Cosmic Ray Albedo Neutron Decay Research Articles

Dynamics of the inner electron radiation belt: A review

The Van Allen radiation belts are an extraordinary science discovery in the Earth magnetosphere and consist of two electron belts. The inner Van Allen belt contains electrons of 10s to 100s keV; the outer belt consists mainly of 0.1−10 MeV electrons. Their dynamics have been analyzed for decades. The newly-launched Van Allen Probes provide unprecedented opportunities to investigate the inner belt more thoroughly. Data from this advanced mission have allowed scientists to demonstrate that the inner belt was formed not only through inward transport of outer belt electrons but Cosmic Ray Albedo Neutron Decay (CRAND) has also played an important role. In addition, the inner belt electrons show energy-dependent variations and present “zebra stripe” structures in the energy spectrum. At the same time, scientists have further confirmed that the electrons in the inner radiation belt get lost through coulomb collision and wave-particle interaction. Despite these advances, important questions remain unanswered and require further investigation. The launch of Macau ScienceSatellite-1 mission, with its low inclination angle and low altitude orbit, will provide advanced radiation belt data for better understanding of the structure and dynamics of the inner electron radiation belt.

Quasi‐Trapped Electron Fluxes Induced by NWC Transmitter and CRAND: Observations and Simulations

AbstractSignals from the NWC ground‐based very low frequency (VLF) transmitter can leak into the magnetosphere and scatter trapped energetic electrons into drift loss cones. Recent studies also suggest that cosmic ray albedo neutron decay (CRAND) is probably an important source for quasi‐trapped electrons in the inner belt. To investigate their relative contributions, this study comprehensively analyzes the long‐term variations of quasi‐trapped 206 keV electrons at L = 1.7, which is roughly the L shell where NWC is located. Furthermore, a drift‐diffusion‐source model is used to reproduce longitudinal distributions of quasi‐trapped electrons and investigate sensitivities of simulation results to VLF transmitter intensities. These results suggest that CRAND is the main source of quasi‐trapped hundreds of keV electrons when the NWC station is at dayside. In contrast, pitch angle diffusions become the main source mechanism of these quasi‐trapped electrons when the NWC station operates at nightside with more VLF transmitter energy leaking into the magnetosphere.

Monte Carlo simulation of CRAND protons trapped at low Earth orbits

The Cosmic Ray Albedo Neutron Decay (CRAND) is believed to be the principal mechanism for the formation of inner proton radiation belt -- at least for relatively higher energy particles. We implement this mechanism in a Monte Carlo simulation procedure to calculate the trapped proton radiation at the low Earth orbits, through event-by-event interaction of the cosmic ray particles in the Earth's atmosphere and their transportation in the magnetosphere. We consider the generation of protons from subsequent decay of the secondary neutrons from the cosmic ray interaction in the atmosphere and their transport (and/or trapping) in the geomagnetic field. We address the computational challenges for this type of calculations and develop an optimized algorithm to minimize the computation time. We consider a full 3D description of the Earth's atmospheric and magnetic-field configurations using the latest available models. We present the spatial and phase-space distribution of the trapped protons considering the adiabatic invariants and other parameters at the low Earth orbits. We compare the simulation results with the trapped proton flux measurements made by PAMELA experiment at low Earth orbit and explain certain features observed by the measurement.

High Latitude Zones of GeV Heavy Ions at the Inner Edge of Jupiter's Relativistic Electron Belt

AbstractJuno has discovered signatures of a high energy heavy ion population (>100 MeV/nucleon) within the inner edge of Jupiter's relativistic electron belt. The particles were detected in narrow zones in the high‐latitude lobes of the synchrotron emission region, a location never explored by prior spacecraft (∼31°–46° magnetic latitude; radial distances 1.12–1.41 Jovian radii, M‐shells 1.5–2.37). The population was revealed by extremely high signal ionization signatures seen in images collected by Juno's Stellar Reference Unit star camera (equivalent to 0.2–0.7 MeV of deposited energy per pixel). Signature morphology indicates a population of GeV ions (Z > 1) with atomic mass as high as sulfur, although species with 2 ≤ Z ≤ 8 potentially account for all of the signatures. The detections have occurred while the spacecraft is magnetically connected to Jupiter's halo and gossamer rings, but not while it is connected to the main ring; similar to equatorial observations of GeV carbon and sulfur ions by the Galileo Probe. We find that the particles reside on magnetic drift shells that suggest they are stably trapped within Jupiter's magnetosphere. Because the cosmic ray albedo neutron decay mechanism can only produce protons and electrons, it cannot be the source of this trapped Z > 1 ion population; other galactic cosmic ray origins are suspected. Inward radial transport of tens of keV heavy ions from the outer magnetosphere is another potential source.

