Electron Flux Enhancements Research Articles

The First Intense Geomagnetic Storm Event Recorded by the China Seismo‐Electromagnetic Satellite

AbstractOn 25 August 2018, the China Seismo‐Electromagnetic Satellite (CSES) encountered the first intense geomagnetic storm event since its launch on 2 February 2018. The main purpose of this work is to check in‐flight performance of the assembled payloads onboard CSES, as well as to investigate the ionosphere perturbations induced by this geomagnetic storm. The study shows that all investigated parameters simultaneously respond to the different phases of the geomagnetic storm, verifying the measuring capabilities of the assembled payloads onboard CSES. Specifically, the magnetic field obtained from the high‐precision magnetometer fits well with that obtained by the Swarm satellite, clearly demonstrating the development phase of the storm; joint analysis using the Langmuir probe, plasma analyzer, electric field detector, and GNSS occultation receiver data demonstrate that this is a positive storm event and that electric field penetration is the possible mechanism for the disturbance in the ionosphere. During this storm event, some significant ELF/VLF waves are also excited and there is enhancement of the energetic electron flux (of energy <1 MeV). These main features are consistent with results from previous works, indicating the excellent performance of the search coil magnetometer and high‐energy particle detector.

Relativistic electron acceleration by surface plasma waves excited with high intensity laser pulses

The process of high energy electron acceleration along the surface of grating targets (GTs) that were irradiated by a relativistic, high-contrast laser pulse at an intensity $I=2.5\times 10^{20}~\text{W}/\text{cm}^{2}$ was studied. Our experimental results demonstrate that for a GT with a periodicity twice the laser wavelength, the surface electron flux is more intense for a laser incidence angle that is larger compared to the resonance angle predicted by the linear model. An electron beam with a peak charge of ${\sim}2.7~\text{nC}/\text{sr}$ , for electrons with energies ${>}1.5~\text{MeV}$ , was measured. Numerical simulations carried out with parameters similar to the experimental conditions also show an enhanced electron flux at higher incidence angles depending on the preplasma scale length. A theoretical model that includes ponderomotive effects with more realistic initial preplasma conditions suggests that the laser-driven intensity and preformed plasma scale length are important for the acceleration process. The predictions closely match the experimental and computational results.

Superposed Epoch Analysis of the Energetic Electron Flux Variations During CIRs Measured by BD‐IES

AbstractThe flux variations of energetic electrons is one of the most important topics to understand dynamic processes in the space environment and the forecast for high‐energy electron burst. BeiDa Image Electron Spectrometer (BD‐IES), an imaging energetic elecstron spectrometer onboard a Chinese navigation satellite at an inclined geosynchronous orbit, can provide 50–600 keV electron flux data, which is used to investigate electron flux variations at GEO orbit during Co‐rotating Interaction Region events (CIRs). The superposed epoch analysis is applied to study the electron flux variations in different energy channels of BD‐IES during CIRs and the results support previous works. It reveals that electron fluxes in different channels all have a dropout before or at CIR interface and then the recovery of electrons with low energy reaches maximum earlier than that of electrons with high energy, with longer time differences for larger energy gap, which is consistent of two major mechanisms for energizing particles: inward radial diffusion and local acceleration. In addition, we investigate the relationship between electron flux enhancement in each energy channel and magnetic locals time and the extent of enhancement for electron flux peak value compared with average value before interface. These results can contribute to our understanding of electron flux variations at GEO during CIRs, and thus lay foundations for forecast research on high‐energy electron enhancement during CIRs.

Comparison of Multiple and Logistic Regression Analyses of Relativistic Electron Flux Enhancement at Geosynchronous Orbit Following Storms

AbstractMany factors influence relativistic outer radiation belt electron fluxes, such as waves in the ultralow frequency (ULF) Pc5, very low frequency (VLF), and electromagnetic ion cyclotron (EMIC) frequency bands, seed electron flux, Dst disturbance levels, substorm occurrence, and solar wind inputs. In this work we compared relativistic electron flux poststorm versus prestorm using three methods of analysis: (1) multiple regression to predict flux values following storms, (2) multiple regression to predict the size and direction of the change in electron flux, and (3) multiple logistic regression to predict only the probability of the flux rising or falling. We determined which is the most predictive model and which factors are most influential. We found that a linear regression predicting the difference in prestorm and poststorm flux (Model 2) results in the highest validation correlations. The logistic regression used in Model 3 had slightly weaker predictive abilities than the other two models but had the most value in providing a prediction of the probability of the electron flux increasing after a storm. Of the variables used (ULF Pc5 and VLF, seed electrons, substorm activity, and EMIC waves), the most influential in the final model were ULF Pc5 waves and the seed electrons. IMF Bz, Dst, and solar wind number density, velocity, and pressure did not improve any of the models, and were deemed unnecessary for effective predictions.

