Electron Flux Dropout Research Articles

Responses of relativistic electron fluxes in the outer radiation belt to geomagnetic storms

AbstractGeomagnetic storms can either increase or decrease relativistic electron fluxes in the outer radiation belt. A statistical survey of 84 isolated storms demonstrates that geomagnetic storms preferentially decrease relativistic electron fluxes at higher energies, while flux enhancements are more common at lower energies. In about 87% of the storms, 0.3–2.5 MeV electron fluxes show an increase, whereas 2.5–14 MeV electron fluxes increase in only 35% of the storms. Superposed epoch analyses suggest that such “energy‐dependent” responses of electrons preferably occur during conditions of high solar wind density which is favorable to generate magnetospheric electromagnetic ion cyclotron (EMIC) waves, and these events are associated with relatively weaker chorus activities. We have examined one of the cases where observed EMIC waves can resonate effectively with >2.5 MeV electrons and scatter them into the atmosphere. The correlation study further illustrates that electron flux dropouts during storm main phases do not correlate well with the flux buildup during storm recovery phases. We suggest that a combination of efficient EMIC‐induced scattering and weaker chorus‐driven acceleration provides a viable candidate for the energy‐dependent responses of outer radiation belt relativistic electrons to geomagnetic storms. These results are of great interest to both understanding of the radiation belt dynamics and applications in space weather.

Relativistic electron flux dropouts in the outer radiation belt associated with corotating interaction regions

AbstractUnderstanding how the relativistic electron fluxes drop out in the outer radiation belt under different conditions is of great importance. To investigate which mechanisms may affect the dropouts under different solar wind conditions, 1.5–6.0 MeV electron flux dropout events associated with 223 corotating interaction regions (CIRs) from 1994 to 2003 are studied using the observations of Solar, Anomalous, Magnetospheric Particle Explorer satellite. According to the superposed epoch analysis, it is found that high solar wind dynamic pressure with the peak median value of about 7 nPa is corresponding to the dropout of the median of the radiation belt content (RBC) index to 20% of the level before stream interface arrival, whereas low dynamic pressure with the peak median value of about 3 nPa is related to the dropout of the median of RBC index to 40% of the level before stream interface arrival. Furthermore, the influences of Russell‐McPherron effect with respect to interplanetary magnetic field orientation on dropouts are considered. It is pointed out that under positive Russell‐McPherron effect (+RM effect) condition, the median of RBC index can drop to 23% of the level before stream interface arrival, while for negative Russell‐McPherron effect (−RM effect) events, the median of RBC index only drops to 37% of the level before stream interface arrival. From the evolution of phase space density profiles, the effect of +RM on dropouts can be through nonadiabatic loss.

Comprehensive analysis of the flux dropout during 7–8 November 2008 storm using multisatellite observations and RBE model

AbstractWe investigate an electron flux dropout during a weak storm on 7–8 November 2008, with Dst minimum value being −37 nT. During this period, two clear dropouts were observed on GOES 11 > 2 MeV electrons. We also find a simultaneous dropout in the subrelativistic electrons recorded by Time History of Events and Macroscale Interactions during Substorms probes in the outer radiation belt. Using the Radiation Belt Environment model, we try to reproduce the observed dropout features in both relativistic and subrelativistic electrons. We found that there are local time dependences in the dropout for both observation and simulation in subrelativistic electrons: (1) particle loss begins from nightside and propagates into dayside and (2) resupply starts from near dawn magnetic local time and propagates into the dayside following electron drift direction. That resupply of the particles might be caused by substorm injections due to enhanced convection. We found a significant precipitation in hundreds keV electrons during the dropout. We observe electromagnetic ion cyclotron and chorus waves both on the ground and in space. We find the drift shells are opened near the beginning of the first dropout. The dropout in MeV electrons at GEO might therefore be initiated due to the magnetopause shadowing, and the followed dropout in hundreds keV electrons might be the result of the combination of magnetopause shadowing and precipitation loss into the Earth's atmosphere.

