAE emission moments are close to this resolution, the characteristic energies of the bunches can be estimated with an accuracy of ≈10%. Examples of energy measurements are given for paraxial and tubular bunches accelerated at cathode potentials of up to −250 kV. The estimated characteristic energies of the bunches, even not corrected for drift losses in the fill gas, turned out to be greater than the energies that could be achieved for bunches accelerated in a vacuum diode with due account for the variations in diode voltage. [ABSTRACT FROM AUTHOR]

Relativistic electrons in the radiation belts can be transported as a result of wave‐particle interactions (WPI) with ultra‐low frequency (ULF) waves. Such WPI are often assumed to be diffusive, parametric models for the radial diffusion coefficient often being used to assess the rates of radial transport. However, these WPI transition from initially coherent interactions to the diffusive regime over a finite time, this time depending on the ULF wave power spectral density, and local resonance conditions. Further, in the real system on the timescales of a single storm, interactions with finite discrete modes may be more realistic. Here, we use a particle‐tracing model to simulate the dynamics of outer radiation belt electrons in the presence of a finite number of discrete frequency modes. We characterize the point of the onset of diffusion as a transition from separate discrete interactions in terms of wave parameters by using the "two‐thirds" overlap criterion (Lichtenberg & Lieberman, 1992, https://doi.org/10.1007/978‐1‐4757‐2184‐3), a comparison between the distance between, and the widths of, the electron's primary resonant islands in phase space. Further, we find the particle decorrelation time in our model system with typical parameters to be on the timescale of hours, which only afterward can the system be modeled by one‐dimensional radial diffusion. Direct comparison of particle transport rates in our model with previous analytic diffusion coefficient formulations show good agreement at times beyond the decorrelation time. These results are critical for determining the time periods and conditions under which ULF wave radial diffusion theory can be applied. Plain Language Summary: The dynamics of Earth's outer Van Allen radiation belt electrons have up to now been almost exclusively modeled using statistical methods. However, such approaches may not be valid for all scenarios. In this work, we defined a criterion separating the regimes where the dynamics of the outer radiation belt electrons can and cannot be modeled statistically, and in particular using a model based around the concepts of diffusion where averaging over many individual interactions leads to an assessment for the overall behavior of a set, or ensemble, of electrons. We use a test particle‐tracing model to assess the actual dynamics of particle ensembles when perturbed by a type of plasma waves with ultra‐low frequency in space. We showed that there is a distinctive qualitative and quantitative difference between diffusive and the more coherent regimes and identified their point of transition. We further verified that once the system has evolved beyond our derived transition criteria it does indeed match the common statistical predictions, verifying the applicability of a diffusion model after that time. Significantly, however, at earlier times the more correlated system behaves differently and may be characterized by a much faster and coherent transport. Key Points: We derive analytic expressions required for the transition from coherent to diffusive transport applied to ultra‐low frequency (ULF) wave‐particle interactionsWe characterize the behavior using an equivalent dynamical system model, highlighting the importance of the particle decorrelation timeFor decorrelation times longer than typical ULF wavetrains, particle transport rates cannot be estimated under a radial diffusion paradigm [ABSTRACT FROM AUTHOR]

Relativistic electron fluxes in the outer radiation belt exhibit significant variability during geomagnetic storms and substorms. This study investigates rapid relativistic electron enhancements (REE) in the outer radiation belt throughout the entire Van Allen Probes (RBSP) era from October 2012 to October 2019. Utilizing RBSP measurements, we identify 182 rapid REE events characterized by a factor of greater than two increase in relativistic electron fluxes within a half RBSP orbit (approximately 4.5 hr) at L = 4.5–5.5. Approximately 76% of rapid REE events occur during geomagnetic storms. Rapid REEs during storms are concentrated within the 12‐hr period preceding and the 24‐hr period following the end of the storm's main phase. Intense REE are more likely found in storm's main phase compared to moderate REE. Sub‐relativistic and relativistic electron injections are commonly observed during rapid REE. Substorm activities (AL/AE, MPB index) and convection (AU index) are more intense before and during REE, in contrast to the intervals following REEs. The intensity of rapid REE correlates with the strength of substorms and convection. This comprehensive survey suggests that rapid REEs in the outer radiation belt are likely associated with, but not strictly tied to, geomagnetic storms. Enhanced convection and substorm appear to create favorable conditions for rapid REE. These substorms and enhanced convection are likely linked to favorable solar wind conditions for REEs, as documented in previous studies. Key Points: Approximately 76% of rapid relativistic electron enhancements (REE) occur during stormsEnhanced convection and substorm appear to be favorable conditions for rapid REEHigher intensity REEs correlate with higher levels of the AL/AU/AE index [ABSTRACT FROM AUTHOR]

