Loss Of Radiation Belt Electrons Research Articles

Statistical Properties of Exohiss Waves and Associated Scattering Losses of Radiation Belt Electrons

AbstractExohiss waves are a type of structureless whistler‐mode waves that exist in the low density plasmatrough outside the plasmapause and may potentially perturb the motions of electrons in the radiation belt and ring current. Using data from Van Allen Probe A, we analyze the distribution of magnetic power spectral density (PSD) of exohiss waves in different magnetic local time (MLT) and L‐shell regions near the geomagnetic equator. The results reveal that the peak magnetic PSD of exohiss waves is the weakest at MLT = 0–6 and the strongest at MLT = 12–18. The magnetic PSDs of exohiss waves are much lower than those of chorus and hiss waves except for the MLT = 12–18 sector. In addition, we calculated the quasi‐linear bounce‐averaged pitch angle and momentum diffusion coefficients (〈Dαα〉 and 〈Dpp〉) of electrons caused by exohiss waves. The diffusion coefficients are then compared with those caused by chorus and hiss waves. The peak 〈Dαα〉 of electrons driven by exohiss waves becomes stronger as L‐shell increases at all MLTs and is the greatest on the dayside, especially in the sector of MLT = 12–18. Exohiss waves have more significant effect on the loss of radiation belt electrons with specific energy levels related to MLT and L‐shell region compared to chorus and hiss waves. On the other hand, 〈Dpp〉 of electrons caused by exohiss waves is very small, which illustrates that exohiss waves have almost no acceleration effect on electrons.

Modeling the Effects of Drift Orbit Bifurcation on the Magnetopause Shadowing Loss of Radiation Belt Electrons

AbstractLast closed drift shell (LCDS) has been identified as a crucial parameter for investigating the magnetopause shadowing loss of radiation belt electrons. However, drift orbit bifurcation (DOB) effects have not been physically incorporated into the LCDS calculation. Here we calculate event‐specific LCDS using different approaches to dealing with the DOB effects, that is, tracing field lines ignoring DOB, tracing test particles rejecting field lines with DOB, and tracing particles including field lines with DOB, and then incorporate them into a radial diffusion model to simulate the fast electron dropout observed by Van Allen Probes in May 2017. The model effectively captures the fast dropout at high L* and exhibits the best agreement with data when LCDS is calculated by tracing test particles with DOB more physically included. This study represents the first quantitative modeling of the DOB effects on radiation belt magnetopause shadowing loss via a more physical specification of LCDS.

Wave analysis during energetic electron microinjections: A case study

The Magnetospheric Multi-scale Mission has frequently observed periodic bursts of counterstreaming electrons with energies ranging from ≈ 30 to 500 keV at the Earth's magnetospheric boundary layers, termed “microinjections.” Recently, a source region for microinjections was discovered at the high-latitude magnetosphere where microinjections showed up simultaneously at all energy channels and were organized by magnetic field variation associated with ultra low frequency mirror mode waves (MMWs) with ≈ 5 min periodicity. These MMWs were associated with strong higher frequency electromagnetic wave activity. Here, we have identified some of these waves as electromagnetic ion cyclotron (EMIC) waves. EMIC waves and parallel electric fields often lead to the radiation belt electron losses due to pitch-angle scattering. We show that, for the present event, the EMIC waves are not responsible for scattering electrons into a loss cone, and thus, they are unlikely to be responsible for the observed microinjection signature. We also find that the parallel electric field potentials within the waves are not adequate to explain the observed electrons with >90 keV energies. While whistler waves may contribute to the electron scattering and may exist during this event, there was no burst mode data available to verify this.

