Electron Flux Enhancements Research Articles

High-speed solar wind with southward interplanetary magnetic field causes relativistic electron flux enhancements of the outer radiation belt via enhanced condition of whistler waves

High-speed solar wind with southward interplanetary magnetic field causes relativistic electron flux enhancements of the outer radiation belt via enhanced condition of whistler waves

Study on high energy electron flux enhancement events and whistler chorus wave

局部加速机制是磁暴期间地球外辐射带高能电子通量增强事件发生的重要原 因. 此加速机制需要两个基本条件, 一是存在种子电子, 二是存在能与种子 电子产生共振的加速波动, 包括哨声模合声波. 通过对2004-2006年 Pi1地磁脉动持续时间与种子电子通量的相关性分析, 更明确提出Pi1地磁脉 动的持续时间可以作为种子电子通量的指示剂. 通过对三个磁暴事例地球同 步轨道的种子电子通量、高能电子通量及哨声模合声波变化情况的分析, 发 现在高能电子通量较强的事例中, 均观测到较高的种子电子通量和较强的 哨声模合声波, 这在一定程度上验证了哨声模合声波对种子电子的回旋加速 机制, 且合声波强度与高能电子通量有正的相关性.

Adiabatic acceleration of suprathermal electrons associated with dipolarization fronts

Recent observations in the inner magnetotail have shown rapid and significant flux increases (usually an order of magnitude of increase within seconds) of suprathermal electrons (tens of keV to hundreds of keV) associated with earthward moving dipolarization fronts. To explain where and how these suprathermal electrons are produced during substorm intervals, two types of acceleration models have been suggested by previous studies: acceleration that localizes near the reconnection site and acceleration that occurs during earthward transport. We perform an analytical analysis of adiabatic acceleration to show that the slope of source differential fluxes is critical for understanding adiabatic flux enhancements during earthward transport. Observationally, two earthward propagating dipolarization fronts accompanied by energetic electron flux enhancements observed by the THEMIS spacecraft have been analyzed; in each event the properties of dipolarization fronts in the inner magnetosphere (XGSM ≈ −10RE) were well correlated with those further down the tail (XGSM ≈ −15RE or XGSM ≈ −20RE). Coupled with theoretical analysis, this enables us to estimate the relative acceleration that occurred as the electrons propagated earthward between the two spacecraft. During the two events studied, the differential fluxes of supra thermal electrons had steep energy spectra with power law indices of −4 to −6.These spectra were much steeper than those at lower energy, as well as those of the supra thermal electrons observed before the fronts arrived. A compression factor of 1.5 as the electrons propagated earthward induced a flux increase of suprathermal electrons by a factor of 7 to 17. Provided these steep spectra, we demonstrate that adiabatic acceleration from the betatron and Fermi mechanisms simultaneously operating can account for these flux increases. Since both analytical analysis and data from the two events show that adiabatic acceleration during earthward transport does not significantly change the power law indices, the steep spectra were likely to be traced back to their source region, presumably near the reconnection site.

Energetic radiation belt electron precipitation showing ULF modulation

The energization and loss processes for energetic radiation belt electrons are not yet well understood. Ultra Low Frequency (ULF) waves have been correlated with both enhancement in outer zone radiation belt electron flux and modulation of precipitation loss to the atmosphere. This study considers the effects of ULF waves in the Pc‐4 to Pc‐5 period range (45 s–600 s) on electron loss to the atmosphere on a time scale of several minutes. Global simulations using magnetohydrodynamics (MHD) model fields as drivers provide a valuable tool for studying the dynamics of these ∼MeV energetic particles. ACE satellite measurements of the MHD solar wind parameters are used as the upstream boundary condition for the Lyon‐Fedder‐Mobarry (LFM) 3D MHD code calculation of fields, used to drive electrons in a 3D test particle simulation that keeps track of attributes like energy, pitch‐angle and L‐shell. The simulation results are compared with balloon observations obtained during the January 21, 2005 CME‐shock event. Rapid loss of 20 keV to 1.5 MeV electrons was detected by balloon‐borne measurements ofbremsstrahlungX‐rays during the MINIS campaign following the shock arrival at Earth. The global precipitation response of the radiation belts to this CME‐shock driven storm was investigated focusing on their interaction with ULF waves. A primary cause for the precipitation modulation seen in both the simulation and the MINIS campaign is suggested based on the lowering of mirror points due to compressional magnetic field oscillations.