Four-component relativistic treatments of the electron paramagnetic resonance g-tensor have so far been based on a common gauge origin and a restricted kinetically balanced basis. The results of such calculations are prone to exhibit a dependence on the choice of the gauge origin for the vector potential associated with uniform magnetic field and a related dependence on the basis set quality. In this work, this gauge problem is addressed by a distributed-origin scheme based on the London atomic orbitals, also called gauge-including atomic orbitals (GIAOs), which have proven to be a practical approach for calculations of other magnetic properties. Furthermore, in the four-component relativistic domain, it has previously been shown that a restricted magnetically balanced (RMB) basis for the small component of the four-component wavefunctions is necessary for achieving robust convergence with regard to the basis set size. We present the implementation of a four-component density functional theory (DFT) method for calculating the g-tensor, incorporating both the GIAOs and RMB basis and based on the Dirac–Coulomb Hamiltonian. The approach utilizes the state-of-the-art noncollinear Kramers-unrestricted DFT methodology to achieve rotationally invariant results and inclusion of spin-polarization effects in the calculation. We also show that the gauge dependence of the results obtained is connected to the nonvanishing integral of the current density in a finite basis, explain why the results of cluster calculations exhibit surprisingly low gauge dependence, and demonstrate that the gauge problem disappears for systems with certain point-group symmetries. [ABSTRACT FROM AUTHOR]

Terrestrial gamma ray flashes (TGFs) are short bursts of gamma rays occurring during thunderstorms. They are believed to be produced by relativistic runaway electron avalanches (RREAs). It is usually admitted that the number of high‐energy electrons produced in the brightest TGFs remains mostly confined within a range from 1017 to 1019. To understand the constraints in the development of RREAs, we perform self‐consistent simulations using a newly developed model with a finite acceleration region and various injection rates. We find that RREAs should naturally self‐quench for a fixed total number of runaway electrons, and hence a fixed number of bremsstrahlung photons. From the idea that TGF sources quench themselves, we derive a simple equation controlling the total number of runaway electrons. In this framework, the existence of a saturation in the electron density discovered in a previous work places a lower limit on TGF durations. Plain Language Summary: Terrestrial gamma ray flashes (TGFs) are short bursts of high‐energy photons occurring during thunderstorms. They are believed to be produced by energetic electrons accelerating due to the intense electric field, forming a Relativistic Runaway Electron Avalanche (RREA). Discovered fairly recently, many of the TGF features remain unexplained. In this article, we aim to understand the constraint on the number of high‐energy electrons produced during TGFs, that always remains confined between 1017 and 1019. Using a newly developed simulation model, we find that RREAs naturally quench themselves when the number of high‐energy electrons and photons reach the range previously mentioned. Based on a limited number of fundamental processes, we were able to derive a simple equation controlling the total number of runaway electrons and deduce a lower limit for TGF durations. Key Points: There is a maximum low‐energy electron density reachable in Terrestrial Gamma Ray Flashe (TGF) sources when relativistic runaway electron avalanches (RREAs) reach saturation: nesat∼1015 ${n}_{e}^{\mathit{sat}}\sim 1{0}^{15}$ m−3There exists a minimum TGF timescale equal to the RREA timescale: τmin ∼ 1 μsThe self‐quenching of the TGF sources implies a maximum number of electrons Ne ∼ 1017 [ABSTRACT FROM AUTHOR]

Using particle and wave measurements from the Van Allen Probes, a 2‐D Fokker‐Planck simulation model driven by the time‐integrated auroral index (AL) value is developed. Simulations for a large sample of 186 storm‐time events are conducted, demonstrating that the AL‐driven model can reproduce flux enhancement of the MeV electrons. More importantly, the relativistic electron flux enhancement is determined by the sustained strong substorm activity. Enhanced substorm activity results in increased chorus wave intensity and reduced background electron density, which creates the required condition for local electron acceleration by chorus waves to MeV energies. The appearance of higher energy electrons in radiation belts requires a higher level of cumulative AL activity after the storm commencement, which acts as a type of switch, turning on progressively higher energies for longer and more intense substorms, at critical thresholds. Plain Language Summary: The Earth's radiation belts are filled with high‐energy electrons (100s keV–10 MeV) that can damage satellites and impact the expanding human presence in space. The flux of these electrons can change by several orders of magnitude in less than a day. Thus, predicting and understanding their dynamics is crucial. We have developed a physical model driven by the integral auroral index (AL), which influences the amplitude of plasma waves and the density of the background plasma. We simulated a large sample of 186 events from 2012 to 2018 for the first time. The results show that this model not only simulates the acceleration of electrons at high energy levels but also reveals that sustained strong substorms act as a switch. High energy electrons are accelerated only when the intensity of sustained substorms exceeds a certain threshold. Key Points: An AL‐driven radiation belt model reproduces the observed MeV electron fluxes for 186 storm‐time events with good performancePost‐storm flux enhancement requires sufficient integrated substorm strength which increases wave activity and decreases plasma densityAccumulated substorm activity is shown to act as a switch, turning on progressively higher energies at certain critical thresholds [ABSTRACT FROM AUTHOR]