The Evolution of Whistler Hiss‐Like Waves Across Plasmapause: Hiss to Exohiss

AbstractThe exohiss waves are hiss‐like whistler mode waves that occur outside plasmapause. Although these waves can cause the loss of radiation belt electrons ∼<MeV through resonant scattering, the formation and evolution processes of exohiss waves remain incompletely covered yet. In this letter, by virtue of measurement from Van Allen Probes and ray tracing simulation, we provide a direct observational evidence for the origin of exohiss waves. We analyze the evolution of power spectrum density of hiss‐like whistler waves during their propagation. The results suggest that exohiss waves originate from mixed result of hiss waves leaked out from different L shells inside the plasmapause. Additionally, we found that frequencies of exohiss waves are primarily below 700 Hz, it is probably because the hiss waves with higher frequencies experience more severe Landau damping. Our results significantly improve understanding of the evolution of whistler waves in radiation belts.

Modeling the Effects of Drift Orbit Bifurcation on Radiation Belt Electrons

AbstractDrift orbit bifurcation (DOB) has been suggested to play a major role in the loss and transport of radiation belt electrons since it violates the second adiabatic invariants of particles and makes the third invariant undefined. Results from our guiding‐center test particle simulations using the Tsyganenko‐1989c magnetic field model show that the DOB could affect a broad region of the outer radiation belt, which can penetrate inside the geosynchronous orbit at Kp ≥ 3, and its effects are more significant further away from the Earth, at a higher Kp and for higher electron energies. Specifically, the short‐term simulation results after one electron drift show both traditional and nontraditional DOB transport of electrons, with the nontraditional DOB, caused by a third minimum of the magnetic field strength near the equator, reported here for the first time. Moreover, our results show large ballistic jumps in the second invariant and radial distance for electrons at high equatorial pitch angles after one drift. In addition, long‐term DOB transport coefficients of electrons over many drifts are calculated based on our simulation results. We find that the pitch angle and radial diffusion coefficients of electrons due to DOB could be comparable to or even larger than those caused by electron interactions with chorus and Ultra‐Low‐Frequency waves, respectively. In sum, our results demonstrate that DOB could cause effective loss and transport of radiation belt electrons even in the absence of waves.

Quantifying Radiation Belt Electron Loss Processes at L

We present a comprehensive analysis of the processes that lead to quasilinear pitch-angle-scattering loss of electrons from the L<4 region of the Earth's inner magnetosphere during geomagnetically quiet times. We consider scattering via Coulomb collisions, hiss waves, lightning-generated whistler (LGW) waves, waves from ground-based very-low-frequency (VLF) transmitters, and electromagnetic ion cyclotron (EMIC) waves. The amplitude, frequency, and wave normal angle spectra of these waves are parameterized with empirical wave models, which are then used to compute pitch-angle diffusion coefficients. From these coefficients, we estimate the decay timescales, or lifetimes, of 30keV to 4MeV electrons and compare the results with timescales obtained from in-situ observations. We demonstrate good quantitative agreement between the two over most of the L and energy range under investigation. Our analysis suggests that the electron decay timescales are very sensitive to the choice of plasmaspheric density model. At L<2, where our theoretical lifetimes do not agree well with the observations, we show that including Coulomb energy drag (ionization energy loss) in our calculations significantly improves the quantitative agreement with the observed decay timescales. We also use an accurate model of the geomagnetic field to provide an estimate of the effect that the drift-loss cone has on the theoretically calculated electron lifetimes, which are usually obtained using an axisymmetric dipole field.

Global Map of Chorus Wave Sizes in the Inner Magnetosphere

AbstractThe dynamics of the radiation belts are largely affected by the resonant interactions of energetic particles with plasma waves, such as whistler mode chorus waves. These waves play a direct and crucial role in the acceleration and loss of radiation belt electrons. The properties of these waves have been investigated thoroughly using various missions that carried wave instruments within the inner magnetosphere. One of the most conspicuous properties of chorus waves is the size of the wave packets. Past studies have estimated the size of chorus wave packets as a function of radial distance, magnetic latitude, and geomagnetic activity. This study takes advantage of the comprehensive wave data from the Van Allen Probes mission to draw a global map of the size of chorus wave packets in the inner magnetosphere. Results show that the size of lower band chorus wave packets varies between ∼1,000 and2,000 km depending on the intensity and activity of chorus waves in the inner magnetosphere. In general, the largest chorus wave packets are observed on the dawn‐dayside where chorus wave activity is also high. The results will help us better understand the interaction region and the generation mechanism of the chorus waves.

