Loss Of Relativistic Electrons Research Articles

Quantification of the precipitation loss of radiation belt electrons observed by SAMPEX

Based on SAMPEX/PET observations, the rates and the spatial and temporal variations of electron loss to the atmosphere in the Earth's radiation belt were quantified using a drift diffusion model that includes the effects of azimuthal drift and pitch angle diffusion. The measured electrons by SAMPEX can be distinguished as trapped, quasi‐trapped (in the drift loss cone), and precipitating (in the bounce loss cone). The drift diffusion model simulates the low‐altitude electron distribution from SAMPEX. After fitting the model results to the data, the magnitudes and variations of the electron lifetime can be quantitatively determined based on the optimum model parameter values. Three magnetic storms of different magnitudes were selected to estimate the various loss rates of ∼0.5–3 MeV electrons during different phases of the storms and at L shells ranging from L = 3.5 to L = 6.5 (L represents the radial distance in the equatorial plane under a dipole field approximation). The storms represent a small storm, a moderate storm from the current solar minimum, and an intense storm right after the previous solar maximum. Model results for the three individual events showed that fast precipitation losses of relativistic electrons, as short as hours, persistently occurred in the storm main phases and with more efficient loss at higher energies over wide range of L regions and over all the SAMPEX‐covered local times. In addition to this newly discovered common feature of the main phase electron loss for all the storm events and at all L locations, some other properties of the electron loss rates, such as the local time and energy dependence that vary with time or locations, were also estimated and discussed. This method combining model with the low‐altitude observations provides direct quantification of the electron loss rate, a prerequisite for any comprehensive modeling of the radiation belt electron dynamics.

Statistical roles of storms and substorms in changing the entire outer zone relativistic electron population

Since storms/substorms can lead to flux enhancements of relativistic electrons (E > 500 keV) in one region of the outer zone (L ∼ 2–7) and simultaneously to flux decreases in another region, the final effects of storms/substorms on changing the entire outer zone relativistic electron population are indicated by the total flux variation of the entire outer zone relativistic electrons. The total flux of the relativistic electrons is the summation of their omnidirectional integral fluxes over the entire outer zone. By analyzing the total flux variations of relativistic electrons in the entire outer zone during about 18 storms from August 1990 to March 1991, this paper investigates the statistical relationships between the total flux variations of the entire outer zone relativistic electrons and the intensities of storms/substorms. The statistical results indicate that the primary impact of a storm development is the net loss of relativistic electrons from the entire outer zone via the main phase losses of relativistic electrons, whereas the continuous intense substorm activity (average AE > 200 nT) can lead to the net increases of the relativistic electrons in the entire outer zone. Furthermore, the more intense the substorm activity, the larger the increases of the relativistic electrons, indicating that the continuous intense substorm activity can effectively supply relativistic electrons for the entire outer zone. Since the net increases of the relativistic electrons usually occur during the continuous intense substorm activity, the effective acceleration of the relativistic electron is correlated with the continuous intense substorm activity, e.g., the stochastic acceleration of electron by whistler mode chorus waves.

Ion observations from geosynchronous orbit as a proxy for ion cyclotron wave growth during storm times

There is still much to be understood about the processes contributing to relativistic electron enhancements and losses in the radiation belts. Wave particle interactions with both whistler and electromagnetic ion cyclotron (EMIC) waves may precipitate or accelerate these electrons. This study examines the relation between EMIC waves and resulting relativistic electron flux levels after geomagnetic storms. A proxy for enhanced EMIC waves is developed using Los Alamos National Laboratory Magnetospheric Plasma Analyzer plasma data from geosynchronous orbit in conjunction with linear theory. In a statistical study using superposed epoch analysis, it is found that for storms resulting in net relativistic electron losses, there is a greater occurrence of enhanced EMIC waves. This is consistent with the hypothesis that EMIC waves are a primary mechanism for the scattering of relativistic electrons and thus cause losses of such particles from the magnetosphere.