The effect of different solar wind parameters upon significant relativistic electron flux dropouts in the magnetosphere

AbstractSuperposed epoch analyses were performed on 193 significant relativistic electron flux dropout events, in order to study the roles of different solar wind parameters in driving the depletion of relativistic electrons, using ~16 years of data from the POES and GOES missions, and the OMNIWEB solar wind database. We find that the solar wind dynamic pressure and interplanetary magnetic field (IMF) Bz play key roles in causing the relativistic electron flux dropouts, but also that either large solar wind dynamic pressure or strong southward IMF Bz by itself is capable of producing the significant depletion of relativistic electrons. The relativistic electron flux dropouts occur not only when the magnetopause is compressed closer to the Earth but also when the magnetopause is located very far (> ~10 RE). Importantly, our results show that in addition to the large solar wind dynamic pressure, which pushes the magnetopause inward strongly and causes the electrons to escape from the magnetosphere, relativistic electrons can also be scattered into the loss cone and precipitate into the Earth's atmosphere during periods of strong southward IMF Bz, which preferentially provides a source of free energy for electromagnetic ion cyclotron (EMIC) wave excitation. This is supported by the fact that the strongest electron precipitation into the atmosphere is found in the dusk sector, where EMIC waves are typically observed in the high‐density plasmasphere or plume and cause efficient electron precipitation down to ~1 MeV.

Relativistic electron fluxes dropout in the outer radiation belt under different solar wind conditions

In this study, we apply superposed epoch analysis to the 1.5–6.0 MeV electron flux dropout events observed on Solar, Anomalous, Magnetospheric Particle Explorer satellite for 110 magnetic storms related to coronal mass ejections (CMEs) associated with interplanetary shocks during 1998–2003, which can help to study one of the scientific objectives of the recently launched Van Allen Probe—to determine and quantify the mechanisms of the losses in the outer radiation belt. Results obtained in this paper show that the impact of high solar wind dynamic pressure (Pdy) on the magnetosphere would lead to much larger electron flux dropout than low dynamic pressure. Furthermore, it is shown that southward interplanetary magnetic field (IMF) condition can result in more significant dropout compared with northward IMF condition. In addition, the largest local dropout is caused by high Pdy with northward IMF at L∼5.1. Dropouts under high Pdy and southward IMF conditions are the largest, whereas dropouts under low Pdy and northward IMF conditions are the smallest. Our study tackles the problem of quantifying the dropouts of electrons by calculating the radiation belt content index and finding the spatial distribution of dropout and the location of maximum dropout. Another finding is that Pdy and IMF affect the dropouts in CME‐driven storms. These new findings provide insight into which mechanisms play a more important role in different dropout events.

Quantifying the effect of magnetopause shadowing on electron radiation belt dropouts

Abstract. Energetic radiation belt electron fluxes can undergo sudden dropouts in response to different solar wind drivers. Many physical processes contribute to the electron flux dropout, but their respective roles in the net electron depletion remain a fundamental puzzle. Some previous studies have qualitatively examined the importance of magnetopause shadowing in the sudden dropouts either from observations or from simulations. While it is difficult to directly measure the electron flux loss into the solar wind, radial diffusion codes with a fixed boundary location (commonly utilized in the literature) are not able to explicitly account for magnetopause shadowing. The exact percentage of its contribution has therefore not yet been resolved. To overcome these limitations and to determine the exact contribution in percentage, we carry out radial diffusion simulations with the magnetopause shadowing effect explicitly accounted for during a superposed solar wind stream interface passage, and quantify the relative contribution of the magnetopause shadowing coupled with outward radial diffusion by comparing with GPS-observed total flux dropout. Results indicate that during high-speed solar wind stream events, which are typically preceded by enhanced dynamic pressure and hence a compressed magnetosphere, magnetopause shadowing coupled with the outward radial diffusion can explain about 60–99% of the main-phase radiation belt electron depletion near the geosynchronous orbit. While the outer region (L* > 5) can nearly be explained by the above coupled mechanism, additional loss mechanisms are needed to fully explain the energetic electron loss for the inner region (L* ≤ 5). While this conclusion confirms earlier studies, our quantification study demonstrates its relative importance with respect to other mechanisms at different locations.