Modeling the Electron Flux Enhancement and Butterfly Pitch Angle Distributions on L Shells <2.5

AbstractWe analyze an energetic electron flux enhancement event in the inner radiation belt observed by Van Allen Probes during an intense geomagnetic storm. The energetic electron flux at L~1.5 increased by a factor of 3 with pronounced butterfly pitch angle distributions (PADs). Using a three‐dimensional radiation belt model, we simulate the electron evolution under the impact of radial diffusion, local wave‐particle interactions including hiss, very low frequency transmitters, and magnetosonic waves, as well as Coulomb scattering. Consistency between observation and simulation suggests that inward radial diffusion plays a dominant role in accelerating electrons up to 900 keV and transporting the butterfly PADs from higher L shells to form the butterfly PADs at L~1.5. However, local wave‐particle interactions also contribute to drive butterfly PADs at L ≳ 1.9. Our study provides a feasible mechanism to explain the electron flux enhancement in the inner belt and the persistent presence of the butterfly PADs at L~1.5.

Mesoscale Convection Structures Associated With Airglow Patches Characterized Using Cluster‐Imager Conjunctions

AbstractPolar cap ionospheric plasma flow studies often focus on large‐scale averaged properties and neglect the mesoscale component. However, recent studies have shown that mesoscale flows are often found to be collocated with airglow patches. These mesoscale flows are typically a few hundred meters per second faster than the large‐scale background and are associated with major auroral intensifications when they reach the poleward boundary of the nightside auroral oval. Patches often also contain ionospheric signatures of enhanced field‐aligned currents and localized electron flux enhancements, indicating that patches are associated with magnetosphere‐ionosphere coupling on open field lines. However, magnetospheric measurements of this coupling are lacking, and it has not been understood what the magnetospheric signatures of patches on open field lines are. The work presented here explores the magnetospheric counterpart of patches and the role these structures have in plasma transport across the open field‐line region in the magnetosphere. Using red‐line emission measurements from the Resolute Bay Optical Mesosphere Thermosphere Imager, and magnetospheric measurements made by the Cluster spacecraft, conjugate events from 2005 to 2009 show that lobe measurements on field lines connected to patches display (1) electric field enhancements, (2) Region 1 sense field‐aligned currents, (3) field‐aligned enhancements in soft electron flux, (4) downward Poynting fluxes, and (5) in some cases enhancements in ion flux, including ion outflows. These observations indicate that patches highlight a localized fast flow channel system that is driven by the magnetosphere and propagates from the dayside to the nightside, most likely being initiated by enhanced localized dayside reconnection.

Variation of Radiation Belt Electron Flux During CME‐ and CIR‐Driven Geomagnetic Storms: Van Allen Probes Observations

AbstractRelativistic electron flux responses in the inner magnetosphere are investigated for 28 magnetic storms driven by corotating interaction region (CIR) and 27 magnetic storms driven by coronal mass ejection (CME), using data from the Relativistic Electron‐Proton Telescope instrument on board Van Allen Probes from October 2012 to May 2017. In this present study we analyze the role of CIRs and CMEs in electron dynamics by sorting the electron fluxes in terms of averaged solar wind parameters, L‐values, and energies. The major outcomes from our study are the following: (i) At L = 3 and E = 3.4 MeV, for >70% cases the electron flux remains stable, while at L = 5, for ~82% cases it changes with the geomagnetic conditions. (ii) At L = 5, ~53% of the CIR storms and 30% of the CME storms show electron flux increase. (iii) At a given L‐value, the tendency for the electron flux variation diminishes with the increasing energies for both categories of storms. (iv) In case of CIR‐driven storms, the electron flux changes are associated with changes in Vsw and Sym‐H. (v) At L ~ 3, CME storms show increased electron flux, while at L ~ 5, CIR storms are responsible for the electron flux enhancements. (vi) During CME‐ and CIR‐driven storms, distinct electron flux variations are observed at L = 3 and L = 5.