Optimization of Radial Diffusion Coefficients for the Proton Radiation Belt During the CRRES Era

AbstractProton flux measurements from the Proton Telescope instrument aboard the CRRES satellite are revisited, and used to drive a radial diffusion model of the inner proton belt at 1.1 ≤ L ≤ 1.65. Our model utilizes a physics‐based evaluation of the cosmic ray albedo neutron decay (CRAND) source, and coulomb collisional loss is driven by a drift averaged density model combining results from the International Reference Ionosphere, NRLMSIS‐00 atmosphere and Radio Plasma Imager plasmasphere models, parameterized by solar activity and season. We drive our model using time‐averaged data at L = 1.65 to calculate steady state profiles of equatorial phase space density, and optimize our choice of radial diffusion coefficients based on four defining parameters to minimize the difference between model and data. This is first performed for a quiet period when the belt can be assumed to represent steady state. Additionally, we investigate fitting steady state solutions to time averages taken during active periods where the data exhibits limited deviation from steady state, demonstrated by CRRES measurements following the March 24, 1991 storm. We also discuss a way to make the optimization process more reliable by excluding periods of variability in plasmaspheric density from any time average. Lastly, we compare our resultant diffusion coefficients to those derived via a similar process in previous work, and diffusion coefficients derived for electrons from ground and in situ observations. We find that higher diffusion coefficients are derived compared with previous work, and suggest more work is required to derive proton diffusion coefficients for different geomagnetic activity levels.

Dynamics of Energetic Electrons in the Slot Region During Geomagnetically Quiet Times: Losses Due to Wave‐Particle Interactions Versus a Source From Cosmic Ray Albedo Neutron Decay (CRAND)

AbstractEarth's slot region, lying between the outer and inner radiation belts, has been identified as due to a balance between inward radial diffusion and pitch angle (PA) scattering induced by waves. However, recent satellite observations and modeling studies indicate that cosmic ray albedo neutron decay (CRAND) may also play a significant role in energetic electron dynamics in the slot region. In this study, using a drift‐diffusion‐source model, we investigate the relative contribution of all significant waves and CRAND to the dynamics of energetic electrons in the slot region during July 2014, an extended period of quiet geomagnetic activity. The bounce‐averaged PA diffusion coefficients from three types of waves (hiss, lightning‐generated whistlers [LGW], and very low frequency [VLF] transmitters) are calculated based on quasi‐linear theory, while the CRAND source follows the results in Xiang et al. (2019, https://doi.org/10.1029/2018GL081730). The simulation results indicate that both LGW and VLF transmitter waves can enhance loss and weaken the top hat PA distribution induced by hiss waves. For 470 keV electrons at L = 2.5, simulation results without CRAND show a much quicker decrease than observations from the Van Allen Probes. After including CRAND, simulated electron flux variations reproduce satellite observations, suggesting that CRAND is an important source for hundreds of keV electrons in the slot region during quiet times. The balance between the CRAND source and loss due to wave‐particle interactions provides a lower limit to relativistic electron fluxes in the slot region, which can act as an important reference point for instrument calibration when a true background level is warranted.