A comparison of magnetic field measurements and a plasma‐based proxy to infer EMIC wave distributions at geosynchronous orbit

Wave‐particle interactions are fundamental to the dynamics of the outer radiation belt. Electromagnetic ion cyclotron (EMIC) waves can resonate with energetic electrons, causing pitch angle diffusion and scattering of the electrons into Earth's atmosphere. These waves act locally; thus, accurately measuring their spatial and temporal distributions is critical to understanding their contribution to radiation belt electron losses. Using Los Alamos National Laboratory Magnetospheric Plasma Analyzer data from geosynchronous orbit, we examine a plasma‐based proxy for enhanced EMIC wave growth during a set of 52 relativistic electron flux dropout events. This proxy is compared to in situ wave measurements from the GOES satellites, also at geosynchronous orbit, for single‐wave events as well as a superposed epoch statistical analysis. The proxy is extended to calculate an amplitude for the inferred waves, to enable a more quantitative comparison to the in situ GOES EMIC measurements. Signatures of EMIC waves are present in both the proxy and the direct wave observations at similar local times as well as epoch times. The waves are most prevalent in the afternoon sector, with enhanced occurrences beginning half a day before the onset of the dropouts and peaking in the day following. We see agreement in occurrence between the proxy and waves both statistically and in individual instances. This study demonstrates the powerful applications of plasma data to infer wave distributions in space.

Resonant Scattering of Relativistic Outer Zone Electrons by Plasmaspheric Plume Electromagnetic Ion Cyclotron Waves

The bounce-averaged Fokker–Planck equation is solved to study the relativistic electron phase space density (PSD) evolution in the outer radiation belt due to resonant interactions with plasmaspheric plume electromagnetic ion cyclotron (EMIC) waves. It is found that the PSDs of relativistic electrons can be depleted by 1–3 orders of magnitude in 5h, supporting the previous finding that resonant interactions with EMIC waves may account for the frequently observed relativistic electron flux dropouts in the outer radiation belt during the main phase of a storm. The significant precipitation loss of ∼MeV electrons is primarily induced by the EMIC waves in H+ and He+ bands. The rapid remove of highly relativistic electrons (> 5 MeV) is mainly driven by the EMIC waves in O+ band at lower pitch-angles, as well as the EMIC waves in H+ and He+ bands at larger pitch-angles. Moreover, a stronger depletion of relativistic electrons is found to occur over a wider pitch angle range when EMIC waves are centering relatively higher in the band.

Numerical calculations of relativistic electron drift loss effect

It has been suggested that drift loss to the magnetopause can be one of the major loss mechanisms contributing to relativistic electron flux dropouts. In this study, we examine details of relativistic electrons' drift physics to determine the extent to which the drift loss through the magnetopause is important to the total loss of the outer radiation belt. We have numerically computed drift paths of relativistic electrons' guiding center for various pitch angles, various measurement positions, and different solar wind conditions using the Tsyganenko T02 model. We specifically demonstrate how the drift loss effect depends on these various parameters. Most importantly, we present various estimates of relative changes of the omnidirectional flux of 1 MeV electrons between two different solar wind conditions based on a simple form of the directional flux function. For a change of the dynamic pressure from 4 nPa to 10 nPa with a fixed IMF BZ = 0 nT, our estimate indicates that after this increase in pressure, the equatorial omnidirectional flux at midnight near geosynchronous altitude decreases by ∼56 to ∼97%, depending on the specific pitch angle dependence of the directional flux. The effect rapidly decreases at regions earthward of geosynchronous orbit and shows a general trend of decrease away from midnight. For a change of the IMF BZ from 0 nT to −15 nT with a fixed dynamic pressure of 4 nPa, the relative decrease of the omnidirectional flux at geosynchronous altitude on the nightside is much smaller than that for the pressure increase, but its effect becomes substantial only beyond geosynchronous orbit. Possibilities exist that our results may change to some extent for a different magnetospheric model than the one used here.