Energy distribution asymmetry of electron precipitation signatures at Mars

The different types of asymmetry observed in the energy distributions of electrons and heavy-ions (M/Q=16-44) during signatures of electron precipitation in the Martian ionosphere have been classified. This has been achieved using the space plasma instrumentation of MEX ASPERA-3 from peri-centre altitude to 2200km. ASPERA-3 ELS observes signatures of electron precipitation on 43.0% of MEX orbits. Unaccelerated electrons in the form of sudden electron flux enhancements are the most common type of electron precipitation signature at Mars and account for ∼70% of the events observed in this study. Electrons that form unaccelerated electron precipitation signatures are either local ionospheric electrons with enhanced density, or electrons transported from another region of ionosphere, solar wind or tail, or a combination of local and transported electrons. The heating of electrons has a strong influence on the shape of most electron energy spectra from accelerated precipitation signatures. On most occasions the general flow of heavy-ions away from Mars is unchanged during the precipitation of electrons, which is thought to be the result of the finite gyroradius effect of the heavy-ions on crustal magnetic field lines. Only ∼17% of events show some form of heavy-ion acceleration that is either concurrent or at the periphery of an electron precipitation signature. The most common combination of electron and heavy-ion energy distributions for signatures of electron precipitation involves electrons that visually have very little asymmetry or are isotropic and heavy-ions that have a upward net flux, and suggest the upward current associated with aurora. Due to a lack of reliable measurements of electrons travelling towards Mars, it is likely we miss further evidence of upward currents. The second most common combination of electron and heavy-ion energy distributions for signatures of electron precipitation, are those distributions of electrons that are asymmetric and have an net upward flux, with distributions of heavy-ions that also have a net upward flux. Energy distributions of heavy-ions with a net flux towards Mars occur half as often as heavy-ions with an upward net flux. There is also evidence to suggest we observe downward currents during electron precipitation signatures when we find energy distributions of electrons that are asymmetric and have an upward net flux, combined with energy distributions of heavy-ions that have a downward net flux. Wave particle interactions and downward parallel electric fields may be responsible for electrons that display a large amount of asymmetry in the upward direction of the energy distribution and have a upward net flux.

Anik‐E1 and E2 satellite failures of January 1994 revisited

The consecutive failures of the geosynchronous Anik‐E1 communication satellite on January 20, 1994, and Anik‐E2 about nine hours later on January 21 (both incidents occurred on January 20 local time) received considerable publicity because the malfunctions of the satellites disrupted television and computer data transmissions across Canada, as well as telephone services to remote northern communities for hours. This often‐cited event is revisited here with materials not covered before. Using publicly available information, Anik‐E failure details, media coverage, recovery effort and cost incurred are first presented. This is then followed by scrutiny of space weather conditions pertinent to the occurrences of the Anik‐E upsets. We trace the space weather episode's inception on the Sun, propagation through interplanetary medium, and manifestation in magnetic field variations as well as in energetic electron flux increases, and its eventual impact on the Anik‐Es. The genesis of the energetic electron enhancements that have been blamed for the satellite malfunctions is thus traceable via high‐speed solar wind stream with Alfven wave fluctuations to a longitudinally wide coronal hole on the Sun. Furthermore, strong magnetic pulsations preceding electron flux peaks indicate Pc5 ULF (Ultra Low Frequency) waves as a probable acceleration mechanism for the energetic electron flux enhancement that resulted in the internal charging of the Anik‐Es. The magnetic fluctuations may even be possible triggers for the subsequent discharge that caused the satellites to malfunction. This incident illustrates that satellite operators should be on alert for elevated high‐energy electron environment that is above established thresholds, as specifications in satellite design may not render a satellite immune from internal charging.