Simulation of EMIC wave excitation in a model magnetosphere including structured high‐density plumes

The HOTRAY code is used to evaluate the path integrated gain of electromagnetic ion cyclotron (EMIC) waves as a function of frequency in two propagation bands above the O+ and He+ gyrofrequencies. Calculations are performed over a range of L shell (3 < L < 7) assuming a cold H+‐He+‐O+ plasma with an additional bi‐Maxwellian hot ring current proton distribution. The cold plasma model includes a plasmasphere and high‐density storm time plume region containing spatial density fluctuations. The strongest wave gain (>40 dB) is found near the plasmapause, within regions with density structure in the plume, and in the low‐density trough at L ≥ 6.5. As a self‐consistent test on whether EMIC waves play an important role in relativistic electron loss from the radiation belts, the minimum cyclotron resonant electron energy is evaluated as a function of wave frequency and L shell for those EMIC waves that exhibit significant gain. The lowest electron resonant energies (approximately a few MeV) are found in structured plumes. The sensitivities of both the wave gain and electron minimum resonant energy to variation in thermal ion compositions, the energetic proton properties, or plume density structure are also investigated.

Modeling of outer radiation belt electrons by multidimensional diffusion process

In this study, we develop a new code which uses a hybrid finite difference (HFD) method to solve a 2‐D bounce‐averaged energy–pitch angle diffusion equation in the outer radiation belts. We implement the numerical algorithm by adopting a split operator technique, an implicit scheme for dealing with diagonal diffusion coefficients, and an alternative direction implicit scheme for off‐diagonal (or cross) diffusion coefficients. We show that our code HFD can successfully overcome the unstable problem when the large and rapidly varying cross diffusion coefficients are included. Particularly, we examine whether resonant interactions with obliquely propagated whistler mode chorus and hiss waves result in a net acceleration or loss of relativistic electrons in the outer radiation belt L = 4.5. Numerical simulations show that chorus waves can yield significant acceleration of relativistic electrons, while hiss waves are primarily responsible for scattering equatorially mirroring electrons toward the loss cone.

On the loss of relativistic electrons at geosynchronous altitude: Its dependence on magnetic configurations and external conditions

The present study statistically examines geosynchronous magnetic configurations and external conditions that characterize the loss of geosynchronous MeV electrons. The loss of MeV electrons often takes place during magnetospheric storms, but it also takes place without any clear storm activity. It is found that irrespective of storm activity, the day‐night asymmetry of the geosynchronous H (north‐south) magnetic component is pronounced during electron loss events. For the loss process, the magnitude, rather than the duration, of the magnetic distortion appears to be important, and its effective duration can be as short as ∼30 min. The solar wind dynamic pressure tends to be high and interplanetary magnetic field BZ tends to be southward during electron loss events. Under such external conditions the dayside magnetopause moves closer to Earth, and the day‐night magnetic asymmetry is enhanced. As a consequence the area of closed drift orbits shrinks. The magnetic field at the subsolar magnetopause, which is estimated from force balance with the solar wind dynamic pressure, is usually stronger than the nightside geosynchronous magnetic field during electron loss events. It is therefore suggested that geosynchronous MeV electrons on the night side are very often on open drift paths when geosynchronous MeV electrons are lost. Whereas the present result does not preclude the widely accepted idea that MeV electrons are lost to the atmosphere by wave‐particle interaction, it suggests that magnetopause shadowing is another plausible loss process of geosynchronous MeV electrons.

Diffuse radio emission from clusters in the MareNostrum Universe simulation

Large-scale diffuse radio emission is observed in some clusters of galaxies. There is ample of evidence that the emission has its origin in synchrotron losses of relativistic electrons that have been accelerated in cluster mergers. In a cosmological simulation, we estimate the radio emission of structure formation shocks as follows: introducing a novel approach to identify strong shock fronts in an smoothed particle hydrodynamics (SPH) simulation, we determine the Mach number as well as the downstream density and temperature in the MareNostrum Universe simulation which has 2 × 10243 particles in a 500 h−1Mpc box. Then, we estimate the radio emission using the formalism derived in Hoeft & Brüggen to produce artificial radio maps of massive clusters and to derive a luminosity function of diffuse radio sources. Several of our clusters show radio objects with similar morphology to observed large-scale radio relics, whereas about half of the clusters show only very little radio emission. In agreement with observational findings, the maximum diffuse radio emission of our clusters depends strongly on their X-ray temperature. We find that the so-called accretion shocks cause only very little radio emission. We conclude that a moderate efficiency of shock acceleration, namely ξe= 0.005, and moderate magnetic fields in the region of the relics, namely 0.07–0.8 μG are sufficient to reproduce the number density and luminosity of radio relics.