Study of correlations between waves and particle fluxes measured on board the DEMETER satellite

The topic of relativistic electron dynamics in the outer radiation belt has received considerable attention for many years. Nevertheless, the problem of understanding the physical phenomenon involved is far from being resolved. In this paper, we use DEMETER observations to examine the variations of the energetic electron fluxes and ELF/VLF wave intensities in the inner magnetosphere during the intense 8 November 2004 magnetic storm. Electron flux spectra and associated wave intensity spectra are analysed throughout the magnetic storm and common characteristics or differences to other storm events are retained. The overall objective of this study is to identify and derive parameters that are relevant for particle flux modelling; the time constant characterizing the persistent decay after particle enhancement was found to be one of these important model parameters. The analysis of the 8 November 2004 event reveals that for L-shell parameter higher than ∼4, an electron flux dropout is observed during the storm’s main phase for electrons in the energy range 0.1–1 MeV, as has been reported from other measurements. Characteristic wave spectra accompanying this phase are analysed. They show a typical enhancement in the frequency range 0.3–10 kHz at onset for all L-shell values under consideration (2 < L < 5). During the first stage of the recovery phase, the electron fluxes are increased to a level higher than the pre-storm level, whereas the level of wave intensity in the frequency range observed below 300 Hz is at its highest. In the second stage, the particle flux decrease goes hand in hand with a global wave activity decline, the relaxation time of the latter being smaller than the former’s one. In some other cases, long-lasting electron enhancement associated with constant wave activity has been observed during this latter stage. For the above mentioned storm, while at low L values the decay time constants are higher for low energy electrons than for high energy electrons, this order is reversed at high L values. At about L = 3.6 the time constant is independent of electron energy.

Relativistic electron precipitation by EMIC waves from self‐consistent global simulations

We study the effect of electromagnetic ion cyclotron (EMIC) wave scattering on radiation belt electrons during the large geomagnetic storm of 21 October 2001 with minimum Dst = −187 nT. We use our global physics‐based model, which solves the kinetic equation for relativistic electrons and H+, O+, and He+ ions as a function of radial distance in the equatorial plane, magnetic local time, energy, and pitch angle. The model includes time‐dependent convective transport and radial diffusion and all major loss processes and is coupled with a dynamic plasmasphere model. We calculate the excitation of EMIC waves self‐consistently with the evolving plasma populations. Particle interactions with these waves are evaluated according to quasi‐linear theory, using diffusion coefficients for a multicomponent plasma and including not only field‐aligned but also oblique EMIC wave propagation. The pitch angle diffusion coefficients increase from 0° to ∼60° during specific storm conditions. Pitch angle scattering by EMIC waves causes significant loss of radiation belt electrons at E ≥ 1 MeV and precipitation into the atmosphere. However, the relativistic electron flux dropout during the main phase at large L ≥ 5 is due mostly to outward radial diffusion, driven by the flux decrease at geosynchronous orbit. We show first results from global simulations indicating significant relativistic electron precipitation within regions of enhanced EMIC instability, whose location varies with time but is predominantly in the afternoon‐dusk sector. The precipitating electron fluxes are usually collocated with precipitating ion fluxes but occur at variable energy range and magnitude. The minimum resonant energy increases at low L and relativistic electrons at E ≤ 1 MeV do not precipitate at L &lt; 3 during this storm.

Solar wind and magnetospheric conditions leading to the abrupt loss of outer radiation belt electrons

The trapped radiation belt electron population is maintained through a competition between multiple source and loss processes occurring within the magnetosphere and driven by the solar wind. In this research we have concentrated on the solar wind and the magnetospheric conditions that lead to the loss of electrons through abrupt energetic electron flux dropouts. We have focused on times when there is only a moderate level of geomagnetic activity, since the magnetospheric response during these conditions is expected to be far less complex than during large geomagnetic storms. We have found that under certain circumstances the radiation belt electrons are remarkably sensitive to the onset of southward IMF and to solar wind dynamic pressure increases. The onset of southward IMF is found to be sufficient to cause the flux dropouts, while increases in solar wind pressure are not necessary but are likely to enhance the loss when they occur in conjunction with southward IMF, as is often the case. It is not clear if an increase in solar wind pressure in the absence of southward IMF is sufficient to cause a flux dropout. The radiation belt fluxes can decrease by more than an order of magnitude with the onset of only minor geomagnetic activity. The level of solar wind forcing (as estimated by the epsilon parameter) and of geomagnetic activity (as estimated by AE, Dst, and the local magnetic field inclination at geosynchronous orbit) responsible for the flux loss is intermediate between lower levels of activity that create localized, adiabatic variations in the flux and large geomagnetic storms that result in both loss and acceleration. The dropout events examined here occurred after one or more days of quiet geomagnetic conditions, which we suggest preconditioned the magnetosphere to be highly sensitive to the onset of new activity. Although it is not known which specific conditions within the magnetosphere lead to this extreme sensitivity of the relativistic electrons, the time periods identified here are ones where the electron loss processes appear to operate in relative isolation of the acceleration processes.