A ROLE OF MAGNETOSONIC PULSES ON VARIATIONS OF VOYAGER-1 MeV ELECTRON INTENSITY IN THE HELIOSHEATH

Voyager 1 (V1) spacecraft observed electrons of 6-14 MeV in the heliosheath which showed several enhancements of significant flux variation. We compare these temporal electron flux variations, from the time when V1 crossed the termination shock (TS) up to mid-2008, with dynamical phenomena in the heliosheath that are obtained from our MHD simulations which are based on Voyager 2 (V2) observed solar-wind data. Our simulations indicate that all electron flux enhancements, except for one, correspond fairly well to the times when a magnetosonic (MS) pulse was driven downstream of the TS due to collision of interplanetary shock (IPS) or shock-driven MS pulse and its reflection in the heliosheath that either passed V1, or collided with the TS or with the plasma sheet near the heliopause (HP). This result suggests that these enhancements in the electron flux should correspond to either direct or indirect effects of MS pulses in the heliosheath driven by IPSs. The scale of the heliosphere is estimated by comparing V1-observed magnetic field intensity with the simulated intensity which suggests that V1 is possibly located near the HP within 4-8 AU at the present time.

Kinetic structure and wave properties associated with sharp dipolarization front observed by Cluster

Abstract. Multiple dipolarization fronts (DFs) were observed by Cluster spacecraft in the magnetotail during a substorm. These DFs were kinetic structures, embedded in the bursty plasma flow, and moved earthward (mainly) and dawnward. Intense electric field, parallel and perpendicular currents were detected in the DF layer. These front layers were energy dissipation region (load region) where the energy of electromagnetic fields were transferred to the plasma thermal and kinetic energy. This dissipation was dominated by electrons. There were enhancements of plasma waves around the DF region: wavelet results show that wave activities around the ion cyclotron frequency in the front layer were generated by Alfvén ion cyclotron instability; whistler waves were also detected before, during and after the DFs, which are triggered by electron temperature anisotropy and coincident with enhancement of energetic electron fluxes. The observation of these waves could be important for the understanding of evolution of DF and electron energization during the substorm. We discuss the generation mechanism of the DFs and suggest that these DFs were generated in the process of transient reconnection, and then traveled toward the Earth.

Analysis on Relativistic Electron Flux Enhancement Event at GEO in April 2010

为研究2010年4月地球同步轨道相对论电子通量异常增强事件的原因, 选取了2004-2010年之间高速太阳风下7个类似事件进行对比分析. 探讨了多种可能导致此次异常事件的太阳风和地磁条件. 结果表明, 较弱的磁暴使得相对论电子高通量区域更接近同步轨道, 此外, 哨声波加速很可能在2010年4月地球同步轨道相对论电子通量异常增强事件中起到重要作用. 磁暴强度与种子电子的注入深度密切相关, 表现为<I>Dst</I>指数曲线的形态与能量为30～100keV的电子高通量区域的下边缘高度吻合. 能量为30～100keV电子的注入深度影响了能量大于300keV的电子出现的磁层区域. 此事件中, 由于磁暴相对较弱, 种子电子向内磁层注入的深度较浅, 更靠近同步轨道区域, 这使得相对论电子大量出现的区域也靠近同步轨道, 最终导致同步轨道相对论电子通量异常增强. 另外, 2010年4月地球同步轨道相对论电子通量异常增强事件中, 高强度的亚暴提供了充足的种子电子并加强了波粒相互作用, 这也是相对论电子增强的必要条件.

Rapid rebuilding of the outer radiation belt

[1] Recent observations by the radiation monitor (RDM) on the spacecraft Akebono have shown several cases of >2.5 MeV radiation belt electron enhancements occurring on timescales of less than a few hours. Similar enhancements are also seen in detectors on board the NOAA/POES and TWINS 1 satellites. These intervals are shorter than typical radial diffusion or wave-particle interactions can account for. We choose two so-called “rapid rebuilding” events that occur during high speed streams (4 September 2008 and 22 July 2009) and simulated them with the Space Weather Modeling Framework configured with global magnetosphere, radiation belt, ring current, and ionosphere electrodynamics model. Our simulations produce a weaker and delayed dipolarization as compared to observations, but the associated inductive electric field in the simulations is still strong enough to rapidly transport and accelerate MeV electrons resulting in an energetic electron flux enhancement that is somewhat weaker than is observed. Nevertheless, the calculated flux enhancement and dipolarization is found to be qualitatively consistent with the observations. Taken together, the modeling results and observations support the conclusion that storm-time dipolarization events in the magnetospheric magnetic field result in strong radial transport and energization of radiation belt electrons.