Long‐Term Variations of Quasi‐Trapped and Trapped Electrons in the Inner Radiation Belt Observed by DEMETER and SAMPEX

AbstractElectrons in the Earth's radiation belts can be categorized into three populations: precipitating, quasi‐trapped, and trapped. We use data from the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) and Solar, Anomalous, and Magnetospheric Particle Explorer satellite (SAMPEX) missions and from ground‐based neutron monitors (NM) and sunspot observations to investigate the long‐term variation of quasi‐trapped and trapped sub‐MeV electrons on different L shells in the inner belt. DEMETER and SAMPEX measurements span over 17 years and show that at L ≤ 1.14 the electron flux is anticorrelated with sunspot number, but proportional to the cosmic ray intensity represented by NM count rates, which suggests that electrons at the inner edge of the inner belt are produced by Cosmic Ray Albedo Neutron Decay (CRAND). The solar cycle variation of cosmic rays increased the electron flux at L ≤ 1.14 by a factor of 2 from solar maximum at 2001 to solar minimum at 2009. At L ≥ 1.2, both quasi‐trapped and trapped electrons are enhanced during geomagnetic storms and decay to a background level during extended quiet times. At L > 2, quasi‐trapped electrons resemble trapped electrons, with correlation coefficients as high as 0.97, indicating that pitch angle scattering is the dominant process in this region.

On Energetic Electron Dynamics During Geomagnetic Quiet Times in Earth's Inner Radiation Belt due to Atmospheric Collisional Loss and CRAND as a Source

AbstractTo investigate the role of atmospheric collisions and cosmic ray albedo neutron decay (CRAND) in the dynamics of energetic electrons in the Earth's inner radiation belt during geomagnetic quiet times, a drift‐collision‐source model that includes azimuthal drift, pitch angle diffusion from elastic collision, energy loss from inelastic collision, and a CRAND source is developed. In the model, the bounce‐averaged pitch angle diffusion coefficients and energy loss rates are calculated based on scattering of electrons with neutrals given by the NRLMSISE‐00 model and with ions and electrons given by International Reference Ionosphere (IRI) 2012 model. The electron source rate from CRAND follows the recently developed drift‐source model in Xiang et al. (2019). For 304‐keV quasi‐trapped electrons at L = 1.25, simulation results with CRAND show good agreement with Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions satellite observations, confirming that CRAND is the main source for these quasi‐trapped electrons, in contrast to the previous understanding that these quasi‐trapped electrons were formed by wide‐angle scattering of the trapped populations. For trapped electrons, 153, 304, and 509 keV at L < 1.3, the simulation results with only azimuthal drift and atmospheric collisions show a much quicker decrease than observations, while simulation results including a CRAND source are generally comparable to the observations, suggesting that CRAND is an important source of trapped hundreds of kiloelectron‐volt electrons at L < 1.3 during quiet times and provides a baseline for the electron flux even during active times as well. Furthermore, these results suggest that actual radial diffusion rates in the inner belt are lower than previous estimates in which CRAND contributions were not considered.

Earth's Van Allen Radiation Belts: From Discovery to the Van Allen Probes Era

AbstractDiscovery of the Earth's Van Allen radiation belts by instruments flown on Explorer 1 in 1958 was the first major discovery of the Space Age. The observation of distinct inner and outer zones of trapped megaelectron volt (MeV) particles, primarily protons at low altitude and electrons at high altitude, led to early models for source and loss mechanisms including Cosmic Ray Albedo Neutron Decay for inner zone protons, radial diffusion for outer zone electrons and loss to the atmosphere due to pitch angle scattering. This scattering lowers the mirror altitude for particles in their bounce motion parallel to the Earth's magnetic field until they suffer collisional loss. A view of the belts as quasi‐static inner and outer zones of energetic particles with different sources was modified by observations made during the Solar Cycle 22 maximum in solar activity over 1989–1991. The dynamic variability of outer zone electrons was measured by the Combined Radiation Release and Effects Satellite launched in July 1990. This variability is caused by distinct types of heliospheric structure that vary with the solar cycle. The launch of the twin Van Allen Probes in August 2012 has provided much longer and more comprehensive measurements during the declining phase of Solar Cycle 24. Roughly half of moderate geomagnetic storms, determined by intensity of the ring current carried mostly by protons at hundreds of kiloelectron volts, produce an increase in trapped relativistic electron flux in the outer zone. Mechanisms for accelerating electrons of hundreds of electron volts stored in the tail region of the magnetosphere to MeVenergies in the trapping region are described in this review: prompt and diffusive radial transport and local acceleration driven by magnetospheric waves. Such waves also produce pitch angle scattering loss, as does outward radial transport, enhanced when the magnetosphere is compressed. While quasilinear simulations have been used to successfully reproduce many essential features of the radiation belt particle dynamics, nonlinear wave‐particle interactions are found to be potentially important for causing more rapid particle acceleration or precipitation. The findings on the fundamental physics of the Van Allen radiation belts potentially provide insights into understanding energetic particle dynamics at other magnetized planets in the solar system, exoplanets throughout the universe, and in astrophysical and laboratory plasmas. Computational radiation belt models have improved dramatically, particularly in the Van Allen Probes era, and assimilative forecasting of the state of the radiation belts has become more feasible. Moreover, machine learning techniques have been developed to specify and predict the state of the Van Allen radiation belts. Given the potential Space Weather impact of radiation belt variability on technological systems, these new radiation belt models are expected to play a critical role in our technological society in the future as much as meteorological models do today.

