Radiation Belt Dynamics Research Articles

Testing the Organization of Lower‐Band Whistler‐Mode Chorus Wave Properties by Plasmapause Location

AbstractLower‐band whistler‐mode chorus waves are important to the dynamics of Earth's radiation belts, playing a key role in accelerating seed population electrons (hundreds of keV) to relativistic (>1 MeV) energies, and in scattering electrons such that they precipitate into the atmosphere. When constructing and using statistical models of lower‐band whistler‐mode chorus wave power, it is commonly assumed that wave power is spatially distributed with respect to magnetic L‐shell. At the same time, these waves are known to drop in power at the plasmapause, a cold plasma boundary which is dynamic in time and space relative to L‐shell. This study organizes wave power and propagation direction data with respect to distance from the plasmapause location to evaluate what role the location of the plasmapause may play in defining the spatial distribution of lower‐band whistler‐mode chorus wave power. It is found that characteristics of the statistical spatial distribution of equatorial lower‐band whistler‐mode chorus are determined by L‐shell and are largely independent of plasmapause location. The primary physical importance of the plasmapause is to act as an Earthward boundary to lower‐band whistler‐mode chorus wave activity. This behavior is consistent with an equatorial lower‐band whistler‐mode chorus wave power spatial distribution that follows the L‐shell organization of the particles driving wave growth.

The influence of various frequency chorus waves on electron dynamics in radiation belts

Non-adiabatic behaviour induced by the chorus-electron interaction is an important contributor to the radiation belt dynamics, and largely relies on wave frequency distribution. During the geomagnetic storm on August 2, 2016, upper-band, lower-band and extremely low frequency (ELF) chorus waves were simultaneously observed within one orbit period of the Van Allen Probe B. Numerical simulations are performed to investigate the electron evolution by the observed chorus with different frequencies. The results show that various frequency chorus waves have different effects on electron dynamics. For chorus in the range f≈0.3fce−0.7fce, energy diffusion is the dominant process in electron evolutions. For chorus in the range 0.1fce⩽f⩽0.25fce, the pitch angle diffusion tends to be comparable to energy diffusion for Ek>0.5 MeV. For ELF chorus below 0.1fce, the pitch angle diffusion rate is much above the energy diffusion rate, leading to potential scattering losses.

Report on VERSIM 2020

The 9th VLF/ELF Remote Sensing of Ionospheres and Magnetospheres Workshop (VERSIM 2020) was successfully held as a virtual meeting during the week of 16–20 November 2020. The meeting had been rescheduled from the original meeting (23–27 March 2020 at the Uji Campus, Kyoto University, Japan), cancelled due to the COVID-19 pandemic. The VERSIM workshop aims to bring together scientists from all across the globe to present and discuss topics related to the excitation, propagation, and effects of electromagnetic waves in the ELF/VLF (300 Hz – 30 kHz) range. The VERSIM group was established as a joint URSI/IAGA work group in 1975. It has been the main scientific body to specifically focus on whistlers in the Earth's ionosphere and magnetosphere (whistler waves were one of the first electromagnetic signals detected by ground instruments, and gave important clues about the Earth's active space environment). The scope of this group has grown substantially over the years. It has grown particularly in recent years with the recognition that whistler-mode chorus and hiss waves play a particularly important role in shaping the dynamics of the Earth's high-energy radiation belts (particularly topical in light of the continuing successful operation of the Van Allen Probes and the Arase (ERG) spacecraft). Whistler waves are also known to be intimately involved in the magnetic reconnection process (which is currently very topical due to the launch of the MMS satellite) and solar flaring. In addition to the traditional VERSIM topics, results of recent space missions, such as Arase, MMS, and the Van Allen Probes, were presented.

Phase Decoherence Within Intense Chorus Wave Packets Constrains the Efficiency of Nonlinear Resonant Electron Acceleration

AbstractElectron resonant interaction with whistler mode waves is traditionally considered as one of the main drivers of radiation belt dynamics. The two main theoretical concepts available for its description are quasi‐linear theory of electron scattering by low‐amplitude waves and nonlinear theory of electron resonant trapping and phase bunching by intense waves. Both concepts successfully describe some aspects of wave‐particle interactions but predict significantly different timescales of relativistic electron acceleration. In this study, we investigate effects that can reduce the efficiency of nonlinear interactions and bridge the gap between the predictions of these two types of models. We examine the effects of random wave phase and frequency variations observed inside whistler mode wave packets on nonlinear interactions. Our results show that phase coherence and frequency fluctuations should be taken into account to accurately model electron nonlinear resonant acceleration and that, along with wave amplitude modulation, they may reduce acceleration rates to realistic, moderate levels.

