Relativistic Electron Enhancements Research Articles

Ultra-relativistic electron flux enhancement under persistent high speed solar wind stream

The physical mechanisms usually applied to explain the relativistic electron enhancement have been delved into to elucidate non-adiabatic electron acceleration resulting in the ultra-relativistic electron population observed in the outer radiation belt. We considered multisatellite observations of the solar wind parameters, magnetospheric waves, and particle flux to report an unusual local acceleration of ultra-relativistic electrons under a prolonged high-speed solar wind stream (HSS). A corotating interaction region reaches the Earth’s bowshock on August 3, 2016, causing a minor geomagnetic storm. Following this, the magnetosphere was driven for 72 h by a long-term HSS propagating at 600 km/s. During this period, the magnetosphere sustained both ultra-low frequency (ULF) and very-low frequency (VLF) waves in the outer radiation belt region. Besides the waves, the relativistic and ultra-relativistic electron fluxes were enhanced with different time lags regarding the magnetic storm main phase. The efficiency of wave-particle interaction in enhancing ultrarelativistic electrons is evaluated by the diffusion coefficient rates, considering both ULF and VLF waves together with phase space density analyses. Results show that local acceleration by whistler mode chorus waves can occur in a time scale of 2–4 h, whereas ULF waves take around 10’s of hours and magnetosonic waves take a time scale of days. This result is confirmed by the phase space density analysis. Accordingly, it shows that peaks of local acceleration of 1 MeV electrons are consistent with the observation of the highest chorus wave amplitude at the same L-shell and MLT. Thus, we argue that whistler mode chorus waves interacting with relativistic electrons are the main physical mechanisms leading to ultra-relativistic electron enhancement, while ULF and fast magnetosonic waves are found as secondary physical processes. Lastly, our analysis contributes to understanding how whistler and ULF waves can contribute to ultra-relativistic electrons showing up in the inner magnetosphere under the HSS driver.

Rapid Relativistic Electron Enhancements During Van Allen Probes Era

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

Rapid Relativistic Electron Enhancements and Associated Particle Injections: A Multi‐Spacecraft Statistical Study

AbstractRapid relativistic electron enhancements (REE) in the outer radiation belt have long been an intriguing phenomenon for space weather. In this study, we investigate rapid REE from October 2012 to December 2017 using multi‐spacecraft observations. A total of 27 rapid REE events are identified from the Van Allen Probes (RBSP) measurements with a 5 times increase of MeV electrons at the center of the outer radiation belt (L = 4.5–5.5) in a half RBSP orbit (∼4.5 hr). All REE events are found to be in association with pulse‐like injections of MeV electrons in the outer radiation belt. Electron fluxes in each injection at L ∼ 6.6 and the overall electron enhancements at L = 4.5–5.5 are quantified. The 500 keV and 0.8–1 MeV electron fluxes are correlated in injections and in overall enhancements. Substorm strength is more intense before/during the REE than intervals after the REE. The statistical study suggests that substorm‐associated MeV electron injections are highly correlated with rapid REE in the outer radiation belt.

Prediction Models of ≥2 MeV Electron Daily Fluences for 3 Days at GEO Orbit Using a Long Short-Term Memory Network

Geostationary satellites are exposed to harsh space weather conditions, including ≥2 MeV electrons from the Earth’s radiation belts. To predict ≥2 MeV electron daily fluences at 75°W and 135°W at geostationary orbit for the following three days, long short-term memory (LSTM) network models have been developed using various parameter combinations. Based on the prediction efficiency (PE) values, the most suitable time step of inputs and best combinations of two or three input parameters of models for predictions are recommended. The highest PE values for the following three days with three input parameters were 0.801, 0.658 and 0.523 for 75°W from 1995 to August 2010, and 0.819, 0.643 and 0.508 for 135°W from 1999 to 2010. Based on yearly PE values, the performances of the above models show the solar cycle dependence. The yearly PE values are significantly inversely correlated with the sunspot number, and they vary from 0.606 to 0.859 in predicting the following day at 75°W from 1995 to 2010. We have proven that the poor yearly PE is related to relativistic electron enhancement events, and the first day of events is the most difficult to predict. Compared with previous models, our models are comparable to the top performances of previous models for the first day, and significantly improve the performance for second and third days.