Modeling the Quasi‐Trapped Electron Fluxes From Cosmic Ray Albedo Neutron Decay (CRAND)

AbstractRecently, cosmic ray albedo neutron decay (CRAND) has been identified as the main source of relativistic electrons measured at the inner edge of inner radiation belt. Here we introduce a drift‐source model that includes azimuthal drift and a CRAND electron source to simulate the quasi‐trapped electron distribution measured by the DEMETER satellite during 20–30 April 2010. The simulated longitude distribution of quasi‐trapped electron fluxes at the inner edge of inner radiation belt successfully reproduces the DEMETER observations, confirming CRAND as the main source for these electrons. Furthermore, a comparison of the energy spectrum and the L distribution of the quasi‐trapped relativistic electrons between simulations and observations further suggests that CRAND is likely the dominant source for 300–700‐keV quasi‐trapped electrons at L < 2 and L ≈ 3.

Cosmic Ray Albedo Neutron Decay (CRAND) as a Source of Inner Belt Electrons: Energy Spectrum Study

AbstractCosmic Ray Albedo Neutron Decay (CRAND) has been recently confirmed as a source of energetic electrons at the inner edge of the inner belt by the Colorado Student Space Weather Experiment (CSSWE) mission. Here we use observations from the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) mission, to investigate the CRAND contribution to inner belt electrons quantitatively over a broad energy range (~100–800 keV). Spectral fitting analysis supports the conclusion that CRAND is the most important electron source at the inner edge of the inner belt. For the first time, we show that CRAND is the dominant source of >250‐keV quasitrapped electrons throughout the inner belt and slot region during quiet times. We suggest that additional sources for <250‐keV electrons exist, perhaps from inward transport. In contrast, dynamics of electrons in the inner belt and slot region is dominated by injections during active times.

Saturn's Innermost Radiation Belt Throughout and Inward of the D‐Ring

AbstractCassini discovered Saturn's innermost radiation belt during the end of its mission. The belt is populated with relativistic protons, probably up to the trapping limit of ≈20 GeV. It extends from Saturn's dense atmosphere into and throughout the D‐ring. The A–C rings separate this belt entirely from the previously known radiation belts, suggesting that the innermost radiation belt is populated entirely via cosmic ray albedo neutron decay. We find that the proton pitch angle distributions are consistent with being shaped by losses to the D‐ring and the upper atmosphere rather than, for example, wave‐particle interactions. This supports that the main loss process of this new radiation belt is energy loss in neutral material, different from Saturn's other radiation belts. This property constrains the overall scale height of Saturn's exosphere to <700 km and the average D‐ring water molecule column density to being about 1 order of magnitude below the Enceladus gas torus.

Measurement of electrons from albedo neutron decay and neutron density in near-Earth space