Parametric Dependence of the Formation of Electron Butterfly Pitch Angle Distribution Driven by Magnetosonic Waves

AbstractUsing the full relativistic test particle (TP) simulation code, we investigate the parametric dependence of electron scattering and phase space density evolution driven by magnetosonic (MS) waves at L = 4.5 both inside and outside the plasmapause. The scattering effects caused by Landau resonance, bounce resonance, and the transit‐time effect are all involved in the study. The net scattering effects are evaluated in the form of diffusion coefficients with different combinations of MS wave parameters, such as frequency bandwidth and wave normal angle, and ambient plasma density. The results demonstrate that (1) Landau resonance and the transit‐time effect dominate the electron scattering inside and outside the plasmapause, respectively, while both are modulated by bounce resonant scattering; (2) bounce resonant scattering becomes more important with narrowband MS waves; (3) electron scattering induced by MS waves is highly sensitive to wave normal angle. The temporal phase space density (PSD) evolution obtained from 2‐D kinetic Fokker‐Planck simulations shows that MS waves with larger wave normal angles are more likely to generate electron butterfly pitch angle distributions (PADs) for hundreds of keV electrons outside the plasmapause. Our study suggests that the electron butterfly distribution has important implications for revealing the combined scattering of MS wave‐particle interactions, and the combination of the multiple scattering mechanisms should be carefully incorporated in future global modeling of radiation belt dynamics.

A Short‐lived Three‐Belt Structure for sub‐MeV Electrons in the Van Allen Belts: Time Scale and Energy Dependence

AbstractIn this study we focus on the radiation belt dynamics driven by the geomagnetic storms during September 2017. Besides the long‐lasting three‐belt structures of ultrarelativistic electrons (>2 MeV, existing for tens of days), which has been studied intensively during the Van Allen Probe era, it is found that magnetospheric electrons of hundreds of keVs can also have three‐belt structures at similar L extent during storm time. Measurements of 500–800 keV electrons from MagEIS instrument onboard Van Allen Probes show double‐peaked (L = 3.5 and 4.5, respectively) flux‐versus‐L‐shell profile in the outer belt, which lasted for 2–3 days. During the time interval of such transient three‐belt structure, the energy‐versus‐L spectrogram shows novel distributions differing from both “S‐shaped” and “V‐shaped” spectrograms reported previously. Such peculiar distribution also illustrates the energy‐dependent occurrence of the three‐belt profile. The gradual formation of “reversed energy spectrum” at L ∼ 3.5 also indicates that hiss scattering inside the plasmapause contributed to the fast decay of sub‐MeV remnant belt.

A Framework for Understanding and Quantifying the Loss and Acceleration of Relativistic Electrons in the Outer Radiation Belt During Geomagnetic Storms

AbstractWe present detailed analysis of the global relativistic electron dynamics as measured by total radiation belt content (RBC) during coronal mass ejection (CME) and corotating interaction region (CIR)‐driven geomagnetic storms. Recent work has demonstrated that the response of the outer radiation belt is consistent and repeatable during geomagnetic storms. Here we build on this work to show that radiation belt dynamics can be divided into two sequential phases, which have different solar wind dependencies and which when analyzed separately reveal that the radiation belt responds more predictably than if the overall storm response is analyzed as a whole. In terms of RBC, in every storm we analyzed, a phase dominated by loss is followed by a phase dominated by acceleration. Analysis of the RBC during each of these phases demonstrates that they both respond coherently to solar wind and magnetospheric driving. However, the response is independent of whether the storm response is associated with either a CME or CIR. Our analysis shows that during the initial phase, radiation belt loss is organized by the location of the magnetopause and the strength of Dst and ultralow frequency wave power. During the second phase, radiation belt enhancements are well organized by the amplitude of ultralow frequency waves, the auroral electroject index, and solar wind energy input. Overall, our results demonstrate that storm time dynamics of the RBC is repeatable and well characterized by solar wind and geomagnetic driving, albeit with different dependencies during the two phases of a storm.