Phase Space Density Analysis of Outer Radiation Belt Electron Energization and Loss During Geoeffective and Nongeoeffective Sheath Regions

AbstractCoronal mass ejection driven sheath regions are one of the key drivers of drastic outer radiation belt responses. The response can however be significantly different based on the sheath properties and the associated inner magnetospheric wave activity. We performed two case studies on the effects of sheaths on outer belt electrons of various energies using data from the Van Allen Probes. One sheath caused a major geomagnetic disturbance and the other had only a minor impact. We especially investigated the phase space density (PSD) of seed, core, and ultrarelativistic electrons to determine the dominant energization and loss processes taking place during the events. Both sheaths produced substantial variation in the electron fluxes from tens of kiloelectronvolts up to ultrarelativistic energies. The responses were however the opposite: the geoeffective sheath mainly led to enhancement, while the nongeoeffective one caused a depletion throughout most of the outer belt. The case studies highlight that both inward and outward radial transport driven by ultra‐low frequency waves played an important role in both electron energization and loss. Additionally, PSD radial profiles revealed a local peak that indicated significant acceleration to core energies by chorus waves during the geoeffective event. The distinct responses and different mechanisms in action during these events were related to the timing of the peaked solar wind dynamic pressure causing magnetopause compression, and the differing levels of substorm activity. The most remarkable changes in the radiation belt system occurred in key sheath sub‐regions near the shock and the ejecta leading edge.

Parametric Dependence of Polarization Reversal Effects on the Particle Pitch Angle Scattering by EMIC Waves

AbstractElectromagnetic ion cyclotron (EMIC) waves play an important role in the loss of radiation belt electrons and ring current protons. After EMIC waves are excited near the magnetic equator, they will propagate to high latitudes and suffer a polarization reversal from left‐handed to right‐handed polarized at the crossover frequency. To improve the current understanding of the EMIC wave‐induced loss of radiation belt electrons and ring current protons, we perform a detailed analysis of the sensitivity of polarization reversal effects to L‐shell, electron number density, and wave frequency spectrum. Our results show that polarization reversal contributes significantly to the pitch angle diffusion coefficients at low pitch angles extending to the loss cone angle for various parameter sets. Compared with the diffusion coefficients only using L‐mode dispersion relation, inclusion of polarization reversal mainly enhances the electron scattering loss at several MeV and weaken the proton scattering loss over a broad energy range from a few keV to 1 MeV. While the increase of electron diffusion coefficients can be up to a factor of 5, the decrease of proton diffusion coefficients can be larger than an order of magnitude. As L‐shell or electron density increases, the affected resonant region by polarization reversal for both electrons and protons tends to expand to lower energies, while an increase of peak wave frequency will shift the resonant region to higher energies. Our results confirm the vital importance of including polarization reversal in understanding the EMIC wave‐induced scattering loss of radiation belt electrons and ring current protons.

Complementary and Catalytic Roles of Man‐Made VLF Waves and Natural Plasma Waves in the Loss of Radiation Belt Electrons

AbstractBy simulating the energy and pitch angle diffusions of radiation belt electrons caused by ground‐based VLF transmitter waves, plasmaspheric hiss and lightning‐generated whistlers (LGWs), we quantitatively estimated the electron change rates caused by individual and multiple waves. We found that man‐made VLF waves and naturally generated hiss or LGWs play complementary and catalytic roles in the loss of radiation belt electrons. Man‐made VLF waves mainly reduce the hundreds of keV electrons in inner radiation belt (L &lt; 2), but they alone cannot effectively remove the MeV electrons in slot region (L ∼ 2–3) because of unefficient diffusions at small pitch angles (&lt;60°). On the contrary, natural hiss and LGWs do not affect the hundreds of keV electrons in the inner belt, but they are able to effectively reduce the hundreds of keV and MeV electrons at small pitch angles in the slot region. The loss of the small pitch electrons caused by hiss or LGWs offer a necessary phase space density gradient for the diffusion of large pitch angle MeV electrons (&gt;60°) driven by man‐made VLF waves toward the loss cone. The combined diffusions by three types of the waves ultimately catalyze the loss of the large pitch angle MeV electrons in the slot region, and thus cause more electron losses in the wider energy and pitch angle ranges.