Precipitation of radiation belt electrons by EMIC waves, observed from ground and space

We show evidence that left‐hand polarised electromagnetic ion cyclotron (EMIC) plasma waves can cause the loss of relativistic electrons into the atmosphere. Our unique set of ground and satellite observations shows coincident precipitation of ions with energies of tens of keV and of relativistic electrons into an isolated proton aurora. The coincident precipitation was produced by wave‐particle interactions with EMIC waves near the plasmapause. The estimation of pitch angle diffusion coefficients supports that the observed EMIC waves caused coincident precipitation of both ions and relativistic electrons. This study clarifies that ions with energies of tens of keV affect the evolution of relativistic electrons in the radiation belts via cyclotron resonance with EMIC waves, an effect that was first theoretically predicted in the early 1970′s.

Pitch angle scattering and synchrotron radiation of relativistic runaway electrons in tokamak stochastic magnetic fields

In a recent work [J. R. Martín-Solís and R. Sánchez, Phys. Plasmas 13, 012508 (2006)], the increase that the presence of stochastic magnetic fields causes on the synchrotron radiation losses of relativistic runaway electrons was quantified using a guiding-center approximation. Here, we complete those studies by considering instead the mechanism which dominates the interaction at the gyromotion level. It is shown that, under typical tokamak conditions, the resonant cyclotron interaction with high enough parallel (to the magnetic field) wave numbers (k∥) modes can create, even for moderate magnetic fluctuation levels, an upper bound on the runaway energy. Implications for disruption-generated runaway electrons will be also discussed.

Observations of relativistic electron precipitation from the radiation belts driven by EMIC waves

For some time theoretical modeling has shown that electromagnetic ion cyclotron (EMIC) waves should play an important role in the loss of relativistic electrons from the radiation belts, through precipitation into the atmosphere. Up to now there has been limited experimental evidence for relativistic electron precipitation driven by EMIC waves. In this paper we present case studies of events showing EMIC waves, observed by ground‐based pulsation magnetometers, which are linked to strong responses in a subionospheric precipitation monitor. This response is consistent with precipitation occurring near the plasmapause, where EMIC waves may resonate with relativistic electrons. At the same time there is only a weak response in a co‐located riometer chain, as expected for relativistic electron precipitation that penetrates deeply into the atmosphere.

Qualitative estimation of magnetic storm efficiency in producing relativistic electron flux in the Earth’s outer radiation belt using geomagnetic pulsations data

It is well known that during many but not all of the geomagnetic storms enhanced fluxes of high-energy electrons are observed in the outer radiation belt. Here we examine relativistic (>2 MeV) electron fluxes measured by GOES at the synchronous orbit and on-ground observations of two types of ULF pulsations during 30 magnetic storms occurred during 1996–2000. To characterize the effectiveness of the chosen magnetic storms in producing relativistic electron fluxes, following to (Reeves, G.D., McAdams, K.L., Friedel, R.H.W., O’Brien, T.R. Acceleration and loss of relativistic electrons during geomagnetic storms. Geophys. Res. Lett. 30, doi:10.1029/2002GL016513, 2003), we calculate a ratio of the maximum daily-averaged electron flux measured during the recovery phase, to the mean pre-storm electron flux. A storm is considered an effective one if its ratio exceeds 2. We compare behavior of Pi1 and Pc5 geomagnetic pulsations during effective and non-effective storms and find a tendency for a storm efficiency to be higher when the mid-latitude Pi1 pulsations are observed for a long time during the magnetic storm main phase. We note also that the prolonged powerful Pc5 pulsation activity during the recovery phase of a magnetic storm is the necessary condition for the storm effectiveness. To interpret the found dependences, we suggest that there are two prerequisites for generating relativistic electron populations during a storm: (1) the availability of seed electrons in the magnetosphere, and Pi1 emissions are indicators of the mid-energy electron interaction with the ionosphere and (2) acceleration of the seed electrons to MeV energies, and interaction of electrons with the MHD wave activity in the Pc5 range is one of the most probable mechanisms proposed in the literature for this purpose.