Solar activity phase dependence of the magnetospheric processes and relativistic electron flux at geostationary orbit

This study aims to investigate the effects of the solar activity phases on the magnetospheric processes in relation to daily relativistic electron ($E>2$ MeV) dynamics at geosynchronous orbit (GEO) during Solar Cycles 23–24. GOES observations indicate that in the descending phase the electron fluxes are seasonally dependent with largest flux in equinoctial periods, relatively high, and 27-day recurrent, while in the maximum phases they are relatively low and nonrecurrent. The electron fluxes are relatively low and partially recurrent in the ascending phases. The cross correlation coefficients (c.c.s) of daily $K_{\textrm{p}}$–$V_{\textrm{sw}}$, $K_{\textrm{p}}$–$AE$, $K_{\textrm{p}}$–$AL$ evolve in the similar trend for both solar cycles: highest during the descending phases, lower in the ascending phases, and lowest in the maximum and minimum phases. The correlation of $K_{\textrm{p}}$–viscous term is strongest during descending phases and weakest around the maximum phases. The correlation of $K_{\textrm{p}}$–merging term slightly varies in a chaotic way from one solar phase to another, but drops to its lowest value in the solar minimum. The correlation analysis results signify the role of the solar activity in controlling the solar wind–magnetosphere couplings. Two case studies during maximum (2000) and descending (2017) phases indicate that the solar activity dependence of the magnetospheric processes and relativistic electrons is characterized by substorm activity and solar wind drivers. Most of the substorms induced by coronal mass ejections in the maximum phase exhibit strong but short and non-repetitive features that associate with low electron fluxes at GEO. In contrast, high intensity, prolonged, and repetitive substorm activities induced by high-speed solar winds are the main cause of the relativistic electron enhancements during the descending phases. The enhancements strongly depend on $V_{\textrm{sw}}$ and $\Sigma K_{\textrm{p}}$ in which only appropriate $V_{\textrm{sw}}$ and $\Sigma K_{\textrm{p}}$ are required. In the descending phase, the requirements for the flux enhancements to $\geq 10^{3}$ cm−2 sr−1 s−1 are the simultaneity of $V_{\textrm{sw}}>500$ km/s and the prolongation of $\Sigma K_{\textrm{p}}>220$ and to $\geq 10^{4}$ cm−2 sr−1 s−1 when $V_{\textrm{sw}}\geq 630$ km/s and $\Sigma K_{\textrm{p}}>227$. Furthermore, the correlations of $\Sigma K_{\textrm{p}}$ and log-electron fluxes are stronger in the descending/ascending phases than in the maximum phase, while the time lag between them is longer in the maximum phase. The remarkable correlations of $V_{\textrm{sw}}$–$K_{\textrm{p}}$ and in the descending phase indicate appropriate magnetospheric convection in association with the repetitive substorms that can effectively trigger the stochastic mechanisms of electron acceleration. The results are extensively discussed in the light of observations and current theories of radiation belt dynamics.

Wave Normal Angle Distribution of Magnetosonic Waves in the Earth's Magnetosphere: 2‐D PIC Simulation

AbstractRadiation belt electrons can be accelerated and scattered by magnetosonic waves in the Earth's magnetosphere, and the scattering rate of electrons is sensitive to the wave normal angle. However, observationally it is difficult to identify the wave normal angle within a few degrees. In this study, using 2‐D particle‐in‐cell (PIC) simulations, we investigate the wave normal angle distribution of magnetosonic waves excited by ring distribution protons. Both the linear theory and simulations have shown that the wave normal angles are distributed over a narrow range (82°–89°) with a major peak at about 85° during the linear growth stage when the proton ring velocity is close to the Alfven speed. In addition, 2‐D PIC simulations further demonstrated that the waves tend to have larger wave normal angles (84°–89°) during the saturation stage since the waves with smaller wave normal angles are dissipated faster. It is also found that wave normal angles decrease with the increase of wave frequency. With the increase of the ring velocity of the proton ring distribution, the perpendicular wavenumber of excited magnetosonic waves decreases, which leads to the decrease of the wave normal angle. The simulation results provide a valuable insight to understand the property of magnetosonic waves, and the findings are useful for the global simulations of radiation belt dynamics.