A Comparison of Radial Diffusion Coefficients in 1‐D and 3‐D Long‐Term Radiation Belt Simulations

AbstractRadial diffusion is one of the dominant physical mechanisms driving acceleration and loss of radiation belt electrons. A number of parameterizations for radial diffusion coefficients have been developed, each differing in the data set used. Here, we investigate the performance of different parameterizations by Brautigam and Albert (2000), https://doi.org/10.1029/1999ja900344, Brautigam et al. (2005), https://doi.org/10.1029/2004ja010612, Ozeke et al. (2014), https://doi.org/10.1002/2013ja019204, Ali et al. (2015), https://doi.org/10.1002/2014ja020419; Ali et al. (2016), https://doi.org/10.1002/2016ja023002; Ali (2016), and Liu et al. (2016), https://doi.org/10.1002/2015gl067398 on long‐term radiation belt modeling using the Versatile Electron Radiation Belt (VERB) code, and compare the results to Van Allen Probes observations. First, 1‐D radial diffusion simulations are performed, isolating the contribution of solely radial diffusion. We then take into account effects of local acceleration and loss showing additional 3‐D simulations, including diffusion across pitch‐angle, energy, and mixed diffusion. For the L* range studied, the difference between simulations with Brautigam and Albert (2000), https://doi.org/10.1029/1999ja900344, Ozeke et al. (2014), https://doi.org/10.1002/2013ja019204, and Liu et al. (2016), https://doi.org/10.1002/2015gl067398 parameterizations is shown to be small, with Brautigam and Albert (2000), https://doi.org/10.1029/1999ja900344 offering the smallest averaged (across multiple energies) absolute normalized difference with observations. Using the Ali et al. (2016), https://doi.org/10.1002/2016ja023002 parameterization tended to result in a lower flux than both the observations and the VERB simulations using the other coefficients. We find that the 3‐D simulations are less sensitive to the radial diffusion coefficient chosen than the 1‐D simulations, suggesting that for 3‐D radiation belt models, a similar result is likely to be achieved, regardless of whether Brautigam and Albert (2000), https://doi.org/10.1029/1999ja900344, Ozeke et al. (2014), https://doi.org/10.1002/2013ja019204, and Liu et al. (2016), https://doi.org/10.1002/2015gl067398 parameterizations are used.

The First Observation of N+ Electromagnetic Ion Cyclotron Waves

AbstractObservations from past space missions report on the significant abundance of N+, in addition to those of O+, outflowing from the terrestrial ionosphere and populating the near‐Earth region. However, instruments on board current space missions lack the mass resolution to distinguish between the two, and often the role of N+ in regulating the magnetosphere dynamics, is lumped together with that of O+ ions. For instance, our understanding regarding the role of electromagnetic ion cyclotron (EMIC) waves in controlling the loss and acceleration of radiation belt electrons and ring current ions has been based on the contribution of He+ and O+ ions only. We report the first observations by Van Allen Probes of linearly polarized N+ EMIC waves, which confirm the presence of N+ in the terrestrial magnetosphere, and open up new avenues to particle energization, loss, and transport mechanisms, based on the altered magnetospheric plasma composition.

Dynamic Mechanisms Associated With High‐Energy Electron Flux Dropout in the Earth's Outer Radiation Belt Under the Influence of a Coronal Mass Ejection Sheath Region