Testing the key processes that accelerate outer radiation belt relativistic electrons during geomagnetic storms

Since the discovery of the Earth’s radiation belts in 1958, it has always been a challenge to determine the dominant physical mechanisms, whether local acceleration by chorus or inward radial diffusion, that leads to outer radiation belt relativistic electron flux enhancements. In this study, we test a chain of processes with several potential successive steps that is believed to accelerate outer belt relativistic electrons. By performing correlation analysis of different part of this chain, including the geomagnetic condition, evolution of source and seed electron fluxes, chorus wave activity, and maximum fluxes (jmax) of relativistic electrons, we aim to identify the critical steps that lead to acceleration of MeV electrons. Based on 5-years of Van Allen Probes observations, our results confirm the repeatable response of both source and seed electrons to the storms, showing a significant flux enhancement during the main phase of storms, followed by either a gradual decay or flux persistence at a stable level. However, it is the intense and prolonged occurrence of substorms that contributes to the long-lasting existence of both source and seed electrons, which is also strongly associated with the jmax of relativistic electrons. The significant correlation (Correlation Coefficient, CC∼0.8) between the seed electron fluxes and jmax reveal that the prolonged and pronounced seed electrons are the prerequisite for the significant flux enhancement of relativistic electrons regardless of the acceleration mechanism. The slightly smaller CC (∼0.5–0.7) between source electron fluxes and jmax of relativistic electrons indicates that while local acceleration by chorus wave plays an important role to accelerate relativistic electrons to jmax, other mechanisms such as inward radial diffusion are still needed in this process. The CC between the source electrons and the chorus wave amplitude increases with increasing levels of substorms, showing (CC)max of ∼0.8, which further supports the crucial role of chorus waves in accelerating the relativistic electrons during intense substroms.

Enhancements of relativistic electron flux at geostationary orbit during high-intensity, long-duration, continuous AE activity (HILDCAA) from 2015 to 2017

This work characterizes the enhancements of relativistic electron flux (MeV) in the geostationary orbit (GEO) in High Intensity, Long Duration, Continuous AE Activity (HILDCAA) occurred during declining phase of solar cycle 24. We employed the relativistic electron (0.8 - 2.0 MeV) flux and low-energy electron (40 - 130 keV) flux measured by GOES-13 and POES satellites, respectively. Typically, the relativistic electron flux increases, while the low-energy electron flux decreases in the long recovery phase of moderate storms. The enhancements of E > 0.8 MeV and > 2.0 MeV occurred promptly and ∼1.0 day after the HILDCAA onset, respectively. A case study of short HILDCAA events shows that low solar wind dynamic pressure and long-lasting high amplitude Alfvén waves are efficient triggers of the relativistic electron enhancement at GEO. Large convection from magnetic reconnection in HILDCAA would induce substorms that injected more seed electrons for the acceleration. The peaks of the E > 2.0 MeV flux are more delayed than of the E > 0.8 MeV. After the onset of short- and prolonged-period HILDCAA, the peak flux of E > 0.8 MeV occurred about 2 and 4 days, respectively, and of E > 2.0 MeV occurred about 2 and 5 days, respectively.

An event of extreme relativistic and ultra-relativistic electron enhancements following the arrival of consecutive corotating interaction regions: Coordinated observations by Van Allen Probes, Arase, THEMIS and Galileo satellites

During July to October of 2019, a sequence of isolated Corotating Interaction Regions (CIRs) impacted the magnetosphere, for four consecutive solar rotations, without any interposed Interplanetary Coronal Mass Ejections. Even though the series of CIRs resulted in relatively weak geomagnetic storms, the net effect of the outer radiation belt during each disturbance was different, depending on the electron energy. During the August-September CIR group, significant multi-MeV electron enhancements occurred, up to ultra-relativistic energies of 9.9 MeV in the heart of the outer Van Allen radiation belt. These characteristics deemed this time period a fine case for studying the different electron acceleration mechanisms. In order to do this, we exploited coordinated data from the Van Allen Probes, the Time History of Events and Macroscale Interactions during Substorms Mission (THEMIS), Arase and Galileo satellites, covering seed, relativistic and ultra-relativistic electron populations, investigating their Phase Space Density (PSD) profile dependence on the values of the second adiabatic invariant K, ranging from near-equatorial to off equatorial mirroring populations. Our results indicate that different acceleration mechanisms took place for different electron energies. The PSD profiles were dependent not only on the μ value, but also on the K value, with higher K values corresponding to more pronounced local acceleration by chorus waves. The 9.9 MeV electrons were enhanced prior to the 7.7 MeV, indicating that different mechanisms took effect on different populations. Finally, all ultra-relativistic enhancements took place below geosynchronous orbit, emphasizing the need for more Medium Earth Orbit (MEO) missions.