Global Occurrences of Auroral Kilometric Radiation Related to Suprathermal Electrons in Radiation Belts

AbstractAuroral kilometric radiation (AKR) can potentially produce serious damage to space‐borne systems by accelerating trapped radiation belt electrons to relativistic energies. Here we examine the global occurrences of AKR emissions in radiation belts based on Van Allen Probes observations from 1 October 2012 to 31 December 2016. The statistical results (1,848 events in total) show that AKR covers a broad region of L= 3–6.5 and 00–24 magnetic local time (MLT), with a higher occurrence on the nightside (20–24 MLT and 00–04 MLT) within L= 5–6.5. All the AKR events are observed to be accompanied with suprathermal (∼1 keV) electron flux enhancements. During active geomagnetic periods, both AKR occurrences and electron injections tend to be more distinct, and AKR emission extends to the dayside. The current study shows that AKR emissions from the remote sources are closely associated with electron injections.

Simulation of Prompt Acceleration of Radiation Belt Electrons During the 16 July 2017 Storm

AbstractWe investigate the prompt enhancement of radiation belt electron flux observed by the Relativistic Electron Proton Telescope instrument on board Van Allen Probes following the 16 July 2017 CME‐shock compression using MHD‐test particle simulations. The prompt enhancements can be explained by the source population interacting with the azimuthally directed electric field impulses induced by CME‐shock compressions of the dayside magnetopause. Electrons in drift resonance with the electric field impulse were accelerated by ∼ 0.6 MeV on a drift period timescale (in minutes) as the impulse propagated from the dayside to the nightside around the flanks of the magnetosphere. MHD test particle simulation of energization and drift phase bunching, due to the bipolar electric field that accompanies the dayside compression and relaxation, is found to be consistent with Van Allen Probes observations. This study reproduces the energy‐dependent drift echoes integrated over pitch angle and observed change in spectra for the first time.

Highly Relativistic Electron Flux Enhancement During the Weak Geomagnetic Storm of April–May 2017

AbstractWe report observations of energetic electron flux and phase space density to show that a relatively weak magnetic storm with Sym‐Hmin≈−50 nT, resulted in a relativistic and ultrarelativistic electron enhancement of two orders of magnitude similar to the St. Patrick's event of 2015, an extreme storm with Sym‐Hmin≈−235 nT. This enhancement appeared at energies up to ≈10 MeV, lasted for at least 24 days, and was not recorded in geosynchronous orbit where most space weather alert data are collected. By combined analysis of phase space density radial profiles and Fokker‐Planck simulation, we show that the enhancement of relativistic and ultrarelativistic electrons is caused by different mechanisms: first, chorus waves during the intense substorm injections of 21–25 April accelerate the seed electron population to relativistic energies and redistribute them while inward diffusion driven by Pc5 Ultra‐Low Frequency (ULF) waves further accelerates them to ultrarelativistic energies.

The Effects of Geomagnetic Storms and Solar Wind Conditions on the Ultrarelativistic Electron Flux Enhancements

AbstractUsing data from the Relativistic Electron Proton Telescope on the Van Allen Probes, the effects of geomagnetic storms and solar wind conditions on the ultrarelativistic electron (E > ~3 MeV) flux enhancements in the outer radiation belt, especially regarding their energy dependence, are investigated. It is showed that, statistically, more intense geomagnetic storms are indeed more likely to cause flux enhancements of ~1.8‐ to 7.7‐MeV electrons, though large variations exist. As the electron energy gets higher, the probability of flux enhancement gets lower. To shed light on which conditions of the storms are preferred to cause ultrarelativistic electron flux enhancement, detailed superposed epoch analyses of solar wind parameters and geomagnetic indices during moderate and intense storms with/without flux enhancements of different energy electrons are conducted. The results suggest that the storms with higher solar wind speed, sustained southward interplanetary magnetic field Bz, lower solar wind number density, higher solar wind Ey, and elevated and sustained substorm activity are more likely to cause ultrarelativistic electron flux enhancements in the outer belt. Comparing results of different energy electrons, the solar wind speed and AE index are the two parameters mostly correlated with the energy‐dependent acceleration of ultrarelativistic electrons: Storms with higher solar wind speed and elevated and sustained substorm activity are more likely to cause flux enhancement of ultrarelativistic electrons with higher energies. This suggests the important roles of inward radial diffusion as well as the source and seed populations provided by substorms on the energy‐dependent acceleration of ultrarelativistic electrons.

