Electrons In Outer Radiation Belt Research Articles

The Response of the Energy Content of the Outer Electron Radiation Belt to Geomagnetic Storms

AbstractUsing the data from the Van Allen Probe‐A spacecraft, the variability of the total outer radiation belt (2.5 < L* < 6) electron energy content (TRBEEC) for energetic electrons (>300 keV) is investigated for the first time during 51 isolated storms spanning from October 2012 to May 2017. The statistical results show that the TRBEEC exhibits no change in 20% of the storms and gets enhanced during 80% of them. The subrelativistic electrons (300–500 keV) and relativistic electrons (0.5–2.0 MeV) equally contribute to the TRBEEC during the main phases, while in the recovery phases, the relativistic electrons contribute up to 80% of the TRBEEC. The results of the superposed epoch analysis of the solar wind parameters and geomagnetic indices indicate that the TRBEEC enhancement events preferably occur during the prolonged southward interplanetary magnetic field period when the solar wind‐magnetosphere coupling is more efficient. Meanwhile, the high AE index with intense injections of several hundreds of kiloelectron volt seed electrons also favors the increase of the TRBEEC. Case study shows that there is a localized growing phase space density peak around L* = 4.3 and the chorus wave energy and the gain of TRBEEC are on the same order of magnitude, which may suggest that the enhancement of the TRBEEC is the consequence of the chorus acceleration. Understanding the energy budget of the outer zone electrons can provide more insight into the energy transfer from plasma waves to the energetic electron population, especially for revealing the underlying physics of the energization of outer radiation belt electrons via chorus wave acceleration.

A 30‐Year Simulation of the Outer Electron Radiation Belt

AbstractAs society becomes more reliant on satellite technology, it is becoming increasingly important to understand the radiation environment throughout the Van Allen radiation belts. Historically most satellites have operated in low Earth orbit or geostationary orbit (GEO), but there are now over 100 satellites in medium Earth orbit (MEO). Additionally, satellites using electric orbit raising to reach GEO may spend hundreds of days on orbits that pass through the heart of the radiation belts. There is little long‐term data on the high‐energy electron flux, responsible for internal charging in satellites, available for MEO. Here we simulate the electron flux between the outer edge of the inner belt and GEO for 30 years. We present a method that converts the >2‐MeV flux measured at GEO by the Geostationary Operational Environmental Satellites spacecraft into a differential flux spectrum to provide an outer boundary condition. The resulting simulation is validated using independent measurements made by the Galileo In‐Orbit Validation Element‐B spacecraft; correlation coefficients are in the range 0.72 to 0.88, and skill scores are between 0.6 and 0.8 for a range of L∗ and energies. The results show a clear solar cycle variation and filling of the slot region during active conditions and that the worst case spectrum overlaps that derived independently for the limiting extreme event. The simulation provides a resource that can be used by satellite designers to understand the MEO environment, by space insurers to help resolve the cause of anomalies and by satellite operators to plan for the environmental extremes.

Relative positions of the polar boundary of the outer electron radiation belt and the equatorial boundary of the auroral oval

Abstract. Finding the position of the polar boundary of the outer electron radiation belt, relative to the position of the auroral oval, is a long-standing problem. Here we analyze it using the data of the METEOR-M1 auroral satellite for the period from 11 November 2009 to 27 March 2010. The geomagnetic conditions during the analyzed period were comparatively quiet. METEOR-M1 has a polar solar-synchronous circular orbit with an altitude of ≈832 km, a period of 101.3 min, and an inclination of 98∘. We analyze flux observations of auroral electrons with energies between 0.03 and 16 keV, and electrons with energies >100 keV, measured simultaneously by the GGAK-M set of instruments, composed of semiconductors, scintillator detectors, and electrostatic analyzers. We assume that in the absence of geomagnetic storms the polar boundary of the outer radiation belt can be identified as a decrease in the count rate of precipitating energetic electrons to the background level. It was found that this boundary can be located both inside the auroral oval or equatorward of the equatorial boundary of the auroral precipitation. It was also found that for slightly disturbed geomagnetic conditions the polar boundary of the outer radiation belt is almost always located inside the auroral oval. We observe that the difference between the position of the polar boundary of the outer radiation belt and the position of the equatorial boundary of the auroral precipitation depend on the AE and PC indices of geomagnetic activity. The implications of these results in the analysis of the formation of the outer radiation belt are discussed.