Ultrarelativistic electrons (Ek > 3 MeV) are the most energetic electrons in the Earth's outer radiation belt, which can cause serious damage to equipments on satellites. The evolutions of ultrarelativistic electrons during geomagnetic storm have been well understood, but the effects of continuous geomagnetic storm on ultrarelativistic electrons are still unclear. Using the data of the Van Allen Probes, we study the evolutions of ultrarelativistic electrons in the Earth's outer radiation belt during the three continuous geomagnetic storm events. These continuous geomagnetic storm events include the two geomagnetic storms. During the recovery phase of the first geomagnetic storm, enhanced relativistic and ultrarelativistic electrons with lower energies (> 3.4 MeV) are observed. These enhanced relativistic electrons could be the source of ultrarelativistic electrons and contribute to ultrarelativistic electron acceleration during the second geomagnetic storm. While 3.4 MeV electrons could be further enhanced during the second geomagnetic storm. During the recovery phase of the second small or moderate geomagnetic storm, ultrarelativistic electrons with higher cutoff energies (> 5.2 MeV) and higher fluxes are observed. Compared to an isolated geomagnetic storm with similar solar wind and geomagnetic conditions, ultrarelativistic electrons with higher cutoff energies and higher fluxes are observed during the recovery phase of the second geomagnetic storm. We also find that continuous geomagnetic storm events may contribute even more to enhancements of ultrarelativistic electrons in the outer radiation belt if the second geomagnetic storm is a small or moderate storm with a low solar wind dynamic pressure and short-duration main phase. These can help us to further understand the evolutions of ultrarelativistic electrons in the Earth's outer radiation belt during geomagnetic storms. [ABSTRACT FROM AUTHOR]

The micro Advanced Stellar Compass is an attitude reference for the MAG investigation onboard Juno. The μASC camera head unit images the star field with a CCD that is also sensitive to particles with enough energy to pass through the camera shielding: >15 MeV electrons and >80 MeV protons. This provides the capability to monitor fluxes of high‐energy particles in Jupiter's magnetosphere. A survey of energetic electron fluxes sampled during the first 47 Juno orbits reveals instances of variations observed when Juno is traversing the M‐shell of the Galilean moons. Juno's traversal of the Europa M‐shell often results in distinctly particle signatures. We present the μASC observations of increased electron flux during the crossing of Europa's plasma wake, and depletion of energetic electron flux on the upstream side. The upstream/downstream differences indicate that the wake environment of Europa drives strong pitch angle scattering on relativistic electrons. Plain Language Summary: The attitude reference (μASC) used for the MAG investigation onboard Juno is also sensitive to particles with enough energy to pass through the camera shielding. This provides the capability to monitor fluxes of high‐energy particles in Jupiter's magnetosphere, as the Juno has done since orbit insertion. A μASC survey of electron fluxes reveals instances of variations observed when Juno is traversing the M‐shell of Jupiter's moon Europa. We present particle signatures related to the interaction with Europa and discuss the implications. Key Points: Europa's wake affects the high‐energy electron driftshell by scattering electrons into the loss‐coneIncrease in e− flux is around x2.3 at distances ∼30 RE, and is gradually dissolved as far as 20° downstreamEuropa will stop the energetic electron drift shells and will be mostly free from hard radiation on the leading side [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron (EMIC) waves can very rapidly and effectively scatter relativistic electrons into the atmosphere. EMIC‐driven precipitation bursts can be detected by low‐altitude spacecraft, and analysis of the fine structure of such bursts may reveal unique information about the near‐equatorial EMIC source region. In this study, we report, for the first time, observations of EMIC‐driven electron precipitation exhibiting energy, E, dispersion as a function of latitude (and hence L‐shell): two predominant categories exhibit dE/dL > 0 and dE/dL < 0. We interpret precipitation with dE/dL < 0 as due to the typical inward radial gradient of cold plasma density and equatorial magnetic field (∼65% of the statistics). Precipitation with dE/dL > 0 is interpreted as due to an outward radial gradient of the equatorial magnetic field, likely produced by energetic ions freshly injected into the ring current (∼35% of the statistics). The observed energy dispersion of EMIC‐driven electron precipitation was reproduced in simulations. Plain Language Summary: Relativistic electron precipitation from the equatorial magnetosphere deposits significant energy fluxes to the atmosphere below 50 km, and thus naturally alters the atmosphere ionization and contributes to ozone destruction in the mesosphere. This precipitation is, in good part, due to electron resonant interactions with electromagnetic ion cyclotron (EMIC) waves. Although basic theories of this interaction have been well understood, the detailed electron precipitation pattern, which depends on the background plasma and magnetic field conditions in the wave source regions, are not well studied. In this study, we demonstrate a new property of electron precipitation driven by EMIC waves—the dispersion in energy versus latitude as observed by the low‐altitude ELFIN CubeSats. Such dispersion can provide information about the EMIC wave source region and, as it turns out, connect relativistic electron precipitation with one of the most powerful phenomena in the magnetosphere, substorm plasma injections. Key Points: We report two types of energy versus latitude (or L‐shell) dispersion of relativistic electron precipitation observed at ELFINBoth types of dispersion signatures can be attributed to electron scattering by electromagnetic ion cyclotron (EMIC) wavesEnergy dispersion is controlled by the magnetic field radial profile in the EMIC wave source region [ABSTRACT FROM AUTHOR]