The importance of atmospheric precipitation in storm‐time relativistic electron flux drop outs

During the sudden decrease of geosynchronous electron flux (&gt;2 MeV) of 17:10–17:20 UT, January 21, 2005 large‐scale precipitation into the atmosphere was observed. Estimates from ground‐based radio propagation experiments at L∼5 in the Northern and Southern hemispheres suggest that the atmospheric precipitation was less than 1/10 of the flux apparently lost during this 10 minute period. However, continuing precipitation losses from 4 &lt; L &lt; 6, observed for the next 2.7 hours, provides about 1/2 of the total relativistic electron content lost.

Radiation belt electron flux dropouts: Local time, radial, and particle‐energy dependence

The radiation belt electrons in Earth's magnetosphere exhibit substantial variability driven by changing solar wind conditions. The electron dynamics are due to a number of different adiabatic and nonadiabatic processes that can result in rapid increases and decreases in the particle flux levels. In this paper we present observations of abrupt flux decreases driven by a moderate geomagnetic storm. The particle dynamics are found to have significant local time and energy dependence that developed over roughly a 10‐hour period beginning with the onset of the storm. The electrons with energies greater than 2 MeV dropped fairly abruptly at various local times, but not simultaneously at different local times. It is shown that the initial flux dropout was due to the development of local taillike magnetic field stretching, rather than due to more global processes such as ring current buildup or large‐scale radial diffusion. It is also found that while the lower energy electrons (E &lt; 300 keV) fully recovered by the end of the storm, the &gt;2 MeV electrons were lost from the magnetosphere and did not recover. These results indicate that the initial dropout of the radiation belt electrons at geostationary orbit was controlled by the adiabatic response to localized changes in the geomagnetic field that develop over many hours, but that eventually nonadiabatic processes acted to cause the loss of electrons from the magnetosphere. It is also shown that during geomagnetically quiet conditions, the energetic electron flux can remain at nearly constant levels for as long as 1 week, suggesting that in the absence of geomagnetic activity either the outer radiation belt electron loss rate becomes quite small or the loss and growth rates are balanced.

Association of waves with narrow particle dropouts in the outer radiation belt

An investigation has been made of the association of plasma waves with narrow dropouts (&lt;30‐min observation time) in the fluxes of trapped energetic electrons and protons within the radiation belts. The experiment, conducted with instruments on the CRRES satellite, indicates that 5 Hz &lt; frequency &lt; 1 kHz waves are nearly always produced at the times and positions of narrow energetic electron flux dropouts. The waves may be produced at the positions of steep particle flux gradients. This phenomenon is best studied during short‐duration events that provide measurements on both sides of a boundary in a relatively quiescent state. In addition to an investigation of the waves associated with narrow dropouts the data presented here show the general correlations between waves and particle fluxes, particularly after new injections or plasmapause crossings. In some cases, energetic proton dropouts also occur and may contribute to the generation of waves.

Properties of spikelike shear flow reversals observed in the auroral plasma by Atmosphere Explorer C

A study of the characteristics of pairs of oppositely directed spikes in ionospheric convection velocities (or shear flow reversals), as first described by Gurnett, has been conducted by using data from Atmosphere Explorer C. These phenomena tend to occur near the large-scale reversal from sunward to antisunward convection on the nightside of the earth. Generally, the spikelike shear flow reversals involve electric field components along the spacecraft orbit that are directed toward the region between them, in which inverted V type electron precipitation is observed. This relationship between the electron precipitation and the electric field spikes is consistent with an upward-flowing field-aligned current that is fed by Pedersen currents from the adjacent regions of strong convection. In one case a divergent equivalent electric field structure was observed, that is, with the spikelike electric fields pointing away from the region in between, which in this case exhibited a sharp electron flux dropout. This opposite configuration may be an example of counterparts to inverted V structures existing in regions of downward-flowing field-aligned currents.