Radiation Belt Radial Diffusion at Earth and Beyond

The year 2019 marks the 60th anniversary of the concept of radial diffusion in magnetospheric research. This makes it one of the oldest research topics in radiation belt science. While first introduced to account for the existence of the Earth’s outer belt, radial diffusion is now applied to the radiation belts of all strongly magnetized planets. But for all its study and application, radial diffusion remains an elusive process. As the theoretical picture evolved over time, so, too, did the definitions of various related concepts, such as the notion of radial transport. Whether data is scarce or not, doubts in the efficacy of the process remain due to the use of various unchecked assumptions. As a result, quantifying radial diffusion still represents a major challenge to tackle in order to advance our understanding of and ability to model radiation belt dynamics. The core objective of this review is to address the confusion that emerges from the coexistence of various definitions of radial diffusion, and to highlight the complexity and subtleties of the problem. To contextualize, we provide a historical perspective on radial diffusion research: why and how the concept of radial diffusion was introduced at Earth, how it evolved, and how it was transposed to the radiation belts of the giant planets. Then, we discuss the necessary theoretical tools to unify the evolving image of radial diffusion, describe radiation belt drift dynamics, and carry out contemporary radial diffusion research.

Magnetic Field Variations and Dynamics of the Outer Electron Radiation Belt of the Earth’s Magnetosphere in February 2014

The paper presents the results of a comparative analysis of the dynamics of relativistic electron fluxes of the Earth’s outer radiation belt and correspondent ring current and magnetospheric magnetic field variations for February 15–22, 2014, based on experimental data obtained in the radiation belt core aboard Van Allen Probes satellites and in geostationary orbit aboard the GOES-15 satellite. The A2000 paraboloid model of the magnetosphere is used to calculate the contributions of main magnetospheric current systems to the Dst variation. The influence of external factors—solar wind and interplanetary magnetic field—is considered. Comparison of the dynamics of electron fluxes in different orbits and the geomagnetic field variations for February 15‒22, 2014 indicate that the main mechanisms of the evolution of outer electron radiation belt are global processes: (i) outward and inward shifts of the trapped electron population inside the Earth’s magnetosphere due to large-scale variations of the magnetospheric magnetic field during geomagnetic disturbances and (ii) ExB drift of electrons from the magnetotail under the influence of electric and magnetic fields. Local particle acceleration is auxiliary process in the dynamics of radiation belts. The southward orientation of the interplanetary magnetic field is the necessary condition for an increase in the intensity of electron fluxes of the Earth’s outer radiation belt, while the function (–BzVsw) is the key external factor.

Identifying Radiation Belt Electron Source and Loss Processes by Assimilating Spacecraft Data in a Three‐Dimensional Diffusion Model

AbstractData assimilation aims to blend incomplete and inaccurate data with physics‐based dynamical models. In the Earth's radiation belts, it is used to reconstruct electron phase space density, and it has become an increasingly important tool in validating our current understanding of radiation belt dynamics, identifying new physical processes, and predicting the near‐Earth hazardous radiation environment. In this study, we perform reanalysis of the sparse measurements from four spacecraft using the three‐dimensional Versatile Electron Radiation Belt diffusion model and a split‐operator Kalman filter over a 6‐month period from 1 October 2012 to 1 April 2013. In comparison to previous works, our 3‐D model accounts for more physical processes, namely, mixed pitch angle‐energy diffusion, scattering by Electromagnetic Ion Cyclotron waves, and magnetopause shadowing. We describe how data assimilation, by means of the innovation vector, can be used to account for missing physics in the model. We use this method to identify the radial distances from the Earth and the geomagnetic conditions where our model is inconsistent with the measured phase space density for different values of the invariants and . As a result, the Kalman filter adjusts the predictions in order to match the observations, and we interpret this as evidence of where and when additional source or loss processes are active. The current work demonstrates that 3‐D data assimilation provides a comprehensive picture of the radiation belt electrons and is a crucial step toward performing reanalysis using measurements from ongoing and future missions.

How Do Ultra‐Low Frequency Waves Access the Inner Magnetosphere During Geomagnetic Storms?

AbstractWave‐particle interactions play a key role in radiation belt dynamics. Traditionally, ultra‐low frequency (ULF) wave‐particle interaction is parameterized statistically by a small number of controlling factors for given solar wind driving conditions or geomagnetic activity levels. Here we investigate solar wind driving of ULF wave power and the role of the magnetosphere in screening that power from penetrating deep into the inner magnetosphere. We demonstrate that during enhanced ring current intensity, the Alfvén continuum plummets, allowing lower frequency waves to penetrate deeper into the magnetosphere than during quiet periods. With this penetration, ULF wave power is able to accumulate closer to the Earth than characterized by statistical models. During periods of enhanced solar wind driving such as coronal mass ejection driven storms, where ring current intensities maximize, the observed penetration provides a simple physics‐based reason for why storm time ULF wave power is different compared to nonstorm time waves.