Electron microburst precipitation has been shown to have significant potential for depletion of the outer radiation belt. We present observations from the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) of six (five unique and one dual‐balloon observation) microburst events, each containing minutes to hours of persistent microbursts. We find that each event included a slowly‐varying smooth precipitation component underlying the bursty component. The smooth component has not yet been fully characterized in the literature; we have written a program to identify microburst events and quantify the relative contributions of each component in the BARREL data. In all six events analyzed, the smooth component contributed more to the total X‐ray counts measured, indicating that the smooth component could contribute significantly to radiation belt loss. Plain Language Summary: Earth's radiation belts have been of interest since their discovery in 1958. Understanding and predicting the particle dynamics of the radiation belts can help us to better understand the space environment around Earth, improving our ability to protect the satellites on which our technological society depends. The microburst events studied here are known to have a significant impact on the radiation belts, with the potential to completely deplete them in as little as a few hours. We report here previously unclassified precipitation that occurs concurrently with microbursts. Unlike the short microbursts (each lasting less than a second) the additional component is less intense but is sustained for much longer periods (minutes to several hours). The analysis presented indicates that this smooth component comprises a significant fraction of the depletion potential, implying that the smooth component should be included in radiation belt models and loss rate estimates. Key Points: Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) microburst events comprise at least two precipitation components: a smooth component (minutes–hours) and a bursty component (∼100 ms)An automated algorithm was created to identify microburst events in BARREL data and separate the smooth and bursty precipitation componentsFor the events studied, the smooth component contributed more to the measured X‐ray count rate than the bursty component [ABSTRACT FROM AUTHOR]

• We investigate the ranks of the driving parameters on the relativistic electrons flux at GEO orbit. • We have used cross-correlation, mutual information, and transfer entropy as statistical techniques. • We have found that AE index and solar wind velocity are the dominant driving parameters for relativistic electrons flux at GEO orbit. The relativistic electron population at MeV energy in the Earth's outer radiation belt at geostationary (GOE) orbit largely varies in association with solar wind disturbances. Although more researches have been done to understand the influence of different mechanisms that cause the variations in the relativistic electron flux in the region, but still the underlying physics is often complex and unclear. The present study uses relativistic electron (E > 2 MeV) fluxes data of Los Alamos National Laboratory (LANL) geosynchronous satellites to understand the variations and its relationship with the driving parameters. The study examines the relativistic electron fluxes variations associated with the interplanetary magnetic field (IMF) and solar wind parameters using daily averaged data from 01 January 1990 to 31 December 2018. The results indicated that the information transfer from solar wind and IMF parameters into relativistic electrons flux is dependent on the kind of driving parameter. The result identifies the rank of driving parameters based on their maximum values in the cross correlation and information transfer analysis. Thus, AE index and v sw are the dominant driving parameters for the dynamics of relativistic electrons flux at GEO orbit. The peak values of coefficient (correlation coefficients, mutual information and entropy transfer) are observed at similar lag times ( τ peak ). For instance, in the case of AE and J e , we have observed the maximum coefficient (associated with lag time) as follows; maximum correlation coefficient; CC max = 0.846 (5 days) , maximum mutual information; MI max = 0.803 (5 days) , maximum transfer entropy; TE max = 0.892 (5 days). Such dependency analysis can help to select the model parameters that influence the relativistic electrons fluxes and improve the existed models of the dynamical system. [ABSTRACT FROM AUTHOR]