The 600 keV electron injections in the Earth’s outer radiation belt: A statistical study

Relativistic electron injections are one of the mechanisms of relativistic (≥0.5 MeV) electron enhancements in the Earth’s outer radiation belt. In this study, we present a statistical observation of 600 keV electron injections in the outer radiation belt by using data from the Van Allen Probes. On the basis of the characteristics of different injections, 600 keV electron injections in the outer radiation belt were divided into pulsed electron injections and nonpulsed electron injections. The 600 keV electron injections were observed at 4.5 &lt; <i>L</i> &lt; 6.4 under the geomagnetic conditions of 450 nT &lt; <i>AE</i> &lt; 1,450 nT. An <i>L</i> of ~4.5 is an inward limit for 600 keV electron injections. Before the electron injections, a flux negative<i> L</i> shell gradient for ≤0.6 MeV electrons or low electron fluxes in the injected region were observed. For 600 keV electron injections at different <i>L</i> shells, the source populations from the Earth’s plasma sheet were different. For 600 keV electron injections at higher <i>L</i> shells, the source populations were higher energy electrons (~200 keV at <i>X</i> ~ –9 <i>R</i>_E), whereas the source populations for 600 keV electron injections at lower <i>L</i> shells were lower energy electrons (~80 keV at <i>X</i> ~ –9 <i>R</i>_E). These results are important to further our understanding of electron injections and rapid enhancements of 600 keV electrons in the Earth’s outer radiation belt.

Relative Contribution of ULF Waves and Whistler‐Mode Chorus to the Radiation Belt Variation During the May 2017 Storm

AbstractWe investigate the time and location where ULF waves and whistler‐mode chorus contributed to the net flux enhancement of relativistic electrons during the magnetic storm of May 2017. During the early recovery phase, both ULF and chorus waves contribute to the enhancement of relativistic electron fluxes, but ULF waves play roles of the inward diffusion. During the late recovery phase, both Van Allen Probe‐B and Arase show that whistler‐mode chorus contributes to the flux enhancement confined in the L‐value. The CRCM coupled with BATS‐R‐US simulation qualitatively reproduces the global evolution of ULF waves. Although the electron flux is underestimated by the simulation, this study reveals a large anisotropy of hot electrons in the region where whistler‐mode chorus waves were actually observed by satellites. In addition, the estimated magnetic field curvature on the dayside is small during the recovery phase. Furthermore, we investigate the control of wave evolution. Both observations and the simulation suggest that the observed ULF waves in the frequency range of ∼2–5 mHz are excited by the enhancement of the solar wind dynamic pressure. Observations also indicate that whistler‐mode chorus on the nightside is predominantly excited by hot electrons with temperature anisotropy, whereas the dayside chorus is enhanced by the change of the magnetic field line configuration. The estimated spatial distributions of electron anisotropy and magnetic field curvature provide an explanation for the presence of enhanced whistler‐mode chorus in the dusk sector, which is far from the usual location of wave generation.

Rapid Injections of MeV Electrons and Extremely Fast Step‐Like Outer Radiation Belt Enhancements

AbstractRapid injection of MeV electrons associated with strong substorm dipolarization has been suggested as a potential explanation for some radiation belt enhancement events. However, it has been difficult to quantify the contribution of MeV electron injections to radiation belt enhancements. This paper presents two isolated MeV electron injection events for which we quite precisely quantify how the entire outer‐belt immediately changed with the injections. Tracking detailed outer‐belt evolution observed by Van Allen Probes, for both events, we identify large step‐like relativistic electron enhancements (roughly 1 order of magnitude increase for ∼2 MeV electron fluxes) for L ≳ 3.8 and L ≳ 4.6, respectively, that occurred on ∼30‐min time scales nearly instantaneously with the injections. The enhancements occurred almost simultaneously for 10s keV to multi‐MeV electrons, with the lowest L of enhancement region located farther out for higher energy. The outer‐belt stayed at these new levels for ≳several hours without substantial subsequent enhancements.