Contribution of ULF Wave Activity to the Global Recovery of the Outer Radiation Belt During the Passage of a High‐Speed Solar Wind Stream Observed in September 2014

AbstractEnergy coupling between the solar wind and the Earth's magnetosphere can affect the electron population in the outer radiation belt. However, the precise role of different internal and external mechanisms that leads to changes of the relativistic electron population is not entirely known. This paper describes how ultralow frequency (ULF) wave activity during the passage of Alfvénic solar wind streams contributes to the global recovery of the relativistic electron population in the outer radiation belt. To investigate the contribution of the ULF waves, we searched the Van Allen Probes data for a period in which we can clearly distinguish the enhancement of electron fluxes from the background. We found that the global recovery that started on 22 September 2014, which coincides with the corotating interaction region preceding a high‐speed stream and the occurrence of persistent substorm activity, provides an excellent scenario to explore the contribution of ULF waves. To support our analyses, we employed ground‐ and space‐based observational data and global magnetohydrodynamic simulations and calculated the ULF wave radial diffusion coefficients employing an empirical model. Observations show a gradual increase of electron fluxes in the outer radiation belt and a concomitant enhancement of ULF activity that spreads from higher to lower L‐shells. Magnetohydrodynamic simulation results agree with observed ULF wave activity in the magnetotail, which leads to both fast and Alfvén modes in the magnetospheric nightside sector. The observations agree with the empirical model and are confirmed by phase space density calculations for this global recovery period.

Local Generation of High‐Frequency Plasmaspheric Hiss Observed by Van Allen Probes

AbstractThe generation of a high‐frequency plasmaspheric hiss (HFPH) wave observed by Van Allen Probes is studied in this letter for the first time. The wave has a moderate power spectral density (∼10−6 nT2/Hz), with a frequency range extended from 2 to 10 kHz. The correlated observations of waves and particles indicate that HFPH is associated with the enhancement of electron flux during the substorm on 6 January 2014. Calculations of the wave linear growth rate driven by the fitted electron phase space density show that the electron distribution after the substorm onset is efficient for the HFPH generation. The energy of the contributing electrons is about 1–2 keV, which is consistent with the observation. These results support that the observed HFPH is likely to be generated locally inside the plasmasphere due to the instability of injected kiloelectron volt electrons.

Effect of Electron Irradiation and Operating Voltage on the Deep Dielectric Charging Characteristics of Polyimide

Space Solar Power Station (SSPS) will operate in geosynchronous orbit. Its operating environment is rather complicated, including space plasma, energetic particles radiation, extreme temperature, and cosmic rays. To reduce power loss, SSPS has to apply high-voltage (HV) power generation and transmission technique. Deep dielectric charging will be a major problem in the design of insulation structure in SSPS. In this paper, the hopping conductivity of polyimide was studied. Hopping distance and hopping barrier height were obtained from surface potential decay analysis. Second, a deep dielectric charging model of polyimide under FLUMIC spectrum electron irradiation was established, in which the hopping conductivity parameters were applied. The effects of electron flux enhancement and operating voltage on the charging characteristics were discussed in four cases, i.e., A) suspended-HV; B) HV-suspended; C) grounded-HV; and D) HV-grounded. When the electron flux enhancement increases, the influence on case A is much larger than the other three cases. When the operating voltage increases, the influence on cases A and B is small, whereas the influence on cases C and D increases fast. Thus, when designing the SSPS, the effect of operating voltage and electron flux enhancement on the insulation system must be carefully considered.