Aims. Recent low-frequency radio observations at 140 MHz discovered a bridge of diffuse emission with a length of 3 Mpc that connects the galaxy clusters Abell 0399 and Abell 0401. We present follow-up observations at 60 MHz to constrain the spectral index of the bridge, which has only been detected at 140 and 144 MHz so far. Methods. We analysed deep (∼18 h) LOw Frequency ARray (LOFAR) Low Band Antenna (LBA) data at 60 MHz to detect the bridge at very low frequencies. We then conducted a multi-frequency study with LOFAR HBA data at 144 MHz and uGMRT data at 400 MHz. Assuming second-order Fermi mechanisms for the re-acceleration of relativistic electrons driven by turbulence in the radio bridge regions, we compared the observed radio spectrum with theoretical synchrotron models. Results. The bridge is detected in the 75″ resolution LOFAR image at 60 MHz, and its emission fully connects the region between the two galaxy clusters. Between 60 MHz and 144 MHz, we found an integrated spectral index value of α60144 = −1.44 ± 0.16 for the bridge emission. For the first time, we produced spectral index and related uncertainties maps for a radio bridge. We produce a radio spectrum that shows a significant steepening between 144 and 400 MHz. Conclusions. This detection at low frequencies provides important information for models of particle acceleration and magnetic field structure on very extended scales. The spectral index gives important clues about the origin of inter-cluster diffuse emission. The steepening of the spectrum above 144 MHz can be explained in a turbulent re-acceleration framework, assuming that the acceleration timescales are longer than ∼200 Myr. [ABSTRACT FROM AUTHOR]

Recent observations have revealed detailed structures of radio relics across a wide range of frequencies. In this work, we performed three-dimensional magnetohydrodynamical (3D MHD) simulations of merger shocks propagating through a turbulent magnetized intracluster medium. We then employed on-the-fly Lagrangian particles to explore the physical processes behind the origination of radio substructures and their appearance in high and low-frequency observations. We employed two cosmic-ray (CR) electron acceleration models, with a fresh injection of electrons from the thermal pool and the re-acceleration of mildly relativistic electrons. We used the relative surface brightness fluctuations, δSν, to define a "degree of patchiness." First, we found that patchiness is produced if the shock's surface has a distribution of Mach numbers, rather than a single Mach number. Second, radio relics appear patchier if the Mach number distribution consists of a large percentage of low Mach numbers (ℳ ≲ 2.5). Furthermore, as the frequency increases, the patchiness also becomes larger. Nevertheless, if radio relics are patchy at high frequencies (e.g., 18.6 GHz), they necessarily will also be patchy at low frequencies (e.g., 150 MHz). Then, to produce noticeable differences in the patchiness at low and high frequencies, the shock front should have a Mach number spread of σℳ ≳ 0.3 − 0.4. Finally, the extent of the patchiness depends on the Mach number distribution as well as the CR acceleration model. We propose δSν as a potential tool for extracting merger shock properties and information on particle acceleration processes at shocks in radio observations. [ABSTRACT FROM AUTHOR]

Electrons in circular motion emit electromagnetic radiation and lose their energy and angular momentum, both of which are carried away by the radiation field. Electromagnetic radiation from such electrons is not only circularly polarized but also, in general, possessing helical phase structure, the former of which corresponds to spin angular momentum and the latter orbital angular momentum. Based on the classical electrodynamics, we show that the chiral topological property related to the orbital angular momentum arises from deformation of the electromagnetic field due to the relativistic effect. [ABSTRACT FROM AUTHOR]

Dispersive characteristics of electromagnetic sound waves with frequencies less than the electron and ion gyro-frequencies are studied herein analytically and numerically at astrophysical scales. Magnetic quantization of Fermi states is concerned with the degenerate relativistic electrons fluid treated by quantum hydrodynamic model (QHD). The quantum features are included from Landau quantized Fermi pressure dependent upon the dc magnetic field, whereas the ions are treated as nondegenerate and classical. The numerical analysis verifies the analytical results. The phase speed of magnetosonic waves for relativistic degenerate plasma typically for white dwarf stars parameters is depicted from the graphical figures. In this manuscript, an overlooked feature of quantization, that is Landau quantization, is mainly focused for magnetoacoustics in plasmas. [ABSTRACT FROM AUTHOR]