The Galaxy is filled with cosmic-ray particles, mostly protons with kinetic energies greater than hundreds of megaelectronvolts. Around Earth, trapped energetic protons, electrons and other particles circulate at altitudes from about 500 to 40,000 kilometres in the Van Allen radiation belts. Soon after these radiation belts were discovered six decades ago, it was recognized that the main source of inner-belt protons (with kinetic energies of tens to hundreds of megaelectronvolts) is cosmic-ray albedo neutron decay (CRAND). In this process, cosmic rays that reach the upper atmosphere interact with neutral atoms to produce albedo neutrons, which, being prone to β-decay, are a possible source of geomagnetically trapped protons and electrons. These protons would retain most of the kinetic energy of the neutrons, while the electrons would have lower energies, mostly less than one megaelectronvolt. The viability of CRAND as an electron source has, however, been uncertain, because measurements have shown that the electron intensity in the inner Van Allen belt can vary greatly, while the neutron-decay rate should be almost constant. Here we report measurements of relativistic electrons near the inner edge of the inner radiation belt. We demonstrate that the main source of these electrons is indeed CRAND, and that this process also contributes to electrons in the inner belt elsewhere. Furthermore, measurement of the intensity of electrons generated by CRAND provides an experimental determination of the neutron density in near-Earth space-2 × 10-9 per cubic centimetre-confirming theoretical estimates.

A heavy ion and proton radiation belt inside of Jupiter's rings

AbstractEnergetic charged particle measurements by the Jupiter Energetic Particle Detector Instrument (JEDI) on board Juno have revealed a radiation belt of hundreds of keV ions up to the atomic mass of sulfur, located between Jupiter's rings and atmosphere. Proton energy spectra display an unusual intensity increase above 300 keV. We suggest that this is because charge exchange in Jupiter's neutral environment does not efficiently remove ions at such high energies. Since this innermost belt includes heavy ions, it cannot be exclusively supplied by cosmic ray albedo neutron decay, which is an important source at Earth and Saturn but only supplies protons and electrons. We find indications that the stripping of energetic neutral atoms in Jupiter's high atmosphere might be the ion source. Since the stripped off electrons are of low energy, this hypothesis is consistent with observations of the ratio of energetic electrons to ions being much less than 1.

Inward diffusion and loss of radiation belt protons

AbstractRadiation belt protons in the kinetic energy range 24 to 76 MeV are being measured by the Relativistic Electron Proton Telescope on each of the two Van Allen Probes. Data have been processed for the purpose of studying variability in the trapped proton intensity during October 2013 to August 2015. For the lower energies (≲32 MeV), equatorial proton intensity near L = 2 showed a steady increase that is consistent with inward diffusion of trapped solar protons, as shown by positive radial gradients in phase space density at fixed values of the first two adiabatic invariants. It is postulated that these protons were trapped with enhanced efficiency during the 7 March 2012 solar proton event. A model that includes radial diffusion, along with known trapped proton source and loss processes, shows that the observed average rate of increase near L = 2 is predicted by the same model diffusion coefficient that is required to form the entire proton radiation belt, down to low L, over an extended (∼103 year) interval. A slower intensity decrease for lower energies near L = 1.5 may also be caused by inward diffusion, though it is faster than predicted by the model. Higher‐energy (≳40 MeV) protons near the L = 1.5 intensity maximum are from cosmic ray albedo neutron decay. Their observed intensity is lower than expected by a factor ∼2, but the discrepancy is resolved by adding an unspecified loss process to the model with a mean lifetime ∼120 years.

High‐energy radiation belt electrons from CRAND

AbstractA calculation of the inner radiation belt electron source from cosmic ray albedo neutron decay (CRAND) is described. High‐energy electrons are included by Lorentz‐transforming the β decay spectrum from the neutron rest frame to the Earth's rest frame and combining with the known high‐energy albedo neutron energy spectrum. Balancing the electron source with energy loss to atmospheric neutral atoms and plasma, and with a decay lifetime representative of plasma wave scattering, then provides an estimate of trapped electron intensity. It is well below measured values for low energies, confirming that CRAND is not a significant source of those trapped electrons. For kinetic energies above the maximum β decay energy (E > 0.8 MeV) a power law energy spectrum ∼E−4 is predicted. For L = 1.5 and MeV the computed omnidirectional trapped electron intensity exceeds an extrapolation of the measured low‐energy exponential energy spectrum.