AbstractThe near‐Earth interplanetary environment conditions affect the dynamics of the relativistic electron population quasitrapped in the radiation belts. A complex chain of processes observed in the magnetosphere can contribute to the variability of these populations when interplanetary structures, such as the interplanetary counterpart of a solar coronal mass ejection (ICME), and high‐speed solar wind streams interact with the magnetosphere. However, as these processes can coexist, it is hard to untangle the relative contribution of each process to the loss of particles and the eventual repopulation. Here we show evidence that it is possible to distinguish the relative contribution of mechanisms related to the loss of the outer radiation belt electrons for an event observed on July 19 and 20, 2016. The interaction of an ICME's turbulent sheath with the Earth's magnetosphere resulted in a decrease in the outer radiation belt relativistic electron population. The ultralow frequency (ULF) and chorus wave activities are detected in the outer radiation belt during the time when the Earth's magnetosphere is under the influence of the ICME's sheath region, as well as the ICME's magnetic cloud region, while the electromagnetic ion cyclotron (EMIC) waves in the outer belt are observed only during the sheath region. Dynamic mechanisms such as magnetopause shadowing, outward radial diffusion driven by ULF waves, pitch‐angle scattering driven by both EMIC and chorus waves are quantitatively analyzed. Our results suggest that the structures of the ICMEs can trigger the drivers to generate the different dynamic mechanisms responsible for the radiation belt population variability.

Multi‐Parameter Chorus and Plasmaspheric Hiss Wave Models

AbstractThe resonant interaction of energetic particles with plasma waves, such as chorus and plasmaspheric hiss waves, plays a direct and crucial role in the acceleration and loss of radiation belt electrons that ultimately affect the dynamics of the radiation belts. In this study, we use the comprehensive wave data measurements made by the Electric and Magnetic Field Instrument Suite and Integrated Science instruments on board the two Van Allen probes, to develop multi‐parameter statistical chorus and plasmaspheric hiss wave models. The models of chorus and plasmaspheric hiss waves are presented as a function of combined geomagnetic activity (AE), solar wind velocity (V), and southward interplanetary magnetic field (Bs). The relatively smooth wave models reveal new features. Despite, the coupling between geomagnetic and solar wind parameters, the results show that each parameter still carries a sufficient amount of unique information to more accurately constrain the chorus and plasmaspheric hiss wave intensities. The new wave models presented here highlight the importance of multi‐parameter wave models, and can improve radiation belt modeling.

Precipitation Loss of Radiation Belt Electrons by Two‐Band Plasmaspheric Hiss Waves

AbstractA two‐band plasmaspheric hiss consisting of a low‐frequency band (normal hiss with the frequency below 2 kHz) and a high‐frequency band (locally generated hiss with the frequency up to 10 kHz) was observed on 6 January 2014 by the Van Allen Probes (He et al., 2019, https://doi.org/10.1029/2018GL081578). The electron scattering effect driven by this kind of two‐band plasmaspheric hiss is evaluated by the quasi‐linear diffusion simulation for the first time. Realistic wave characteristic parameters of the two‐band plasmaspheric hiss from statistics are adopted for driving our simulation. The pitch angle diffusion rates of the low‐frequency band hiss present a “gap” with minimum magnitude at pitch angle αe ∼ 70°, a condition not favoring the transport of large pitch angle electrons toward the loss cone. However, the diffusion rates of the high‐frequency band hiss have peak values at αe ∼ 70°, filling up for the “gap” of the low‐frequency hiss diffusion rates. The realistic wave‐driven electron PSD evolutions demonstrate that the collaborated effect of the low‐frequency band and high‐frequency band hiss can cause significant precipitation losses of energetic electrons of tens to several hundred keV within 2 days.

Effects of Polarization Reversal on the Pitch Angle Scattering of Radiation Belt Electrons and Ring Current Protons by EMIC Waves

AbstractIn this study, we investigate the effects of polarization reversal of electromagnetic ion cyclotron (EMIC) waves at the crossover frequencies on computations of bounce‐averaged pitch angle diffusion coefficients of radiation belt electrons and ring current protons. We find that inclusion of polarization reversal can cause significant changes of H+ band‐induced particle diffusion coefficients, while scattering by the He+ band is almost unaffected. Our results show that the pure L‐mode approach, which has been widely implemented in previous studies, tends to underestimate the diffusion coefficients of ultrarelativistic (&gt;4 MeV) electrons and overestimate those of 10–50 and &gt;100 keV protons caused by H+ band EMIC waves. Especially for &gt;100 keV protons, the differences in diffusion coefficients can be larger by an order of magnitude. We confirm that the polarization reversal can contribute importantly to the scattering loss of radiation belt electrons and ring current protons by H+ band EMIC waves.