The investigation of waves and instabilities in ultra-relativistic charged particle beam–plasma systems is considerable for a wide class of astrophysical environments. Hence, in this study, using relativistic kinetic plasma theory and Lorentz transformations, we study the interaction of a relativistic electron beam with an ultra-relativistic hot plasma. First, we obtain the dispersion relation of fast and slow longitudinal waves and consequently the growth rate of the slow wave (negative-energy wave) instability. Then, we plot the instability growth rate in terms of different physical parameters, such as the Lorentz factor, the electron beam density, the thermal velocity of electrons, and the temperature of the beam particles, and discuss them further. Our results in the ultra-relativistic regime show that the growth rate of instability decreases by increasing the Lorentz factor and thermal velocity of particles, while the beam density enhances the growth rate of instability. Moreover, a comparison with the results obtained from previous studies shows that the growth rate of instability in the cold electron beam–cold plasma interaction is higher than the instability growth rate in the case of the relativistic electron beam and ultra-relativistic hot plasma. [ABSTRACT FROM AUTHOR]

We present experimental results for the controlled mitigation of the electromagnetic pulses (EMPs) produced in the interactions of a 1 PW high-power 30 fs Ti:Sa laser VEGA-3 with solid-density targets transparent to laser-forward-accelerated relativistic electrons. This study aims at the band of very high frequencies (VHFs), i.e., those in the hundreds of MHz, which comprise the fundamental cavity modes of the rectangular VEGA-3 vacuum chamber. We demonstrate mode suppression by a tailoring of the laser-produced space charge distribution. [ABSTRACT FROM AUTHOR]

Electromagnetic Ion Cyclotron (EMIC) waves mediate energy transfer from the solar wind to the magnetosphere, relativistic electron precipitation, or thermalization of the ring current population, to name a few. How these processes take place depends on the wave properties, such as the wavevector and polarization. However, inferring the wavevector from in‐situ measurements is problematic since one needs to disentangle spatial and time variations. Using 8 years of Magnetospheric Multiscale (MMS) mission observations in the dayside magnetosphere, we present an algorithm to detect proton‐band EMIC waves in the Earth's dayside magnetosphere, and find that they are present roughly 15% of the time. Their normalized frequency presents a dawn‐dusk asymmetry, with waves in the dawn flank magnetosphere having larger frequency than in the dusk, subsolar, and dawn near subsolar region. It is shown that the observations are unstable to the ion cyclotron instability. We obtain the wave polarization and wavevector by comparing Single Value Decomposition and Ampere methods. We observe that for most waves the perpendicular wavenumber (k⊥) is larger than the inverse of the proton gyroradius (ρi), that is, k⊥ρi > 1, while the parallel wavenumber is smaller than the inverse of the ion gyroradius, that is, k‖ρi < 1. Left‐hand polarized waves are associated with small wave normal angles (θBk < 30°), while linearly polarized waves are associated with large wave normal angles (θBk > 30°). This work constitutes, to our knowledge, the first attempt to statistically infer the full wavevector of proton‐band EMIC waves observed in the outer magnetosphere. Key Points: We conduct a statistical analysis of proton‐band Electromagnetic Ion Cyclotron (EMIC) waves in dayside magnetosphere using 8 years of Magnetospheric Multiscale data and measure full wavevectorsThe normalized frequency of EMIC waves presents a dawn‐dusk asymmetryThe perpendicular wavevector is, for most of the waves, larger than the inverse of proton gyroradius and ion inertial lengths [ABSTRACT FROM AUTHOR]