Relativistic electron losses related to EMIC waves during CIR and CME storms

The losses of radiation belt electrons to the atmosphere due to wave–particle interactions with electromagnetic ion-cyclotron (EMIC) waves during corotating interaction region (CIR) storms compared to coronal mass ejections (CME) storms is investigated. Geomagnetic storms with extended ‘recovery’ phases due to large-amplitude Alfvén waves in the solar wind are associated with relativistic electron flux enhancements in the outer radiation belt. The corotating solar wind streams following a CIR in the solar wind contain large-amplitude Alfvén waves, but also some CME storms with high-speed solar wind can have large-amplitude Alfvén waves and extended ‘recovery’ phases. During both CIR and CME storms the ring current protons are enhanced. In the anisotropic proton zone the protons are unstable for EMIC wave growth. Atmospheric losses of relativistic electrons due to weak to moderate pitch angle scattering by EMIC waves is observed inside the whole anisotropic proton zone. During storms with extended ‘recovery’ phases we observe higher atmospheric loss of relativistic electrons than in storms with fast recovery phases. As the EMIC waves exist in storms with both extended and short recovery phases, the increased loss of relativistic electrons reflects the enhanced source of relativistic electrons in the radiation belt during extended recovery phase storms. The region with the most unstable protons and intense EMIC wave generation, seen as a narrow spike in the proton precipitation, is spatially coincident with the largest loss of relativistic electrons. This region can be observed at all MLTs and is closely connected with the spatial shape of the plasmapause as revealed by simultaneous observations by the IMAGE and the NOAA spacecraft. The observations in and near the atmospheric loss cone show that the CIR and CME storms with extended ‘recovery’ phases produce high atmospheric losses of relativistic electrons, as these storms accelerate electrons to relativistic energies. The CME storm with short recovery phase gives low losses of relativistic electrons due to a reduced level of relativistic electrons in the radiation belt.

Solar-wind–magnetosphere coupling, including relativistic electron energization, during high-speed streams

High geomagnetic activity occurs continuously during high-speed solar wind streams, and fluxes of relativistic electrons observed at geosynchronous orbit enhance significantly. High-speed streams are preceded by solar wind compression regions, during which time there are large losses of relativistic electrons from geosynchronous orbit. Weak to moderate geomagnetic storms often occur during the passage of these compression regions; however, we find that the phenomena that occur during the ensuing high-speed streams do not depend on whether or not a preceding storm develops. Large-amplitude Alfvén waves occur within the high-speed solar wind streams, which are expected to lead to intermittent intervals of significantly enhanced magnetospheric convection and to thus also lead to repetitive substorms due to repetitively occurring reductions in the strength of convection. We find that such repetitive substorms are clearly discernible in the LANL geosynchronous energetic particle data during high-speed stream intervals. Global auroral images are found to show unambiguously that these events are indeed classical substorms, leading us to conclude that substorms are an important contributor to the enhanced geomagnetic activity during high-speed streams. We used the onsets of these substorms as indicators of preceding periods of enhanced convection and of reductions in convection, and we have used ground-based chorus observations from the VELOX instrument at Halley station as an indicator of magnetospheric chorus intensities. These data show evidence that it is the periods of enhanced convection that precede substorm expansions, and not the expansions themselves, that lead to the enhanced dawn-side chorus wave intensity that has been postulated to cause the energization of relativistic electrons. If this inference is correct, and if it is chorus that energizes the relativistic electrons, then high-speed solar wind streams lead to relativistic electron flux enhancements because the embedded large-amplitude Alfvén waves give multi-day periods of intermittent significantly enhanced convection.

