Polar-orbiting Operational Environmental Research Articles

Ring current electron precipitation during the 17 March 2013 geomagnetic storm: Underlying mechanisms and their effect on the atmosphere

Electron and ion fluxes at energies of ∼10 keV−1 MeV can change by orders of magnitude during geomagnetically active periods. This can lead to intensification of particle precipitation into the Earth's atmosphere. The process further affects atmospheric chemistry, which may impact weather and climate on the Earth’s surface. In this study, we concentrate on ring current electrons, and investigate precipitation mechanisms using a numerical model based on the Fokker-Planck equation. We focus on a study of the main precipitation mechanisms, and their connection with atmospheric parameters. We investigate the 17 March 2013 storm using the convection–diffusion 4-Dimensional Versatile Electron Radiation Belt (VERB-4D) code, that allows us to quantify the impact of the storm on the electron ring current, and the resulting electron precipitation. We validate our results against observations from the Polar Operational Environmental Satellites (POES) mission, the low Earth orbiting meteorological satellites National Oceanic and Atmospheric Administration (NOAA-15,-16,-17,-18,-19), and Meteorological Operational Satellite MetOp-02, as well as the Van Allen Probes, and produce a data set of precipitating fluxes that covers an energy range from 10 keV to 1 MeV. We calculate the altitude-dependent atmospheric ionization rates, a prerequisite for atmospheric models to estimate effects of geomagnetically active periods on chemical and physical variability of the atmosphere at high latitudes. Atmospheric ionization rates are validated against Atmospheric Ionization during Substorm (AIMOS 2.1-Aisstorm) and Special Sensor Ultraviolet Spectrographic Imagers (SSUSI) values, and show good agreement at high geomagnetic latitudes during the storm time.

Prompt Appearance of Large‐Amplitude EMIC Waves Induced by Solar Wind Dynamic Pressure Enhancement and the Subsequent Relativistic Electron Precipitation

AbstractElectromagnetic ion cyclotron (EMIC) waves play an important role in relativistic electron dynamics. In this study, we find a large‐amplitude EMIC wave event induced by the prompt enhancement of solar wind dynamic pressure on 6 November 2015. These large‐amplitude EMIC waves are simultaneously observed by multiple satellites over 13 hr in magnetic local time (MLT) with a peak amplitude of ∼4 nT. Satellites at different locations observed different bands of EMIC waves, implying the importance of background plasma density in EMIC wave generation. Electron pitch angle distributions show obvious responses to EMIC wave activities. During EMIC wave appearance, the fluxes of relativistic electrons with pitch angles around 90° increase, while the fluxes of field‐aligned relativistic electrons decrease, showing distinct “bite‐out” signatures, indicating pitch angle scattering by EMIC waves, and the scattering efficiency depends on the amplitude and polarization of EMIC waves. Combined with phase space density profiles of electrons that are nearly constant at energies below the minimum resonant energy of electrons (Emin) but show dropout at energies above the Emin after EMIC wave activities, we conclude that large‐amplitude EMIC waves can cause rapid electron loss down to several hundred keV. In addition, simultaneous observations of hundreds of keV electron precipitation and tens of keV proton precipitation by Polar Operational Environmental Satellites near the region where EMIC waves are observed, provide direct evidence of relativistic electron precipitation caused by the large‐amplitude EMIC waves, ultimately driven by solar wind structures.

Types of pulsating aurora: comparison of model and EISCAT electron density observations

Abstract. Energetic particle precipitation associated with pulsating aurora (PsA) can reach down to lower mesospheric altitudes and deplete ozone. It is well documented that pulsating aurora is a common phenomenon during substorm recovery phases. This indicates that using magnetic indices to model the chemistry induced by PsA electrons could underestimate the energy deposition in the atmosphere. Integrating satellite measurements of precipitating electrons in models is considered to be an alternative way to account for such an underestimation. One way to do this is to test and validate the existing ion chemistry models using integrated measurements from satellite and ground-based observations. By using satellite measurements, an average or typical spectrum of PsA electrons can be constructed and used as an input in models to study the effects of the energetic electrons in the atmosphere. In this study, we compare electron densities from the EISCAT (European Incoherent Scatter scientific radar system) radars with auroral ion chemistry and the energetics model by using pulsating aurora spectra derived from the Polar Operational Environmental Satellite (POES) as an energy input for the model. We found a good agreement between the model and EISCAT electron densities in the region dominated by patchy pulsating aurora. However, the magnitude of the observed electron densities suggests a significant difference in the flux of precipitating electrons for different pulsating aurora types (structures) observed.