The two most important wave modes responsible for energetic electron scattering to the Earth's ionosphere are electromagnetic ion cyclotron (EMIC) waves and whistler‐mode waves. These wave modes operate in different energy ranges: whistler‐mode waves are mostly effective in scattering sub‐relativistic electrons, whereas EMIC waves predominately scatter relativistic electrons. In this study, we report the direct observations of energetic electron (from 50 keV to 2.5 MeV) scattering driven by the combined effect of whistler‐mode and EMIC waves using ELFIN measurements. We analyze five events showing EMIC‐driven relativistic electron precipitation accompanied by bursts of whistler‐driven precipitation over a wide energy range. These events reveal an enhancement of relativistic electron precipitation by EMIC waves during intervals of whistler‐mode precipitation compared to intervals of EMIC‐only precipitation. We discuss a possible mechanism responsible for such precipitation. We suggest that below the minimum resonance energy (Emin) of EMIC waves, the whistler‐mode wave may both scatter electrons into the loss‐cone and accelerate them to higher energy (1–3 MeV). Electrons accelerated above Emin resonate with EMIC waves that, in turn, quickly scatter those electrons into the loss‐cone. This enhances relativistic electron precipitation beyond what EMIC waves alone could achieve. We present theoretical support for this mechanism, along with observational evidence from the ELFIN mission. We discuss methodologies for further observational investigations of this combined whistler‐mode and EMIC precipitation. Plain Language Summary: Energetic electron precipitation into the upper atmosphere is an important loss process of outer radiation belt fluxes. Whistler‐mode and electromagnetic ion cyclotron (EMIC) waves are two of the most important wave modes responsible for energetic electron scattering to the Earth's ionosphere through wave‐particle interaction. These wave modes typically drive losses of electrons in different energy ranges (above 1 MeV for EMIC waves and tens to hundreds of keV for whistler‐mode waves), occurring in different spatial regions. We report the first observations of energetic electron scattering driven by the combined effect of whistler‐mode and EMIC waves. Our results from equatorial and low‐altitude observations, and a data‐driven test particle simulation explain the wide energy range of electron precipitation from tens of keVs to a few MeVs due to the combined whistler‐mode and EMIC waves effect and explain the unusually high intensity of relativistic electron precipitation at such times. Key Points: We report observations of energetic electron precipitation likely driven by concurrent whistle‐mode and electromagnetic ion cyclotron (EMIC) wavesThe combined scattering of whistler‐mode and EMIC waves leads to electron precipitation over a wide energy range of 50 keVs to a few MeVsThis study highlights the potential nonlinear effects for explaining the observed energetic electron fluxes in the inner magnetosphere [ABSTRACT FROM AUTHOR]

Certain forms of solar wind transients contain significant enhancements of dynamic pressure and may effectively drive magnetosphere dynamics, including substorms and storms. An integral element of such driving is the generation of a wide range of electromagnetic waves within the inner magnetosphere, either by compressionally heated plasma or by substorm plasma sheet injections. Consequently, solar wind transient impacts are traditionally associated with energetic electron scattering and losses into the atmosphere by electromagnetic waves. In this study, we show the first direct measurements of two such transient‐driven precipitation events as measured by the low‐altitude Electron Losses and Fields Investigation CubeSats. The first event demonstrates storm‐time generated electromagnetic ion cyclotron waves efficiently precipitating sub‐relativistic and relativistic electrons from >300 keV to 2 MeV at the duskside. The second event demonstrates whistler‐mode waves leading to scattering of electrons from 50 to 700 keV on the dawnside. These observations confirm the importance of solar wind transients in driving energetic electron losses and subsequent dynamics in the ionosphere. Key Points: A corotating interaction region (CIR) and interplanetary coronal mass ejection (ICME), both including interplanetary shocks, are observed resulting in relativistic electron precipitation from low‐altitude orbitDuskside precipitation after CIR impact is driven by intense electromagnetic ion cyclotron waves, showing distinct energy‐L dispersion from magnetic field distortionDawnside precipitation after ICME impact is driven by intense whistler‐mode waves resonating with electrons at high latitudes [ABSTRACT FROM AUTHOR]