Effect of solar wind density on relativistic electrons at geosynchronous orbit

We examined the relationship between relativistic (>2 MeV) electron fluxes at geostationary orbit and two solar wind parameters, the speed and density. For the analysis, we used data of relativistic electron fluxes measured with GOES‐8 spacecraft for 1997–2002 and GOES‐10 spacecraft for 2003–2006, and solar wind data, provided by Goddard Space Flight Center. Using the correlation analysis, we found that not only solar wind speed but also solar wind density provides a strong effect on relativistic electrons: the relativistic electron fluxes increase with increasing solar wind speed and decrease with increasing solar wind density. In contrast to solar wind speed that shows the best correlation with electron fluxes after about two days only, the solar wind density shows the best correlation with the electron fluxes after about 15 hours. To separate the effects of solar wind speed and density, we examined events when solar wind speed varied insignificantly; in these events, the effect of solar wind density on the relativistic electrons remained strong. We found that the solar wind density, which may affect the loss of relativistic electrons, effectively controls the magnitude of relativistic electron fluxes at geostationary orbit, so that the strongest fluxes occurred almost exclusively during low‐density solar wind conditions. Thus, our study showed that solar wind density is an important parameter that may significantly affect the loss of relativistic electrons and, as a result, the resulting magnitude of the electron fluxes at geostationary orbit.

Resonant diffusion of radiation belt energetic electrons by field-aligned propagation whistler-mode chorus waves

Adopting the dipole geomagnetic field， Gaussian spectral density for the waves， and semi-empirical latitudinal electron density models obtained from available in situ data， this paper has calculated the local and bounce-averaged quasi-linear resonant electron diffusion coefficients due to chorus and then determined the timescales for electron precipitation loss and stochastic acceleration， in the range of 4≤L≤7 outside the plasmapause. The results indicate that the spatial extent where gyroresonance occurs depends on electron energy， equatorial pitch angle， wave spectrum， and the local electron number density and magnetic field. Besides these five parameters， the actual values of resonant frequency rely on magnetic latitude where resonance occurs. The acceleration of radiation belt energetic electrons occurs predominantly due to equatorial chorus， and the mid-latitude chorus preferably contributes to the precipitation loss of relativistic electrons. The timescales for both electron loss and acceleration due to chorus-driven diffusion have been evaluated to be of hours for lower-energy electrons (about 200keV) and of days for higher-energy electrons (about 1MeV). It is also found that variation of latitudinal density distributions contributes importantly to chorus-driven electron resonant diffusion. In general， an increasing latitudinal electron density increases the loss timescales for untrapped electrons with small equatorial pitch angles， but has negligible effect on the acceleration of trapped electrons with large equatorial pitch angles. The variations of chorus wave amplitude and wave spectrum with magnetic latitude and L-shell also make important contributions to the lifetime and acceleration of radiation belt electrons， which are generally greater than the effects of varying latitudinal distribution of cold plasma density.

Loss of relativistic electrons: Evidence for pitch angle scattering by electromagnetic ion cyclotron waves excited by unstable ring current protons

During geomagnetic storms the flux of radiation belt electrons can increase, decrease, or stay constant, depending on the competition between acceleration and loss mechanisms. We focus on loss of relativistic electrons. We use low‐altitude polar‐orbiting spacecraft and analyze fluxes of tens to hundreds of keV protons and relativistic (>1.5 MeV) electrons during a moderate geomagnetic storm, with a long‐lasting recovery phase (4–5 d). Using data from four local times, we find that the loss of relativistic electrons is confined within the anisotropic proton zone and that a spatially limited loss of relativistic electrons is spatially collocated with increased loss of protons. The proton pitch angle distributions within these peaks are consistent with moderate to strong pitch angle scattering due to electromagnetic ion cyclotron (EMIC) waves. The loss of relativistic electrons collocated with protons is found at all four local times considered (0300, 0700, 1400, 1700 MLT). Since anisotropic proton distributions can under certain conditions generate EMIC waves, we find strong indications that the observed relativistic electrons are scattered into the atmospheric loss cone by EMIC waves. EMIC wave scattering is less efficient at high equatorial pitch angles but very efficient near the loss cone, thereby controlling the loss rate of relativistic electrons to the atmosphere. Our observations in and near the loss cone support theoretical work suggesting that EMIC waves can cause scattering loss to the atmosphere of relativistic electrons over the course of a geomagnetic storm.

