The Acceleration of Ultrarelativistic Electrons During a Small to Moderate Storm of 21 April 2017

Zhao, Hong; Baker, D. N.; Li, Xinlin; Jaynes, Allison; Kanekal, S.

doi:10.1029/2018gl078582

Cited by 35 publications

(47 citation statements)

References 20 publications

(43 reference statements)

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“…The results shown in Figures 1 and 2 suggest the presence of energy-dependent acceleration mechanism of ultrarelativistic electrons in the center of outer belt at L *~3 -5. These results statistically confirm the results of Zhao et al (2018) based on an event study. Zhao et al (2018) studied the evolution of radiation belt electron fluxes during a small to moderate storm on 22 April 2017.…”

Section: Statistical Analysis Of the Ultrarelativistic Electron Psd Esupporting

confidence: 88%

On the Acceleration Mechanism of Ultrarelativistic Electrons in the Center of the Outer Radiation Belt: A Statistical Study

Zhao

Baker

et al. 2019

JGR Space Physics

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Using energetic particle and wave measurements from the Van Allen Probes, Polar Orbiting Environmental Satellites (POES), and Geostationary Operational Environmental Satellite (GOES), the acceleration mechanism of ultrarelativistic electrons (>3 MeV) in the center of the outer radiation belt is investigated statistically. A superposed epoch analysis is conducted using 19 storms, which caused flux enhancements of 1.8-7.7 MeV electrons. The evolution of electron phase space density radial profile suggests an energy-dependent acceleration of ultrarelativistic electrons in the outer belt. Especially, for electrons with very high energies (~7 MeV), prevalent positive phase space density radial gradients support inward radial diffusion being responsible for electron acceleration in the center of the outer belt (L *~3 -5) during most enhancement events in the Van Allen Probes era. We propose a two-step acceleration process to explain the acceleration of~7 MeV electrons in the outer belt: intense and sustained chorus waves locally energize core electron populations to ultrarelativistic energies at high L region beyond the Van Allen Probes' apogee, followed by inward radial diffusion which further energizes these populations to even higher energies. Statistical results of chorus wave activity inferred from POES precipitating electron measurements as well as core electron populations observed by the Van Allen Probes and GOES support this hypothesis.

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Section: Statistical Analysis Of the Ultrarelativistic Electron Psd Esupporting

confidence: 88%

On the Acceleration Mechanism of Ultrarelativistic Electrons in the Center of the Outer Radiation Belt: A Statistical Study

Zhao

Baker

et al. 2019

JGR Space Physics

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“…Their observed energy‐dependent long‐term behavior of ultrarelativistic electrons suggests that the underlying physical mechanisms must be energy dependent. This energy‐dependent mechanism has also been suggested by Zhao, Baker, et al () through a detailed event study during a small to moderate storm of April 2017. Their results showed that the acceleration mechanism of ultrarelativistic electrons are clearly energy dependent during this storm: For ~3‐ to 5‐MeV electrons, the local acceleration is the most important acceleration mechanism at L ~ 4–6, while for electrons with even higher energies, the inward radial diffusion is the dominant acceleration mechanism in this region though local acceleration was very likely first causing energization at L > 6.…”

Section: Introductionsupporting

confidence: 68%

“…Results shown in Figures and suggest that, solar wind speed is, statistically, the most effective solar wind parameters in causing the energy‐dependent acceleration of ultrarelativistic electrons. Zhao, Baker, et al () studied the acceleration mechanism of ultrarelativistic electrons during a small to moderate storm and showed the energy‐dependent acceleration mechanism: For ~3‐ to 5‐MeV electrons, the local acceleration plays the most important role at L ~ 4–6, while for ~7‐MeV electrons, inward radial diffusion was identified as the main acceleration mechanism in this region, which further accelerated electrons that energized through local acceleration at L > ~6. This is consistent with our results, which shows the importance of high solar wind speed in accelerating ultrarelativistic electrons with very high energies and could suggest a more dominating role of inward radial diffusion for the higher‐energy ultrarelativistic electron flux enhancements.…”

Section: The Superposed Epoch Analysis Of Solar Wind/geomagnetic Paramentioning

confidence: 99%

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

Zhao

Baker

et al. 2019

JGR Space Physics

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

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“…The key to such rapid acceleration is the interaction of “seed” populations of ~30‐ to ~300‐keV electrons with electromagnetic waves in the lower band whistler‐mode chorus frequency range (e.g., Jaynes et al, ). On somewhat longer timescales ultralow frequency (ULF) waves can play a strong role in driving radial diffusion as well (Li et al, ; Mann et al, ; Zhao et al, ; Jaynes et al, ).…”

Section: Introductionmentioning

confidence: 99%

Multiyear Measurements of Radiation Belt Electrons: Acceleration, Transport, and Loss

et al. 2019

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In addition to clarifying morphological structures of the Earth's radiation belts, it has also been a major achievement of the Van Allen Probes mission to understand more thoroughly how highly relativistic and ultrarelativistic electrons are accelerated deep inside the radiation belts. Prior studies have demonstrated that electrons up to energies of 10 megaelectron volts (MeV) can be produced over broad regions of the outer Van Allen zone on timescales of minutes to a few hours. It often is seen that geomagnetic activity driven by strong solar storms (i.e., coronal mass ejections, or CMEs) almost inexorably leads to relativistic electron production through the intermediary step of intense magnetospheric substorms. In this study, we report observations over the 6‐year period 1 September 2012 to 1 September 2018. We focus on data about the relativistic and ultrarelativistic electrons (E≥5 MeV) measured by the Relativistic Electron‐Proton Telescope sensors on board the Van Allen Probes spacecraft. This work portrays the radiation belt acceleration, transport, and loss characteristics over a wide range of geomagnetic events. We emphasize features seen repeatedly in the data (three‐belt structures, “impenetrable” barrier properties, and radial diffusion signatures) in the context of acceleration and loss mechanisms. We especially highlight solar wind forcing of the ultrarelativistic electron populations and extended periods when such electrons were absent. The analysis includes new display tools showing spatial features of the mission‐long time variability of the outer Van Allen belt emphasizing the remarkable dynamics of the system.

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The Acceleration of Ultrarelativistic Electrons During a Small to Moderate Storm of 21 April 2017

Cited by 35 publications

References 20 publications

On the Acceleration Mechanism of Ultrarelativistic Electrons in the Center of the Outer Radiation Belt: A Statistical Study

On the Acceleration Mechanism of Ultrarelativistic Electrons in the Center of the Outer Radiation Belt: A Statistical Study

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

Multiyear Measurements of Radiation Belt Electrons: Acceleration, Transport, and Loss

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