Reanalysis of radiation belt electron phase space density using various boundary conditions and loss models

Data assimilation is becoming an increasingly important tool for understanding the near Earth hazardous radiation environments. Reanalysis of the radiation belts can be used to identify the electron acceleration mechanism and distinguish local acceleration from radial diffusion. However, for any practical applications we need to determine how reliable is reanalysis, and how significant is the dependence of the results on the assumptions of the code and choice of boundary conditions. We present the sensitivity of reanalysis of the radiation belt electron phase space density (PSD) to the assumed location of the outer boundary, using the VERB code and a Kalman filter. We analyze the sensitivity of reanalysis to changes in the electron-loss throughout the domain, and the sensitivity to the assumed boundary condition and its effect on the innovation vector. All the simulations presented in this study for all assumed loss models and boundary conditions, show that peaks in the phase space density of relativistic electrons build up between 4.5 and 6 R E during relativistic electron flux enhancements in the outer radiation belt. This clearly shows that peaks build up in the heart of the electron radiation belt independent of the assumptions in the model, and that local acceleration is operating there. The work here is also an important step toward performing reanalysis using observations from current and future missions.

Radiation belt electron dynamics driven by adiabatic transport, radial diffusion, and wave-particle interactions

The adiabatic transport process is introduced into our recently developed three-dimensional physics-based electron radiation belt model (STEERB, Storm-Time Evolution of Electron Radiation Belt) via adopting a time-varying Hilmer-Voigt geomagnetic field. The current STEERB model contains more complete physical processes: adiabatic transport, radial diffusion, and various in situ wave-particle interactions. In particular, the influence of adiabatic transport on storm time radiation belt electron dynamics is investigated by some idealized simulations. It is found that the adiabatic transport alone (without plume hiss and electromagnetic ion cyclotron (EMIC) waves) is unable to reproduce the observed main phase loss of energetic outer radiation belt electron fluxes in the presence of a strong chorus-driven acceleration process. However, these adiabatic and nonadiabatic processes for radiation belt electron dynamics are coupled to each other. The adiabatic transport, together with radial diffusion and cyclotron resonant interactions with chorus, plume hiss, and EMIC waves, contributes significantly to the main phase loss and the recovery phase enhancement of energetic electron fluxes. In the absence of adiabatic transport, the energetic outer radiation belt electron fluxes are found to be overestimated by a factor of 5-30 over all the pitch angles during the main phase and to be underestimated by a factor of 2-5 at larger pitch angles (alpha(e) > 50 degrees) during the recovery phase. These numerical results suggest that the adiabatic transport in a time-varying geomagnetic field model should be incorporated into the future radiation belt models for space weather application.

Energetic electron flux distribution model in the inner and middle magnetosphere

Based on the magnetospheric kinetic theory, a model is developed to specify the flux of energetic electrons in the inner and middle magnetosphere. Under the assumption of adiabatic motion and isotropic particle distribution maintained by pitch-angle scattering, the model calculates the electron flux by following bounce-averaged electric field, gradient, and curvature drift in the time dependent electric and magnetic field, meanwhile it counts the electron loss caused by pitch angle scattering. Using the model, the electron flux distribution during a magnetic storm was calculated and compared with the observation data from the geosynchronous orbit. It is shown that the model can successfully reproduce most of the major electron flux enhancements observed at the geosynchronous orbit and generally tracks the satellite data well. The rms errors of the modeled logarithm of flux are between 0.5–1.0.