Processes in auroral oval and outer electron radiation belt

We have analyzed the role of auroral processes in the formation of the outer radiation belt, considering that the main part of the auroral oval maps to the outer part of the ring current, instead of the plasma sheet as is commonly postulated. In this approach, the outer ring current is the region where transverse magnetospheric currents close inside the magnetosphere. Specifically, we analyzed the role of magnetospheric substorms in the appearance of relativistic electrons in the outer radiation belt. We present experimental evidence that the presence of substorms during a geomagnetic storm recovery phase is, in fact, very important for the appearance of a new radiation belt during this phase. We discuss the possible role of adiabatic acceleration of relativistic electrons during storm recovery phase and show that this mechanism may accelerate the relativistic electrons by more than one order of magnitude.

Modeling the Depletion and Recovery of the Outer Radiation Belt During a Geomagnetic Storm: Combined MHD and Test Particle Simulations

AbstractDuring geomagnetic storms the intensities of the outer radiation belt electron population can exhibit dramatic variability. Deep depletions in intensity during the main phase are followed by increases during the recovery phase, often to levels that significantly exceed their prestorm values. To study these processes, we simulate the evolution of the outer radiation belt during the 17 March 2013 geomagnetic storm using our newly developed radiation belt model (Conservative Hamiltonian Integrator for Magnetospheric Particles) based on test particle and coupled 3‐D ring current and global magnetohydrodynamic (MHD) simulations, and driven solely with solar wind and F10.7 flux data. Our approach differs from previous work in that we use MHD information to identify regions of strong, bursty, and azimuthally localized earthward convection in the magnetotail where test particles are then seeded. We validate our model using in situ Van Allen Probe electron intensities over a multiday period and show that our model is able to reproduce meaningful qualitative and quantitative agreement. Analysis of our model enables us to study the processes that govern the transition from the prestorm to poststorm outer belt. Our analysis demonstrates that during the early main phase of the storm the preexisting outer belt is largely wiped out via magnetopause losses, and subsequently, a new outer belt is created during a handful of discrete, mesoscale injections. Finally, we demonstrate the potential importance of magnetic gradient trapping in the transport and energization of outer belt electrons using a controlled numerical experiment.

Nonlinear Dynamics of Radiation Belt Electrons Interacting With Chorus Emissions Localized in Longitude

AbstractUsing results of test particle simulations of electrons interacting with whistler mode chorus emissions, we numerically obtain Green's functions to model evolution of the electron distribution function after all of the possible interactions with the waves. In both wave models with and without subpackets, electrons undergoing the cyclotron resonance with the waves are efficiently accelerated by nonlinear wave trapping. Since the strong modulation of the wave amplitude (in the case of the waves with subpackets) affects dynamics of resonant electrons, the electrons are detrapped from the wave potential or entrapped into it more frequently than those interacting with the waves without subpackets. As a result of interaction with the waves with subpackets, the number of electrons undergoing the acceleration is increased, while the acceleration efficiency in energy is decreased because of shorter interaction time. Modifying the numerical Green's function method with the simplified model of chorus waves uniform in longitude (Omura et al., 2015, https://doi.org/10.1002/2015JA021563), we compute the formation process of the outer radiation belt electron fluxes induced by the interaction with the chorus waves localized in longitude occurring for a time scale of 1 hr. The formation of MeV electron fluxes is characterized by large acceleration rates and butterfly pitch angle distributions, which are found in satellite observation results.

Formation of Butterfly Pitch Angle Distributions of Relativistic Electrons in the Outer Radiation Belt With a Monochromatic Pc5 Wave

AbstractRadial transport of relativistic electrons in the inner magnetosphere has been considered one of the acceleration mechanisms of the outer radiation belt electrons and can be driven by the drift resonance with Pc5 ultralow‐frequency waves. In this study, we focus on the pitch angle dependences of the radial transport and investigate the pitch angle distributions (PADs) of the relativistic electrons after interaction with a monochromatic Pc5 wave, using two simulation models of the inner magnetosphere: Geomagnetic Environment Modeling System for Integrated Studies (GEMSIS)‐Ring Current and GEMSIS‐Radiation Belt models. The results show that butterfly‐like PADs (two peaks in small pitch angles) are produced at a fixed radial distance (L‐shell) and energy, where the electrons with small pitch angles satisfy the resonance condition. The radial range and timescale of the butterfly PAD appearance correspond to the maximum radial transport of the electron (i.e., resonant width) and its oscillation period in the radial direction, respectively. We analytically derive the resonant width and oscillation period of relativistic electrons interacting with a monochromatic Pc5 wave, assuming conservation of first and second adiabatic invariants, and compare them with the simulation result. The results show that electrons with small equatorial pitch angles at a fixed magnetic moment can interact with a monochromatic Pc5 wave in a wider L‐shell range than perpendicular electrons due to the pitch angle changes, in the course of the radial transport.