Observations of the inner radiation belt: CRAND and trapped solar protons

AbstractMeasurements of inner radiation belt protons have been made by the Van Allen Probes Relativistic Electron‐Proton Telescopes as a function of kinetic energy (24 to 76 MeV), equatorial pitch angle, and magnetic L shell, during late 2013 and early 2014. A probabilistic data analysis method reduces background from contamination by higher‐energy protons. Resulting proton intensities are compared to predictions of a theoretical radiation belt model. Then trapped protons originating both from cosmic ray albedo neutron decay (CRAND) and from trapping of solar protons are evident in the measured distributions. An observed double‐peaked distribution in L is attributed, based on the model comparison, to a gap in the occurrence of solar proton events during the 2007 to 2011 solar minimum. Equatorial pitch angle distributions show that trapped solar protons are confined near the magnetic equator but that CRAND protons can reach low altitudes. Narrow pitch angle distributions near the outer edge of the inner belt are characteristic of proton trapping limits.

Direct observation of the CRAND proton radiation belt source

Observations of geomagnetically trapped 27–45 MeV protons following the November 2003 magnetic storm show a gradual intensity rise that is interpreted as a direct measurement of the cosmic ray albedo neutron decay (CRAND) source strength. The intensity rise is simulated by combining the detector response function with a model CRAND source, obtained by drift‐averaging neutron intensity from Monte Carlo simulation of cosmic ray interactions in the atmosphere. The simulation, for 2.4<L<2.7, matches the data within statistical uncertainties, verifying the CRAND model. Another simulation includes the effects of solar proton trapping and other known radiation belt processes. It further shows that the CRAND source was predominant, while radial diffusion and magnetic storm losses effected minor corrections in certain L ranges.

Processes forming and sustaining Saturn’s proton radiation belts

Saturn’s proton radiation belts extend over the orbits of several moons that split this region of intense radiation into several distinct belts. Understanding their distribution requires to understand how their particles are created and evolve. High-energy protons are thought to be dominantly produced by cosmic ray albedo neutron decay (CRAND). The source of the lower energies and the role of other effects such as charge exchange with the gas originating from Enceladus is still an open question. There is also no certainty so far if the belts exist independently from each other and the rest of the magnetosphere or if and how particles are exchanged between these regions. We approach these problems by using measurements acquired by the MIMI/LEMMS instrument onboard the Cassini spacecraft. Protons in the range from 500keV to 40MeV are considered. Their intensities are averaged over 7years of the mission and converted to phase space densities at constant first and second adiabatic invariant. We reproduce the resulting radial profiles with a numerical model that includes radial diffusion, losses from moons and interactions with gas, and a phenomenological source. Our results show that the dominating effects away from the moon sweeping corridors are diffusion and the source, while interactions with gas are secondary. Based on a GEANT4 simulation of the interaction of cosmic rays with Saturn’s rings, we conclude that secondary particles produced within the rings can only account for the high-energy part of the source. A comparison with the equivalent processes within Earth’s atmosphere shows that Saturn’s atmosphere can contribute to the production of the lower energies and might be even dominating at the higher energies. Other possibilities to supply the belts and exchange particles between them, as diffusion and injections from outside the belts, or stripping of ENAs, can be excluded.

Energetic particle phase space densities at Saturn: Cassini observations and interpretations

[1] Saturn's magnetosphere has been studied extensively by the Cassini spacecraft during the last 6 years. We present mission-averaged energetic proton and electron measurements obtained by the MIMI/LEMMS instrument onboard Cassini in an energy range from several 10 keV to several 10 MeV separated by equatorial pitch angle. We discuss the resulting radial profiles and energy spectra. The measured intensities are converted to phase space densities. The distribution of energetic particles is governed by a large variety of processes. For instance, moons absorb energetic particles, creating macrosignatures or microsignatures. We have found that the moon Rhea is partly responsible for a change in gradient of electron phase space densities. We show that, in contrast to larger distances, the particle distribution for L < 8 is not driven by radial diffusion alone. There, the particle profiles are significantly modified due to Saturn's Neutral Torus, plasma environment, E ring, injection events, and cosmic ray albedo neutron decay. Large parts of our analysis are focused near L = 7. There, protons are lost within the Neutral Torus and not the E ring. For electrons, we find that these two losses are of comparable rate but have discovered that neither process is the dominant driver of loss. We point out that intensity measured by a energy channel, such as in a particle instrument, can actually increase in the region of ring and torus instead of decrease. The importance of injection events is shown to be at least of similar importance as radial diffusion.