Magnetic Local Time‐Resolved Examination of Radiation Belt Dynamics during High‐Speed Solar Wind Speed‐Triggered Substorm Clusters

AbstractParticle observations from low Earth orbiting satellites are used to undertake superposed epoch analysis around clusters of substorms, in order to investigate radiation belt dynamical responses to mild geomagnetic disturbances. Medium energy electrons and protons have drift periods long enough to discriminate between processes occurring at different magnetic local time, such as magnetopause shadowing, plasma wave activity, and substorm injections. Analysis shows that magnetopause shadowing produces clear loss in proton and electron populations over a wide range of L‐shells, initially on the dayside, which interacts with nightside substorm‐generated flux enhancements following charge‐dependent drift directions. Inner magnetospheric injections recently identified as an important source of tens to hundreds keV electrons at low L (L<3), occurring during similar solar wind‐driving conditions as recurrent substorms, show similar but more enhanced geomagnetic AU‐index signatures. Twofold increases in substorm occurrence at the time of the sudden particle enhancements at low L shells (SPELLS) suggest a common linkage.

The Storm Time Development of Source Electrons and Chorus Wave Activity During CME‐ and CIR‐Driven Storms

AbstractWhistler mode chorus waves influence the dynamics of the Earth's radiation belts and the inner magnetosphere through gyroresonant wave particle interactions. Chorus waves are generated by anisotropic hot electrons from a few to tens of keV, called source electrons, which have increased access from the nightside plasma sheet to the inner magnetosphere during geomagnetic storms. The primary drivers of geomagnetic storms are coronal mass ejections (CMEs) and corotating interaction regions (CIRs). Through differences in their characteristic physical parameters, they can each impact the nightside plasma sheet differently. Using Van Allen Probes observations, we have conducted a superposed epoch analysis of chorus wave activity and source electron development across all local times between L = 2–6 during 25 CME‐ and 35 CIR‐driven storms. The superposed epoch analysis shows that chorus wave power follows the storm phase‐dependent access of the source electron population. Chorus waves and source electrons are observed on the dawnside during the main phase, when open drift path access via eastward convective drift from the plasma sheet is enhanced. During the recovery phase, chorus waves and source electrons are observed at all magnetic local times with low intensities, exemplifying the formation of a weak symmetric, trapped electron population. A linear theory approximation for wave growth from source electron observations shows that increased wave growth follows the enhanced source electrons during each storm phase. CME and CIR storms display similar behavior and levels of average wave power; however, chorus wave activity reaches lower L‐shells during CME storms on average.

Review of Controlled Excitation of Non-linear Wave-Particle Interactions in the Magnetosphere

Controlled experiments involving injection of 0.5 Hz – 8 kHz electromagnetic waves into the Earth’s magnetosphere have played an important role in discovering and elucidating wave-particle interactions in near-Earth space. Due to the significant engineering challenges of efficiently radiating in the ELF/VLF: 300 Hz – 30 kHz band, few experiments have been able to provide sustained transmissions of sufficient power to excite observable effects for scientific studies. Two noteworthy facilities that were successful in generating a large database of pioneering and repeatable observations were the Siple Station Transmitter in Antarctica and the High Frequency Active Auroral Research Program (HAARP) facility in Alaska. Both facilities were able to excite Doppler shifted cyclotron resonance interactions leading to linear and nonlinear wave amplification, triggering of free running emissions, and pitch angle scattering of energetic electrons. Amplified and triggered waves were primarily observed on the ground in the geomagnetic conjugate region after traversal of the magnetosphere along geomagnetic field aligned propagation paths or in the vicinity of the transmitter following two traversals of the magnetosphere. In several cases, spacecraft observations of the amplified and triggered signals were also made. The observations show the amplifying wave particle interaction to be dynamically sensitive to specific frequency and also specific frequency-time format of the transmitted wave. Transmission of multiple coherent waves closely spaced in frequency showed that the wave particle interaction requires a minimum level of coherency to enter the nonlinear regime. Theory and numerical simulations point to cyclotron resonance with counter streaming particles in the 10-100 keV range as the dominant process. A key feature of the nonlinear interaction is the phase-trapping of resonant particles by the wave that is believed to drive non-linear wave amplification and the triggering of free-running emissions. Observations and modeling of controlled wave injections have important implications for naturally occurring whistler mode emissions of hiss and chorus and the broader phenomena of radiation belt dynamics. A review of observational, theoretical, and numerical results is presented and suggestions for future studies are made.