Explaining the apparent impenetrable barrier to ultra-relativistic electrons in the outer Van Allen belt

Recent observations have shown the existence of an apparent impenetrable barrier at the inner edge of the ultra-relativistic outer electron radiation belt. This apparent impenetrable barrier has not been explained. However, recent studies have suggested that fast loss, such as associated with scattering into the atmosphere from man-made very-low frequency transmissions, is required to limit the Earthward extent of the belt. Here we show that the steep flux gradient at the implied barrier location is instead explained as a natural consequence of ultra-low frequency wave radial diffusion. Contrary to earlier claims, sharp boundaries in fast loss processes at the barrier are not needed. Moreover, we show that penetration to the barrier can occur on the timescale of days rather than years as previously reported, with the Earthward extent of the belt being limited by the finite duration of strong solar wind driving, which can encompass only a single geomagnetic storm.

Internal Charging Characteristics in Typical Navigation Satellite Orbits

Internal charging is an important hazard for navigation satellites, which operate in outer electron radiation belt and experience a very variable radiation environment because of the inclination of orbits. A rapid analysis method of internal charging is introduced, including physical model, conductivity model, shielding and electron transportation algorithm, and charging calculation method. Using this method, we calculate the charging processes of dielectric in medium earth orbit (MEO), geostationary earth orbit (GEO), and inclined geosynchronous orbit (IGSO). The results show that charging field fluctuates a lot in orbit and variation rate of field is proportional to electron flux. The saturated charging field in MEO is higher than that in GEO, and the field in GEO is higher than that in IGSO. So, the internal electrostatic discharging risk in MEO, GEO, and IGSO decreases in turn. If conductivity of dielectric is smaller, saturated field is larger and the difference of field in MEO, GEO, and IGSO is smaller. The dark and radiation-induced conductivity (RIC) of dielectric are important parameters in simulation, so we study the impact of conductivity in charging processes. In IGSO and MEO, when dark conductivity-induced time constant of charging is greater than orbital period and RIC is not large, the deposited electrons between different orbits accumulate and the charging field increase constantly. In this situation, the dark conductivity has a big influence on internal charging and it is important to measure it accurately.

Radiation Belt “Dropouts” and Drift‐Bounce Resonances in Broadband Electromagnetic Waves

AbstractObservations during the main phase of geomagnetic storms reveal an anticorrelation between the occurrence of broadband low‐frequency electromagnetic waves and outer radiation belt electron flux. We show that the drift‐bounce motion of electrons in the magnetic field of these waves leads to rapid electron transport. For observed spectral energy densities it is demonstrated that the wave magnetic field can drive radial diffusion via drift‐bounce resonance on time scales less than a drift orbit. This process may provide outward transport sufficient to account for electron “dropouts” during storm main phase and more generally modulate the outer radiation belt during geomagnetic storms.

The Role of Localized Compressional Ultra‐low Frequency Waves in Energetic Electron Precipitation

AbstractTypically, ultra‐low frequency (ULF) waves have historically been invoked for radial diffusive transport leading to acceleration and loss of outer radiation belt electrons. At higher frequencies, very low frequency waves are generally thought to provide a mechanism for localized acceleration and loss through precipitation into the ionosphere of radiation belt electrons. In this study we present a new mechanism for electron loss through precipitation into the ionosphere due to a direct modulation of the loss cone via localized compressional ULF waves. We present a case study of compressional wave activity in tandem with riometer and balloon‐borne electron precipitation across keV‐MeV energies to demonstrate that the experimental measurements can be explained by our new enhanced loss cone mechanism. Observational evidence is presented demonstrating that modulation of the equatorial loss cone can occur via localized compressional wave activity, which greatly exceeds the change in pitch angle through conservation of the first and second adiabatic invariants. The precipitation response can be a complex interplay between electron energy, the localization of the waves, the shape of the phase space density profile at low pitch angles, ionospheric decay time scales, and the time dependence of the electron source; we show that two pivotal components not usually considered are localized ULF wave fields and ionospheric decay time scales. We conclude that enhanced precipitation driven by compressional ULF wave modulation of the loss cone is a viable candidate for direct precipitation of radiation belt electrons without any additional requirement for gyroresonant wave‐particle interaction. Additional mechanisms would be complementary and additive in providing means to precipitate electrons from the radiation belts during storm times.