Relativistic electron precipitation (REP) is a relatively high‐latitude phenomenon where high‐energy electrons trapped in the outer radiation belt are lost into the Earth's atmosphere. REP events observed at low Earth orbit show varying temporal profiles and global distributions. While the precipitation origin has been attributed to specific wave modes or scattering sources, the sorting of REP events by type or driver remains an unsolved challenge. In this study, we analyze the temporal profile of relativistic electron precipitation events observed by the CALorimetric Electron Telescope (CALET) experiment on board the International Space Station. We use an unsupervised machine learning technique called Self‐Organizing‐Maps (SOM) to automatically detect and then classify relativistic electron events observed by the two scintillator layers at the top of the apparatus, sensitive to electrons with energies >1.5 MeV and >3.4 MeV, respectively. We calculate the power spectral density (PSD) of the count rates observed by both sensors and use them as an input for the SOM. The SOM technique groups the PSDs by their similarity, resulting in a classification of relativistic electron events by the periodicity of the observed precipitation. We investigate the L‐shell and magnetic local time distribution of the resulting classification, and energy spectral index associated with the observations. Clear precipitation patterns are observed and compared to past precipitation categorization attempts as well as known distributions of various scattering mechanisms. The classification reveals features through the sorting of the variability of the rapid precipitation, allowing the identification of different precipitation populations with varying properties. Plain Language Summary: Fast electrons are normally trapped by the Earth's magnetic field. However, they often get released in bursts and impact the upper layers of the atmosphere near the poles. The underlying processes are still not well understood and debated. In this study we use an unsupervised artificial intelligence technique called Self‐Organizing‐Maps (SOM) to automatically detect and classify the observations made by a charged particle detector onboard the International Space Station (ISS). The SOM categorizes the bursts based on their variability and group together observations by their similarity. We compare the categorization with the spatial location of the electron bursts. Clear patterns are observed and compared with past categorizations attempts. Key Points: Relativistic Electron Precipitation (REP) is observed by the CALET experiment from the International Space StationThe Self‐Organizing‐Map technique is used for automatic detection and classification of rapidly varying REP intervalsThe Self‐Organizing‐Map distinguish between different REP populations [ABSTRACT FROM AUTHOR]

We observed five clusters of upper‐level compact intracloud discharges (CIDs) moving positive charge up over land and over water in Florida. The clusters each contained 3 to 6 CIDs, and the overall cluster duration ranged from 27 to 58 s. On average, the CIDs in a given cluster occurred 11 s apart and were separated by a 3D distance of about 1.5 km. All the clustered CIDs were located above the tropopause and were likely associated with convective surges that penetrated the stratosphere. The average periodicity of CID occurrence within a cluster (every 11 s) was comparable to the periodicity at which the average cluster area is expected to be bombarded by ≥1016 eV cosmic‐ray particles (every 5 s). Each of such energetic particles gives rise to a cosmic ray shower (CRS) and, in the presence of sufficiently strong electric field over a sufficiently large distance, to a relativistic runaway electron avalanche (RREA). We infer that each of our upper‐level CIDs is likely to be caused by a CRS‐RREA traversing, at nearly the speed of light, the electrified overshooting convective surge and triggering, within a few microseconds, a multitude of streamer flashes along its path, over a distance of the order of hundreds of meters (as per the mechanism recently proposed for lightning initiation by Kostinskiy et al., 2020, https://doi.org/10.1029/2020JD033191). The upper‐level CID clustering was likely made possible by the recurring action of energetic cosmic rays and the rapid recovery of the negative screening charge layer at stratospheric altitudes. Plain Language Summary: Compact intracloud discharges (CIDs) are non‐conventional lightning discharges inside the cloud that are characterized by a relatively small spatial extent (less than 1 km), very strong HF‐VHF (3–300 MHz) electromagnetic emissions, and characteristic wideband radiofrequency field pulses. It has been recently discovered, using space‐based detectors, that CIDs also produce optical signals, the so‐called blue flashes at cloud tops. Our understanding of the physics of CIDs is still poor. CIDs are usually solitary events, but we observed five clusters of CIDs, mostly at altitudes in the 18–20 km range, which were likely associated with kilometer‐scale convective surges that penetrated the stratosphere. Each cluster contained 3 to 6 CIDs, and the cluster duration ranged from 27 to 58 s. On average, the CIDs in a given cluster occurred 11 s apart and were separated by a 3D distance of about 1.5 km. Some CIDs within a cluster were essentially collocated, while being separated in time by up to several tens of seconds. We inferred that the CID clustering observed in our study was made possible by (a) the recurring action of omnipresent energetic cosmic rays and (b) the rapid recovery of the negative screening charge layer at stratospheric altitudes. Key Points: 5 clusters of 3–6 compact intracloud discharges (CIDs) moving positive charge up in convective surges overshooting the tropopause in Florida were examinedCID clustering was tight in both time and space: average time interval was 11 s and average 3D distance within a cluster was 1.5 kmCID clustering was made possible by recurring action of ≥1016 eV cosmic‐ray particles and rapid recovery of negative screening charge layer [ABSTRACT FROM AUTHOR]

The presence of the orbital collapse of the g-orbitals in the excited state of the atom with the nuclear charge number Z = 124 and in the ground state of the atom with Z = 125, which belong to the 8th period of the extended periodic table, is demonstrated. In both cases, the orbital collapse occurs within the same configuration when increasing the total angular momentum J, which characterizes the relativistic term of the atom in the jj coupling. All calculations are performed by means of the relativistic Dirac–Fock method. [ABSTRACT FROM AUTHOR]