The March 2015 Superstorm Revisited: Phase Space Density Profiles and Fast ULF Wave Diffusive Transport

AbstractWe present the temporal evolution of electron Phase Space Density (PSD) in the outer radiation belt during the intense March 2015 geomagnetic storm. Comparing observed PSD profiles as a function of L* at fixed first, M, and second, K, adiabatic invariants with those produced by simulations is critical for determining the physical processes responsible for the outer radiation belt dynamics. Here we show that the bulk of the accelerated and enhanced outer radiation belt population consists of electrons with K < 0.17 G1/2Re. For these electrons, the observed PSD versus L* profiles during the recovery phase of the storm have a positive radial gradient. We compare the observed temporal evolution of the PSD profiles during the recovery phase with those produced by radial diffusion simulations driven by observed Ultralow Frequency wave power as measured on the ground. Our results indicate that the dominant flux enhancement, inside L* < 5, in the heart of the outer radiation belt during the March 2015 geomagnetic storm is consistent with that produced by fast inward radial diffusion of electrons from a dynamic outer boundary driven by enhanced Ultralow Frequency wave power.

Quantitative Assessment of Radiation Belt Modeling

AbstractThe “Quantitative Assessment of Radiation Belt Modeling” focus group was in place at Geospace Environment Modeling from 2014 to 2018. The overarching goals of this focus group were to bring together the current state‐of‐the‐art models for the acceleration, transport, and loss processes in Earth's radiation belts; develop event‐specific and global inputs of wave, plasma, and magnetic field to drive these models; and combine all these components to achieve a quantitative assessment of radiation belt modeling by validating against contemporary radiation belt measurements. This article briefly reviews the current understanding of radiation belt dynamics and related modeling efforts, summarizes the activities and accomplishments of the focus group, and discusses future directions.

Monitoring, analysis and post-casting of the Earth’s particle radiation environment during February 14–March 5, 2014

Internet-based system of Space Monitoring Data Center (SMDC) of Skobeltsyn Institute of Nuclear Physics of Moscow State University (SINP MSU) has been developed to predict and analyze radiation conditions in near-Earth space. This system contains satellite measurement databases and operational models and devoted to collect, store and process space weather monitoring data in the near real-time. SMDC operational services acquire data from ACE, SDO, GOES, Electro-L, Meteor-M satellites and use them for forecasting, now-casting and post-casting of space weather factors. This paper is intended to give overview of operational services of SMDC Internet-based system and demonstrate their possibilities and limitations to analyze space weather phenomena and predict radiation and geomagnetic conditions in the near-Earth space during February 14–March 5, 2014. This prolonged period of high level solar and geomagnetic activity demonstrates various manifestations of the space weather: solar proton events, geomagnetic storms and outer radiation belt (RB) dynamics. Solar sources of interplanetary space disturbances and their influence on geomagnetic and radiation state of the Earth’s magnetosphere were described using output coming from SMDC’ Web-based applications. Validation of SMDC’s operational models was performed based on the quality of description of the physical conditions in near-Earth space during space weather events observed from February 14 to March 5, 2014. The advantages and disadvantages of SMDC operational services are illustrated and discussed based on comparison with data obtained from satellites.

The Outer Radiation Belt Response to the Storm Time Development of Seed Electrons and Chorus Wave Activity During CME and CIR Driven Storms

AbstractGyroresonant wave‐particle interactions with very low frequency whistler mode chorus waves can accelerate subrelativistic seed electrons (hundreds of keV) to relativistic energies in the outer radiation belt during geomagnetic storms. In this study, we conduct a superposed epoch analysis of the chorus wave activity, the seed electron development, and the outer radiation belt electron response between L* = 2.5 and 5.5, for 25 coronal mass ejection and 35 corotating interaction region storms using Van Allen Probes observations. Electron data from the Magnetic Electron Ion Spectrometer and Relativistic Electron Proton Telescope instruments are used to monitor the storm‐phase development of the seed and relativistic electrons, and magnetic field measurements from the Electric and Magnetic Field Instrument Suite and Integrated Science instrument are used to identify the chorus wave activity. Our results show a deeper (lower L*), stronger (higher flux), and earlier (epoch time) average seed electron enhancement and a resulting greater average radiation belt electron enhancement in coronal mass ejection storms compared to the corotating interaction region storms despite similar levels and lifetimes of average chorus wave activity for the two storm drivers. The earlier and deeper seed electron enhancement during the coronal mass ejection storms, likely driven by greater convection and substorm activity, provides a higher probability for local acceleration. These results emphasize the importance of the timing and the level of the seed electron enhancements in radiation belt dynamics.