The Contribution of Compressional Magnetic Pumping to the Energization of the Earth's Outer Electron Radiation Belt During High‐Speed Stream‐Driven Storms

AbstractCompressional magnetic pumping is an interaction between cyclic magnetic compressions and pitch angle scattering with the scattering acting as a catalyst to allow the cyclic compressions to energize particles. Compressional magnetic pumping of the outer electron radiation belt at geosynchronous orbit in the dayside magnetosphere is analyzed by means of computer simulations, wherein solar wind compressions of the dayside magnetosphere energize electrons with electron pitch angle scattering by chorus waves and by electromagnetic ion cyclotron (EMIC) waves. The magnetic pumping is found to produce a weak bulk heating of the electron radiation belt, and it also produces an energetic tail on the electron energy distribution. The amount of energization depends on the robustness of the solar wind compressions and on the amplitude of the chorus and/or EMIC waves. Chorus‐catalyzed pumping is better at energizing medium‐energy (50–200 keV) electrons than it is at energizing higher‐energy electrons; at high energies (500 keV–2 MeV) EMIC‐catalyzed pumping is a stronger energizer. The magnetic pumping simulation results are compared with energy diffusion calculations for chorus waves in the dayside magnetosphere; in general, compressional magnetic pumping is found to be weaker at accelerating electrons than is chorus‐driven energy diffusion. In circumstances when solar wind compressions are robust and when EMIC waves are present in the dayside magnetosphere without the presence of chorus, EMIC‐catalyzed magnetic pumping could be the dominant energization mechanism in the dayside magnetosphere, but at such times loss cone losses will be strong.

Overview of the ISS Radiation Environment Observed during the ESA EXPOSE‐R2 Mission in 2014–2016

AbstractThe radiation risk radiometer‐dosimeter (R3D)‐R2 solid‐state detector performed radiation measurements at the European Space Agency EXPOSE‐R2 platform outside of the Russian “Zvezda” module at the International Space Station (ISS) from 24 October 2014 to 11 January 2016. The ISS orbital parameters were average altitude of 415 km and 51.6° inclination. We developed special software and used experimentally obtained formulas to determine the radiation flux‐to‐dose ratio from the R3DR2 Liulin‐type deposited‐energy spectrometer. We provide for the first time simultaneous, long‐term estimates of radiation dose external to the ISS for four source categories: (i) galactic cosmic ray particles and their secondary products; (ii) protons in the South Atlantic Anomaly region of the inner radiation belt (IRB); (iii) relativistic electrons and/or bremsstrahlung in the outer radiation belt (ORB); and (iv) solar energetic particle (SEP) events. The latter category is new in this study. Additionally, in this study, secondary particles (SP) resulting from energetic particle interaction with the detector and nearby materials are identified. These are observed continuously at high latitudes. The detected SPs are identified using the same sorting requirements as SEP protons. The IRB protons provide the highest consistent hourly dose, while the ORB electrons and SEPs provide the most extreme hourly doses. SEPs were observed 11 times during the study interval. The R3DR2 data support calculation of average equivalent doses. The 30 day and 1 year average equivalent doses are much smaller than the skin and eyes doses recommendations by the National Council on Radiation Protection (Report 132), which provides radiation protection guidance for Low Earth Orbit.

Acceleration of radiation belt electrons and the role of the average interplanetary magnetic field Bz component in high‐speed streams

AbstractIn this study we examine the recovery of relativistic radiation belt electrons on 15–16 November 2014, after a previous reduction in the electron flux resulting from the passage of a corotating interaction region (CIR). Following the CIR, there was a period of high‐speed streams characterized by large, nonlinear fluctuations in the interplanetary magnetic field (IMF) components. However, the outer radiation belt electron flux remained at a low level for several days before it increased in two major steps. The first increase is associated with the IMF background field turning from slightly northward on average to slightly southward on average. The second major increase is associated with an increase in the solar wind velocity during a period of southward average IMF background field. We present evidence that when the IMF Bz is negative on average, the whistler mode chorus wave power is enhanced in the outer radiation belt, and the amplification of magnetic integrated power spectral density in the ULF frequency range, in the nightside magnetosphere, is more efficient as compared to cases in which the mean IMF Bz is positive. Preliminary analysis of the time evolution of phase space density radial profiles did not provide conclusive evidence on which electron acceleration mechanism is the dominant. We argue that the acceleration of radiation belt electrons requires (i) a seed population of keV electrons injected into the inner magnetosphere by substorms and both (ii) enhanced whistler mode chorus waves activity as well as (iii) large‐amplitude MHD waves.