Contrasting the efficiency of radiation belt losses caused by ducted and nonducted whistler‐mode waves from ground‐based transmitters

It has long been recognized that whistler‐mode waves can be trapped in plasmaspheric whistler ducts which guide the waves. For nonguided cases these waves are said to be “nonducted”, which is dominant for L &lt; 1.6. Wave‐particle interactions are affected by the wave being ducted or nonducted. In the field‐aligned ducted case, first‐order cyclotron resonance is dominant, whereas nonducted interactions open up a much wider range of energies through equatorial and off‐equatorial resonance. There is conflicting information as to whether the most significant particle loss processes are driven by ducted or nonducted waves. In this study we use loss cone observations from the DEMETER and POES low‐altitude satellites to focus on electron losses driven by powerful VLF communications transmitters. Both satellites confirm that there are well‐defined enhancements in the flux of electrons in the drift loss cone due to ducted transmissions from the powerful transmitter with call sign NWC. Typically, ∼80% of DEMETER nighttime orbits to the east of NWC show electron flux enhancements in the drift loss cone, spanning a L range consistent with first‐order cyclotron theory, and inconsistent with nonducted resonances. In contrast, ∼1% or less of nonducted transmissions originate from NPM‐generated electron flux enhancements. While the waves originating from these two transmitters have been predicted to lead to similar levels of pitch angle scattering, we find that the enhancements from NPM are at least 50 times smaller than those from NWC. This suggests that lower‐latitude, nonducted VLF waves are much less effective in driving radiation belt pitch angle scattering.

Observations of energetic electrons up to 200 keV associated with a secondary island near the center of an ion diffusion region: A Cluster case study

An antiparallel reconnection event is recognized in the magnetotail, and a secondary island with a strong core magnetic field is identified near the center of the ion diffusion region. In the island, the electron density peaks in the outer region while the dip is in the core region with a strong core magnetic field. A strong electron beam parallel to the magnetic field, as well as an obvious current antiparallel to the magnetic field with density up to ∼40 nA/m2, is observed in the outer region of the island. This suggests that the strong core magnetic field inside the island is generated by the electron beam and then the antiparallel current in the outer region of the island. The electron density dip in the core region is formed due to the existing strong core field, which expels electrons out of the core region. The electron flat‐top distributions are detected in the ion diffusion region except the core region of the island, and the shoulder energy range of the flat‐top distributions is from 100 eV to 4 keV. An enhancement of the energetic electron flux up to 200 keV is found in the ion diffusion region, and a further increase of energetic electron fluxes is observed inside the island. Waves at the lower hybrid frequency are intensified in the ion diffusion region, while the intensification is strong in the outer region of the island and becomes very weak in the core region.

Solar cycle variations of outer radiation belt and its relationship to solar wind structure dependences

The outer radiation belt shows solar cycle variation: the L-shell of the electron flux peak in the outer belt shifts inward during the period between the rising phase and the solar maximum, while it shifts outward between the beginning of the declining phase and the solar minimum. We show a possible mechanism which considers two typical types of magnetic storms categorized in accordance with solar wind drivers, namely coronal mass ejections (CMEs) and corotating interaction regions (CIRs). Large flux enhancements at the inner portion of the outer belt tend to occur during the recovery phase of great storms driven by CMEs, while large flux enhancements at the outer portion and at geosynchronous orbit tend to occur during the recovery phase of relatively moderate storms driven by CIRs. High-speed coronal hole streams which do not always cause large magnetic storms also effectively enhance the electron flux enhancement at the outer portion and in geosynchronous orbit. In this framework, the plasmapause always plays an important role in both flux enhancement and flux loss in the outer belt. The average plasmapause position depends on the storm amplitude, and the plasmapause reaches closest to the Earth during great storms driven by CMEs. CMEs themselves and CME-driven storms occur during maximum periods of solar activity, while CIRs themselves and CIR-driven storms occur during the solar declining phase. The observed long-term variations of the outer belt can therefore be understood in terms of their dependence on the large-scale interplanetary structures, varying depending on the phase of the solar cycle.