Numerical simulation of chorus-driving acceleration of relativistic electrons at extremely low L-shell during geomagnetic storms* *Project supported by the National Natural Science Foundation of China (Grant Nos. 41904149 and 12173038), Stable-Support Scientific Project of ChinaResearch Institute of Radiowave Propagation (Grant No. A132001W07), and the National Institute of Natural Hazards, Ministry of Emergency Management of China (Grant No. 2021-JBKY-11).

During 2018 major geomagnetic storm, relativistic electron enhancements in extremely low L-shell regions (reaching L∼3) have been reported based on observations of ZH-1 and Van Allen probes satellites, and the storm is highly likely to be accelerated by strong whistler-mode waves occurring near very low L-shell regions where the plasmapause was suppressed. It is very interesting to observe the intense chorus-accelerated electrons locating in such low L-shells and filling into the slot region. In this paper, we further perform numerical simulation by solving the two-dimensional Fokker–Planck equation based on the bounce-averaged diffusion rates. Numerical results demonstrate the evolution processes of the chorus-driven electron flux and confirm the flux enhancement in low pitch angle ranges (20°–50°) after the wave-particle interaction for tens of hours. The simulation result is consistent with the observation of potential butterfly pitch angle distributions of relativistic electrons from both ZH-1 and Van Allen probes.

Interplay of source/seed electrons and wave-particle interactions in producing relativistic electron PSD enhancements in the outer Van Allen belt

Abstract We perform a superposed epoch analysis on two groups of selected geospace disturbance events, emerging from single and isolated interplanetary drivers, and resulting in either the enhancement or the depletion of the average relativistic electron Phase Space Density (PSD). We investigate the occurrence of behavioural/temporal patterns of solar wind and geomagnetic parameters, chorus and ULF Pc5 wave activity, and the source/seed electron PSD in the outer Van Allen radiation belt. Our results indicate that the source electron population of μ = 10 MeV/G exhibits a similar behaviour during both event groups, while the μ = 1 MeV/G population can be considered as negligible for the whole process of wave excitation and electron energisation. Moreover, events that result in relativistic electron enhancement are characterised by statistically stronger and prolonged storm, substorm and wave activity, combined with an abundance of seed electrons of μ = 100 MeV/G, mostly at L* = 4–5, the nominal heart of the outer radiation belt.

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.

Radial Response of Outer Radiation Belt Relativistic Electrons During Enhancement Events at Geostationary Orbit

AbstractForecasting relativistic electron fluxes at geostationary Earth orbit (GEO) has been a long‐term goal of the scientific community, and significant advances have been made in the past, but the relation to the interior of the radiation belts, that is, to lower L‐shells, is still not clear. In this work we have identified 60 relativistic electron enhancement events at GEO to study the radial response of outer belt fluxes and the correlation between the fluxes at GEO and those at lower L‐shells. The enhancement events occurred between 1 October 2012 and 31 December 2017 and were identified using Geostationary Operational Environmental Satellite (GOES) 15 &gt;2 MeV fluxes at GEO, which we have used to characterize the radial response of the radiation belt, by comparing to fluxes measured by the Van Allen probes Energetic Particle, Composition and Thermal Plasma Suite Relativistic Electron‐Proton Telescope (ECT‐REPT) between 2.5&lt;L&lt;6.0 at E=2.1 MeV. We have found that in general the response of the radiation belts during enhancement events is cohesive for L&gt;5.0 and generally similar for L&gt;4.5. Post‐enhancement maximum fluxes show a remarkable correlation for all L&gt;4.0 although the magnitude of the pre‐existing fluxes on the outer belt plays a significant role and makes the ratio of pre‐enhancement to post‐enhancement fluxes less predictable in the region 4.0&lt;L&lt;4.5. For L&lt;4 the fluxes are poorly correlated with geostationary orbit, but they also tend to be less variable. We have also examined SYM‐H, Kp, and AE indices and found that depending on their magnitude, the response of different parts of the outer belt can be better quantified.