Empirical radiation belt models: Comparison with in situ data and implications for environment definition

AbstractThe empirical AP8/AE8 model has been the de facto Earth's radiation belts engineering reference for decades. The need from the community for a better model incubated the development of AP9/AE9/SPM, which addresses several shortcomings of the old model. We provide additional validation of AP9/AE9 by comparing in situ electron and proton data from Jason‐2, Polar Orbiting Environmental Satellites (POES), and the Van Allen Probes spacecraft with the 5th, 50th, and 95th percentiles from AE9/AP9 and with the model outputs from AE8/AP8. The relatively short duration of Van Allen Probes and Jason‐2 missions means that their measurements are most certainly the result of specific climatological conditions. In low Earth orbit (LEO), the Jason‐2 proton flux is better reproduced by AP8 compared to AP9, while the POES electron data are well enveloped by AE9 5th and 95th percentiles. The shape of the South Atlantic anomaly (SAA) from Jason‐2 data is better captured by AP9 compared to AP8, while the peak SAA flux is better reproduced by AP8. The <1.5 MeV inner belt electrons from Magnetic Electron Ion Spectrometer (MagEIS) are well enveloped by AE9 5th and 95th percentiles, while AE8 overpredicts the measurements. In the outer radiation belt, MagEIS and Relativistic Electron and Proton Telescope (REPT) electrons closely follow the median estimate from AE9, while AP9 5th and 95th percentiles generally envelope REPT proton measurements in the inner belt and slot regions. While AE9/AP9 offer the flexibility to specify the environment with different confidence levels, the dose and trapped proton peak flux for POES and Jason‐2 trajectories from the AE9/AP9 50th percentile and above are larger than the estimates from the AE8/AP8 models.

On the cause of two prompt shock-induced relativistic electron depletion events

Dayside interplanetary (IP) shock-induced injections are known to be a source of highly relativistic electrons in Earth's outer radiation belt, and are possibly the only source of >1 MeV electrons in the inner belt. The associated electron energization process is well understood and modeled. Recently, relativistic electron depletion echoes have also been associated with IP shocks, but the processes driving the depletions are less well understood. In this study, we investigate in detail two shock induced >1 MeV electron depletion events observed by the Van Allen Probes, March 17, 2015 and May 24, 2013, and draw similarities to night-side substorm related enhancements and depletions. Both events exhibit shock induced enhancements on one of the Van Allen Probes and depletions on the other in >1 MeV channels, such observations have not previously been reported. The depletion of >1 MeV electrons during the March 17, 2015 event is associated with enhancements in 10 s–100 s keV electrons on the same spacecraft. The depletion is consistent with the effects of a lack of seed electrons at larger radial distances combined with inward motion due to asymmetric compression by the shock impact. The immediate enhancements and depletions of 75 keV–2.6 MeV electrons are explained by the local phase space density radial profile. Observations of electron flux dynamics during the May 24, 2013 event can also be explained by a lack of a seed population at larger radial distances, supported by butterfly distributions observed during the event. The electron's inward radial motion can be attributed to the inward propagating impulse also associated with the >1 MeV electron enhancements observed on the complementary probe, rather than global asymmetric compression. This causal mechanism has parallels to substorm related depletions. Alternatively, evidence is provided to attribute the sudden depletions to losses due to a sudden but brief inward motion of the magnetopause.

Detailed characteristics of radiation belt electrons revealed by CSSWE/REPTile measurements: Geomagnetic activity response and precipitation observation

AbstractEarth's outer radiation belt electrons are highly dynamic. We study the detailed characteristics of relativistic electrons in the outer belt using measurements from the Colorado Student Space Weather Experiment (CSSWE) mission, a low Earth orbit (LEO) CubeSat, which traverses the radiation belt four times in one orbit (~1.5 h) and has the advantage of measuring the dynamic activities of the electrons including their rapid precipitation. We focus on the measured electron response to geomagnetic activity for different energies to show that there are abundant sub‐MeV electrons in the inner belt and slot region. These electrons are further enhanced during active times, while there is a lack of >1.63 MeV electrons in these regions. We also show that the variation of measured electron flux at LEO is strongly dependent on the local magnetic field strength, which is far from a dipole approximation. Moreover, a specific precipitation band, which happened on 19 January 2013, is investigated based on the conjunctive measurement of CSSWE, the Balloon Array for Radiation belt Relativistic Electron Losses, and one of the Polar Operational Environmental Satellites. In this precipitation band event, the net loss of the 0.58–1.63 MeV electrons (L = 3.5–6) is estimated to account for 6.8% of the total electron content.