Recent developments in the radiation belt environment model

The fluxes of energetic particles in the radiation belts are found to be strongly controlled by the solar wind conditions. In order to understand and predict the radiation particle intensities, we have developed a physics-based Radiation Belt Environment (RBE) model that considers the influences from the solar wind, ring current and plasmasphere. Recently, an improved calculation of wave-particle interactions has been incorporated. In particular, the model now includes cross diffusion in energy and pitch-angle. We find that the exclusion of cross diffusion could cause significant overestimation of electron flux enhancement during storm recovery. The RBE model is also connected to MHD fields so that the response of the radiation belts to fast variations in the global magnetosphere can be studied. We are able to reproduce the rapid flux increase during a substorm dipolarization on 4 September 2008. The timing is much shorter than the time scale of wave associated acceleration.

Combined radial diffusion and adiabatic transport of radiation belt electrons with arbitrary pitch angles

Storm‐time radial diffusion of radiation belt electrons with arbitrary pitch angles in a time‐varying geomagnetic field is simulated based on our recently developed STEERB code. In particular, the fully adiabatic response of energetic electrons to the variation of geomagnetic field is self‐consistently incorporated. Simulation results show that the outward adiabatic transport (instead of outward radial diffusion) is primarily responsible for the main phase depletion of energetic electron fluxes at large pitch angles beyond 5Re (Re is the Earth's radius). However, combined radial diffusion and adiabatic transport contributes insignificantly to the main phase depletion of energetic electron fluxes within 5Re, or the recovery phase enhancement of energetic electron fluxes in the outer radiation belt. Moreover, the simulation with both radial diffusion and adiabatic transport shows that the pitch angle distribution of energetic outer zone electrons can evolve from a rounded 90°‐peaked distribution to a butterfly‐shaped distribution during the main phase, and back to a rounded 90°‐peaked distribution during the recovery phase. Such essential changes of pitch angle distribution may further affect the efficiency of other local loss and energization mechanisms.

Simultaneous FAST and Double Star TC1 observations of broadband electrons during a storm time substorm

Broadband electrons (BBEs) exhibit remarkable electron flux enhancements over a broad energy range (0.03–30 keV) near the equatorward edge of the auroral oval during geomagnetic storms. Here, we report a BBE event observed by the Fast Auroral Snapshot (FAST) satellite at 1355–1359 UT, ∼61°–66° invariant latitudes, ∼0600 magnetic local time (MLT), and ∼3800 km altitude during a storm on 25 July 2004. The Double Star (DS) TC1 satellite was located near the magnetic equator at L = 5.7, close to the same local time as FAST. We investigate the acceleration process of BBEs from the inner magnetosphere to near the ionosphere by comparing electron data obtained by FAST and DS TC1. We also investigate both plasma and field variations in the inner magnetosphere associated with substorm onset using DS TC1 data to examine the relationship between the BBEs and the storm time substorm. Ground geomagnetic field data show a positive H‐bay at ∼1349 UT at ∼0600 MLT, indicating that a storm time substorm started just before the appearance of the BBEs. At ∼1350 UT, a tailward ion flow was observed by DS TC1. Then, DS TC1 observed a local dipolarization and a drastic ion density enhancement at ∼1351 UT, indicating that particle heating associated with the substorm was occurring in the inner magnetosphere. From ∼1352 UT, electron fluxes were isotropically enhanced at energies above ∼0.5 keV as observed by DS TC1. On the other hand, the pitch angle distribution of BBEs at the FAST altitude showed field‐aligned lower‐energy electrons below ∼0.5 keV and isotropic higher‐energy electrons above ∼0.5 keV. From these data, it was inferred that the BBEs might consist of two energy components due to the acceleration or heating of electrons at different altitudes in association with the storm time substorm.

In SituObservations of a Secondary Magnetic Island in an Ion Diffusion Region and Associated Energetic Electrons

Numerical simulations have predicted that an extended current sheet may be unstable to secondary magnetic islands in the vicinity of the X line, and these islands can dramatically influence the reconnection rate. In this Letter, we present the first evidence of such a secondary island near the center of an ion diffusion region, which is consistent with the action of the secondary island instability occurring in the vicinity of the X line. The island is squashed in the z direction with a strong core magnetic field. Energetic electrons with anisotropic or field-aligned bidirectional fluxes are found in the ion diffusion region, and the enhancement of energetic electron fluxes is more obvious inside the secondary island.