Experimental study of fast electron generation from intense laser irradiated mylar foil with thin metal coating on front or rear surfaces

We report angular and spectral distribution of fast electrons in the interaction of a high intensity laser pulse (30 fs, I∼1019 W/cm2) with uncoated transparent mylar foil (thickness: 8 μm). The effect of thin Al coating (50 nm) on either the front or rear surface of the mylar foil on fast electron generation was also investigated. An electron flux enhancement of ∼1.8× (from ∼45 pC to ∼80 pC) and an increase in the maximum electron energy from ∼ 400 keV to ∼ 800 keV were observed in the case of front coated foil compared to the uncoated one. In the case of rear coated foil, an enhancement of ∼1.4× in the electron flux with no change in maximum electron energy was observed compared to the uncoated foil. The observations are understood in terms of possible different preplasma conditions for various target configurations used, which is also supported by 1D hydrodynamic simulation carried out for the present experimental conditions. The observed enhancement in electron flux and temperature is also supported by 2D Particle in Cell (PIC) simulation.

Ultralow Frequency Waves as an Intermediary for Solar Wind Energy Input Into the Radiation Belts

AbstractEnhancements of electron fluxes in the outer radiation belt have been closely linked to increases in solar wind speed and density as well as to prolonged intervals of southward interplanetary magnetic field. Periodic oscillations in the Earth's magnetic field with frequencies in the range of a few millihertz (ultralow frequency or ultralow frequency waves) may be an intermediary through which these solar wind drivers influence radiation belt dynamics due to their potential for resonant interactions with energetic electrons causing the radial migration of resonant electrons. Using data from more than 180 ground magnetometers contributing to the worldwide SuperMAG collaboration, we explore possible relationships between relativistic electron flux variations and the spatial and temporal profiles of ultralow frequency wave power contained in the Pc5 frequency band (2–7 mHz). During 19 geomagnetic storms marked by relativistic (1.5 MeV < E < 6 MeV) electron flux enhancements and 19 storms that led to prolonged electron flux depletions, Pc5 wave power is found penetrating to L shells as low as 2–3. The enhancement of Pc5 wave power starts almost simultaneously with the storm onset. The depth of wave activity penetration was found associated with the strength of geomagnetic activity (Spearman's ρ = 0.54), which is also related to the location of electron flux maximum observed in the recovery phase. Pc5 wave activity persists longer (for up to ≈62 hr) for those storms that produced relativistic electrons. We also investigate the combination of interplanetary conditions necessary to differentiate the response of relativistic electron fluxes to geomagnetic storms. A coupling function that captures the increased reconnection rate at the dayside magnetopause affecting magnetospheric processes which may produce Pc5 wave power offers an additional key to further understanding the outer belt dynamics.

Electron Flux Enhancements at L = 4.2 Observed by Global Positioning System Satellites: Relationship With Solar Wind and Geomagnetic Activity

AbstractDetermining solar wind and geomagnetic activity parameters most favorable to strong electron flux enhancements is an important step toward forecasting radiation belt dynamics. Using electron flux measurements from Global Positioning System satellites at L = 4.2 in 2009–2016, we seek statistical relationships between flux enhancements at different energies and solar wind dynamic pressure Pdyn, AE, and Kp, from hundreds of events inside and outside the plasmasphere. Most ≥1‐MeV electron flux enhancements occur during nonstorm (or weak storm) times. Flux enhancements of 4‐MeV electrons outside the plasmasphere occur during periods of low Pdyn and high AE. We perform superposed epoch analyses of Global Positioning System electron fluxes, along with solar wind and geomagnetic indices, 40‐keV electron flux, ultralow frequency (ULF) wave index from Geostationary Operational Environmental Satellite, and chorus wave intensity from the Van Allen Probes and Time History of Events and Macroscale Interactions during Substorms mission. We demonstrate that 4‐MeV electron flux enhancements outside the plasmasphere start when the interplanetary magnetic field (Bz) reaches a minimum and develop during periods of low Pdyn, high AE, low but increasing Dst, moderate ULF wave index, and intense chorus waves. Flux enhancements at 100 keV occur under conditions with higher Pdyn, higher ULF wave index, and elevated 40‐keV electron flux at L = 6.6. Moreover, electron flux enhancements take much more time to develop at higher energies. This suggests that 100‐keV flux enhancements are dominated by injections, while MeV electron energization is predominantly induced by chorus waves with further amplification by inward transport.