A comparison of outer electron radiation belt dropouts during solar wind stream interface and magnetic cloud driven storms

Energetic electrons are trapped in the Earth’s radiation belts which occupy a toroidal region between 3 and 7 \(\hbox {R}_{\mathrm{E}}\) above the Earth’s surface. Rapid loss of electrons from the radiation belts is known as dropouts. The source and loss mechanisms regulating the radiation belts population are not yet understood entirely, particularly during geomagnetic storm times. Nevertheless, the dominant loss mechanism may require an event based study to be better observed. Utilizing multiple data sources from the year 1997–2007, this study identifies radiation belt electron dropouts which are ultimately triggered when solar wind stream interfaces (SI) arrived at Earth, or when magnetic clouds (MC) arrived. Using superposed epoch analysis (SEA) technique, a synthesis of multiple observations is performed to reveal loss mechanism which might, perhaps, be a major contributor to radiation belt losses under SI and MC driven storms. Results show an abrupt slower decaying precipitation of electron peak (about 3000 counts/sec) on SI arrival within 5.05 \(< L\) < 6.05, which persist till 0.5 day before gradual recovery. This pattern is interpreted as an indication of depleted electrons from bounce lost cone via precipitating mechanism known as relativistic electron microburst. On the other hand, MC shows a pancake precipitating peak extending to lower L (Plasmapause); indicating a combination of electron cyclotron harmonic (ECH) and whistler mode waves as the contributing mechanisms.

Roles of whistler mode waves and magnetosonic waves in changing the outer radiation belt and the slot region

AbstractUsing the Van Allen Probe long‐term (2013–2015) observations and quasi‐linear simulations of wave‐particle interactions, we examine the combined or competing effects of whistler mode waves (chorus or hiss) and magnetosonic (MS) waves on energetic (&lt;0.5 MeV) and relativistic (&gt;0.5 MeV) electrons inside and outside the plasmasphere. Although whistler mode chorus waves and MS waves can singly or jointly accelerate electrons from the hundreds of keV energy to the MeV energy in the low‐density trough, most of the relativistic electron enhancement events are best correlated with the chorus wave emissions outside the plasmapause. Inside the plasmasphere, intense plasmaspheric hiss can cause the net loss of relativistic electrons via persistent pitch angle scattering, regardless of whether MS waves were present or not. The intense hiss waves not only create the energy‐dependent electron slot region but also remove a lot of the outer radiation belt electrons when the expanding dayside plasmasphere frequently covers the outer zone. Since whistler mode waves (chorus or hiss) can resonate with more electrons than MS waves, they play dominant roles in changing the outer radiation belt and the slot region. However, MS waves can accelerate the energetic electrons below 400 keV and weaken their loss inside the plasmapause. Thus, MS waves and plasmaspheric hiss generate different competing effects on energetic and relativistic electrons in the high‐density plasmasphere.

Generation of extremely low frequency chorus in Van Allen radiation belts

AbstractRecent studies have shown that chorus can efficiently accelerate the outer radiation belt electrons to relativistic energies. Chorus, previously often observed above 0.1 equatorial electron gyrofrequency fce, was generated by energetic electrons originating from Earth's plasma sheet. Chorus below 0.1 fce has seldom been reported until the recent data from Van Allen Probes, but its origin has not been revealed so far. Because electron resonant energy can approach the relativistic level at extremely low frequency, relativistic effects should be considered in the formula for whistler mode wave growth rate. Here we report high‐resolution observations during the 14 October 2014 small storm and firstly demonstrate, using a fully relativistic simulation, that electrons with the high‐energy tail population and relativistic pitch angle anisotropy can provide free energy sufficient for generating chorus below 0.1 fce. The simulated wave growth displays a very similar pattern to the observations. The current results can be applied to Jupiter, Saturn, and other magnetized planets.