Chorus Acceleration of Relativistic Electrons in Extremely Low L‐Shell During Geomagnetic Storm of August 2018

AbstractBased on data from the Van Allen Probes and ZH‐1 satellites, relativistic electron enhancements in extremely low L‐shell regions (reaching ) were observed during August 2018 major geomagnetic storm (minimum nT). Contrary to what occurs in the outer belt, such an intense and deep electron penetration event is rare and more interesting. Strong whistler‐mode (chorus and hiss) waves, with amplitudes 81–126 pT, were also observed in the extremely low L‐shell simultaneously (reaching ) where the plasmapause was suppressed. The bounce‐averaged diffusion coefficient calculations support that the chorus waves can play a significantly important role in diffusing and accelerating the 1–3 MeV electrons even in such low L‐shells during storms. This is the first time that the electron acceleration induced by chorus waves in the extremely low L‐shell region is reported. This new finding will help to deeply understand the electron acceleration process in radiation belt physics.

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

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

Interplanetary Parameters Leading to Relativistic Electron Enhancement and Persistent Depletion Events at Geosynchronous Orbit and Potential for Prediction

AbstractWe have identified 61 relativistic electron enhancement events and 21 relativistic electron persistent depletion events during 1996 to 2006 from the Geostationary Operational Environmental Satellite (GOES) 8 and 10 using data from the Energetic Particle Sensor (EPS) &gt;2 MeV fluxes. We then performed a superposed epoch time analysis of the events to find the characteristic solar wind parameters that determine the occurrence of such events, using the OMNI database. We found that there are clear differences between the enhancement events and the persistent depletion events, and we used these to establish a set of threshold values in solar wind speed, proton density and interplanetary magnetic field (IMF) Bz that can potentially be useful to predict sudden increases in flux. Persistent depletion events are characterized by a low solar wind speed, a sudden increase in proton density that remains elevated for a few days, and a northward turning of IMF Bz shortly after the depletion starts. We have also found that all relativistic electron enhancement or persistent depletion events occur when some geomagnetic disturbance is present, either a coronal mass ejection or a corotational interaction region; however, the storm index, SYM‐H, does not show a strong connection with relativistic electron enhancement events or persistent depletion events. We have tested a simple threshold method for predictability of relativistic electron enhancement events using data from GOES 11 for the years 2007–2010 and found that around 90% of large increases in electron fluxes can be identified with this method.

The effects of magnetospheric processes on relativistic electron dynamics in the Earth's outer radiation belt

AbstractUsing the electron phase space density (PSD) data measured by Van Allen Probe A from January 2013 to April 2015, we investigate the effects of magnetospheric processes on relativistic electron dynamics in the Earth's outer radiation belt during 50 geomagnetic storms. A statistical study shows that the maximum electron PSDs for various μ (μ = 630, 1096, 2290, and 3311 MeV/G) at L*~4.0 after the storm peak have good correlations with storm intensity (cc~0.70). This suggests that the occurrence and magnitude of geomagnetic storms are necessary for relativistic electron enhancements at the inner edge of the outer radiation belt (L* = 4.0). For moderate or weak storm events (SYM‐Hmin &gt; ~−100 nT) with weak substorm activity (AEmax &lt; 800 nT) and strong storm events (SYM‐Hmin ≤ ~−100 nT) with intense substorms (AEmax ≥ 800 nT) during the recovery phase, the maximum electron PSDs for various μ at different L* values (L* = 4.0, 4.5, and 5.0) are well correlated with storm intensity (cc &gt; 0.77). For storm events with intense substorms after the storm peak, relativistic electron enhancements at L* = 4.5 and 5.0 are observed. This shows that intense substorms during the storm recovery phase are crucial to relativistic electron enhancements in the heart of the outer radiation belt. Our statistics study suggests that magnetospheric processes during geomagnetic storms have a significant effect on relativistic electron dynamics.

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.

Combined effects of concurrent Pc5 and chorus waves on relativistic electron dynamics

Abstract. We present electron phase space density (PSD) calculations as well as concurrent Pc5 and chorus wave activity observations during two intense geomagnetic storms caused by interplanetary coronal mass ejections (ICMEs) resulting in contradicting net effect. We show that, during the 17 March 2013 storm, the coincident observation of chorus and relativistic electron enhancements suggests that the prolonged chorus wave activity seems to be responsible for the enhancement of the electron population in the outer radiation belt even in the presence of pronounced outward diffusion. On the other hand, the significant depletion of electrons, during the 12 September 2014 storm, coincides with long-lasting outward diffusion driven by the continuous enhanced Pc5 activity since chorus wave activity was limited both in space and time.