Three-dimensional hot electron photovoltaic device with vertically aligned TiO2 nanotubes

Titanium dioxide (TiO2) nanotubes with vertically aligned array structures show substantial advantages in solar cells as an electron transport material that offers a large surface area where charges travel linearly along the nanotubes. Integrating this one-dimensional semiconductor material with plasmonic metals to create a three-dimensional plasmonic nanodiode can influence solar energy conversion by utilizing the generated hot electrons. Here, we devised plasmonic Au/TiO2 and Ag/TiO2 nanodiode architectures composed of TiO2 nanotube arrays for enhanced photon absorption, and for the subsequent generation and capture of hot carriers. The photocurrents and incident photon to current conversion efficiencies (IPCE) were obtained as a function of photon energy for hot electron detection. We observed enhanced photocurrents and IPCE using the Ag/TiO2 nanodiode. The strong plasmonic peaks of the Au and Ag from the IPCE clearly indicate an enhancement of the hot electron flux resulting from the presence of surface plasmons. The calculated electric fields and the corresponding absorbances of the nanodiode using finite-difference time-domain simulation methods are also in good agreement with the experimental results. These results show a unique strategy of combining a hot electron photovoltaic device with a three-dimensional architecture, which has the clear advantages of maximizing light absorption and a metal–semiconductor interface area.

Quantitative Evaluation of Radial Diffusion and Local Acceleration Processes During GEM Challenge Events

AbstractWe simulate the radiation belt electron flux enhancements during selected Geospace Environment Modeling (GEM) challenge events to quantitatively compare the major processes involved in relativistic electron acceleration under different conditions. Van Allen Probes observed significant electron flux enhancement during both the storm time of 17–18 March 2013 and non–storm time of 19–20 September 2013, but the distributions of plasma waves and energetic electrons for the two events were dramatically different. During 17–18 March 2013, the SYM‐H minimum reached −130 nT, intense chorus waves (peak Bw ~140 pT) occurred at 3.5 < L < 5.5, and several hundred keV to several MeV electron fluxes increased by ~2 orders of magnitude mostly at 3.5 < L < 5.5. During 19–20 September 2013, the SYM‐H remained higher than −30 nT, modestly intense chorus waves (peak Bw ~80 pT) occurred at L > 5.5, and electron fluxes at energies up to 3 MeV increased by a factor of ~5 at L > 5.5. The two electron flux enhancement events were simulated using the available wave distribution and diffusion coefficients from the GEM focus group Quantitative Assessment of Radiation Belt Modeling. By comparing the individual roles of local electron heating and radial transport, our simulation indicates that resonant interaction with chorus waves is the dominant process that accounts for the electron flux enhancement during the storm time event particularly near the flux peak locations, while radial diffusion by ultralow‐frequency waves plays a dominant role in the enhancement during the non–storm time event. Incorporation of both processes reasonably reproduces the observed location and magnitude of electron flux enhancement.

Antioxidant protection and PSII regulation mitigate photo-oxidative stress induced by drought followed by high light in cashew plants

We tested the hypothesis that effective antioxidant and photoprotective mechanisms are able to avoid photodamage induced by prolonged water deficit (WD) followed by high light (HL). We employed cashew plants (Anacardium occidentale L.), a semiarid adapted species, as a model plant. WD-plants exposed to HL did not show alterations in maximum quantum efficiency of photosystem II (Fv/Fm), cellular integrity, H2O2 and thiobarbituric acid reactive species (TBARS) contents, evidencing that they did not suffered photoinhibition and oxidative stress. These responses were associated with increases in ascorbate peroxidase (APX) and superoxide dismutase (SOD) activities, glutathione (GSH) oxidation and ascorbate (ASC) synthesis. This effective oxidative protection occurred in parallel to strong decrease in photosystem II and I (PSII and PSI) activities, increase in heat dissipation (qE), which was related to enhancement in cyclic electron flux. These favorable photoprotective changes were associated with efficient water use in response to HL, all contributing to avoid excess energy in chloroplasts of drought-exposed leaves. These protective features were associated with a tight regulation in D1 protein accumulation during HL, contributing to avoid reactive oxygen species over-accumulation and a subsequent effective PSII recovery during darkness. Our results indicate that cashew plants are able to avoid photoinhibition, tolerating extreme conditions of drought combined with HL. The displayed mechanisms involve essentially integrated responses to balance energy input and output, avoiding oxidative stress.