Dynamic evolution of energetic outer zone electrons due to wave‐particle interactions during storms

Relativistic electrons in the outer radiation belt are subjected to pitch angle and energy diffusion by chorus, electromagnetic ion cyclotron (EMIC), and hiss waves. Using quasi‐linear diffusion coefficients for cyclotron resonance with field‐aligned waves, we examine whether the resonant interactions with chorus waves produce a net acceleration or loss of relativistic electrons. We also examine the effect of pitch angle scattering by EMIC and hiss waves during the main and recovery phases of a storm. The numerical simulations show that wave‐particle interactions with whistler mode chorus waves with realistic wave spectral properties result in a net acceleration of relativistic electrons, while EMIC waves, which provide very fast scattering near the edge of the loss cone, may be a dominant loss mechanism during the main phase of a storm. In addition, hiss waves are effective in scattering equatorially mirroring electrons and may be an important mechanism of transporting high pitch angle electrons toward the loss cone.

Modeling the propagation characteristics of chorus using CRRES suprathermal electron fluxes

In the present paper, phase space density functions of the form f(v) = AN/vn are fitted to statistical distributions of suprathermal electron fluxes (E = 0.213–16.5 keV) from the CRRES satellite, parameterized by L‐shell, Magnetic Local Time (MLT), and geomagnetic activity. The fitted distributions are used in conjunction with ray tracing to calculate the Landau damping rates of an ensemble of rays representing whistler‐mode chorus waves. The modeled propagation characteristics are compared with observations of chorus wave power from the CRRES satellite, as a function of L‐shell, MLT, and magnetic latitude, in various frequency bands, and under various geomagnetic conditions. It is shown that the model results are remarkably consistent with many aspects of the observed wave distributions, including frequency, L‐shell, MLT, and latitudinal dependence. In addition, the MLT distribution of wave power becomes characteristically asymmetric during active geomagnetic conditions, with small propagation lengths on the nightside which increase with MLT and maximize on the dayside. This asymmetry is shown to be directly related to the dynamics of the Landau resonant suprathermal electrons which drift around the Earth whilst undergoing scattering and loss due to a variety of plasma waves. Consequently, the suprathermal electrons play an important role in radiation belt dynamics, by controlling the distribution of chorus, which in turn contributes to the acceleration and loss of relativistic electrons in the recovery phase of storms.

Parameterization of radiation belt electron loss timescales due to interactions with chorus waves

Wave‐particle interactions lead to the loss of relativistic electrons from the outer radiation belt on timescales ranging from hours to weeks. For a fixed value of chorus wave amplitudes pitch‐angle diffusion coefficients are computed for a range of energies and L, and are related to the loss rates of radiation belt electrons. By analyzing the dependence of the loss rates on L‐value and energy we find functional dependencies for the lifetime of the radiation belt electrons. Parameters of the functional dependences are obtained using a linear regression technique. To create parameterizations of loss rate as a function of geomagnetic indices, we also analyzed the statistical data from day‐side lower band chorus observations in the range of geomagnetic latitudes from 20° to 30°. The combined parameterizations of the wave amplitudes and scattering rates indicate that electron loss due to chorus waves strongly depends on energy and geomagnetic activity. During storm‐time conditions the lifetimes of relativistic electrons, in the heart of the outer zone are on the order of a day and are on the scale of hours at lower energies. Pitch‐angle scattering by chorus waves thus plays an important role in radiation belt dynamics. The developed parameterizations may be used in particle tracing codes and radial diffusion codes. The limitations of the parameterization, effect of scattering by other waves, and local acceleration processes are also discussed.

Review of radiation belt relativistic electron losses

We present a brief review of radiation belt electron losses which are vitally important for controlling the dynamics of the radiation belts. A historical overview of early observations is presented, followed by a brief description of important known electron loss mechanisms. We describe key theoretical results and observations related to pitch-angle scattering by resonant interaction with plasmaspheric hiss, whistler-mode chorus and electromagnetic ion cyclotron waves, and review recent work on magnetopause losses. In particular, we attempt to organize recent observational data by loss mechanism and their relative importance to the overall rate of loss. We conclude by suggesting future observational and theoretical work that would contribute to our understanding of this important area of radiation belt research.