Properties of Lightning Generated Whistlers Based on Van Allen Probes Observations and Their Global Effects on Radiation Belt Electron Loss

AbstractLightning generated whistlers (LGWs) play an important role in precipitating energetic electrons in the Earth's inner radiation belt and beyond. Wave burst data from the Van Allen Probes are used to unambiguously identify LGWs and analyze their properties at L &lt; 4 by extending their frequencies down to ~100 Hz for the first time. The statistical results show that LGWs typically occur at frequencies from 100 Hz to 10 kHz with the major wave power below the equatorial lower hybrid resonance frequency, and their wave amplitudes are typically strong at L &lt; 3 with an occurrence rate up to ~30% on the nightside. The lifetime calculation indicates that LGWs play an important role in scattering electrons from tens of keV to several MeV at L &lt; ~2.5. Our newly constructed LGW models are critical for evaluating the global effects of LGWs on energetic electron loss at L &lt; 4.

Hot Plasma Effects on the Pitch-angle Scattering Rates of Radiation Belt Electrons Due to Plasmaspheric Hiss

Abstract Plasmaspheric hiss is known to be a major contributor to the dynamic losses of Earth’s radiation belt electrons. While previous computation attempts of hiss-driven electron losses are limited to the cold plasma approximation, in this study we find that hot plasma effects will modify the hiss dispersion relation and result in changes in the electron bounce-averaged electron pitch angle diffusion coefficients. Cold plasma approximation tends to overestimate the diffusion coefficients of ≲100 keV electrons by orders of magnitude, while the scattering efficiency of higher energy electrons is not greatly affected. As the L-shell decreases or the parameter decreases (where is the electron gyrofrequency and is the plasma frequency), the decrease of diffusion coefficients of low energy electrons caused by hot plasma effects become more pronounced. It is also shown that both the increase of hot electron abundance and temperature anisotropy can weaken the scattering efficiency of ≲100 keV electrons at almost all pitch angles, while the diffusion coefficients of higher energy electrons decrease at large pitch angles. Our study confirms the importance of including hot plasma effects in evaluations of hiss-driven scattering loss of radiation belt electrons.

The Role of the Dynamic Plasmapause in Outer Radiation Belt Electron Flux Enhancement

AbstractThe plasmasphere is a highly dynamic toroidal region of cold, dense plasma around Earth. Plasma waves exist both inside and outside this region and can contribute to the loss and acceleration of high energy outer radiation belt electrons. Early observational studies found an apparent correlation on long time scales between the observed inner edge of the outer radiation belt and the modeled innermost plasmapause location. More recent work using high‐resolution Van Allen Probes data has found a more complex relationship. For this study, we determine the standoff distance of the location of maximum electron flux of the outer belt MeV electrons from the plasmapause following rapid enhancement events. We find that the location of the outer radiation belt based on maximum electron flux is consistently outside the plasmapause, with a peak radial standoff distance of ∆L ~ 1. We discuss the implications this result has for acceleration mechanisms.

Persistent EMIC Wave Activity Across the Nightside Inner Magnetosphere

AbstractElectromagnetic ion cyclotron (EMIC) waves can act as a loss process for both ring current ions and radiation belt electrons, and the spatial and temporal characteristics of these waves are important for quantifying their effects on energetic particles. Here we utilize observations from multiple spacecraft to constrain the azimuthal and radial dimensions as well as the duration of an EMIC wave event occurring on the nightside of the inner magnetosphere on 7 July 2013. These combined observations reveal waves limited to a narrow radial extent but persisting ~10+ hr and spanning ~12 hr in local time. The solar wind conditions, geomagnetic activity, and plasma environment are also examined to better understand the conditions under which persistent nightside EMIC waves can occur. Relativistic electron phase space density profiles during this event reveal local minima concurrent with the wave activity, consistent with EMIC‐driven scattering and loss of radiation belt electrons.