Long‐Term Dropout of Relativistic Electrons in the Outer Radiation Belt During Two Sequential Geomagnetic Storms

AbstractOn 31 January 2016, the flux of >2 MeV electrons observed by Geostationary Operational Environmental Satellite (GOES)‐13 dropped to the background level during a minor storm main phase (−48 nT). Then, a second storm (−53 nT) occurred on 2 February; during the 3 days after its main phase, the flux remained at background level. Using data from various instruments on the GOES, Polar Operational Environmental Satellites (POES), Radiation Belt Storm Probes (RBSP), Meteor‐M2, and Fengyun‐series spacecraft, we study this long‐term dropout of MeV electrons during two sequential storms of similar magnitude under lightly disturbed solar wind conditions. Observations from low‐altitude satellites show that the fluxes decreased first at higher L‐shells and then gradually propagated inward. Moreover, the fluxes were almost completely lost and dropped to the background level at L > 5, while the fluxes at 4 < L < 5 were partly lost, as observed by RBSP and low‐altitude satellites. Finally, observations show that on 5 February, only the fluxes at L > 5.5 recovered, while the fluxes at 4 < L < 5 did not return to the prestorm levels. These observations indicate that the loss and recovery processes developed first at higher L‐shells. Phase space density (PSD) analysis shows that radial outward diffusion was the main reason for the dropout at higher L‐shells. Regarding electron enhancement, stronger inward diffusion was accompanied by ultra‐low‐frequency (ULF) wave activities at higher L‐shells, and chorus waves observed at outer L‐shells provided conditions for relativistic electron flux recovery to the prestorm levels.

Electron Precipitation From the Outer Radiation Belt During the St. Patrick's Day Storm 2015: Observations, Modeling, and Validation

AbstractRecently, a model for medium‐energy (30–1000 keV) radiation belt‐driven electron precipitation (ApEEP) has been put forward for use in decadal to century‐long climate model runs as part of the Climate Modelling Intercomparison Project, phase 6 (CMIP6). The ApEEP model is based on directly observed precipitation data spanning 2002–2012 from the constellation of low‐Earth‐orbiting Polar Operational Environmental Satellites (POES). Here, we test the ApEEP model's ability using its magnetic local time variant, ApEEP_MLT, to accurately represent electron precipitation fluxes from the radiation belts during a large geomagnetic storm that occurred outside of the span of the development data set. In a study of narrowband subionospheric very low frequency (VLF) transmitter data collected during March 2015, continuous phase observations have been analyzed throughout the entire St. Patrick's Day geomagnetic storm period for the first time. Using phase data from the U.K. transmitter, call‐sign GVT (22.1 kHz), received in Reykjavik, Iceland, electron precipitation fluxes from L = 2.8 to 5.4 are calculated around magnetic local noon (12 MLT) and magnetic midnight (00 MLT). VLF‐inferred >30‐keV fluxes are similar to the equivalent directly observed POES fluxes. The ApEEP_MLT >30‐keV fluxes for L < 5.5 describe the overall St. Patrick's Day geomagnetic storm‐driven flux enhancement well, although they are a factor of 1.7 (1.3) lower than POES (VLF‐inferred) fluxes during the recovery phase. Such close agreement in >30‐keV flux levels during a large geomagnetic storm, using three different techniques, indicates this flux forcing are appropriate for decadal climate simulations for which the ApEEP model was created.

Cloud_cci Advanced Very High Resolution Radiometer post meridiem (AVHRR-PM) dataset version 3: 35-year climatology of global cloud and radiation properties

Abstract. We present version 3 of the Cloud_cci Advanced Very High Resolution Radiometer post meridiem (AVHRR-PM) dataset, which contains a comprehensive set of cloud and radiative flux properties on a global scale covering the period of 1982 to 2016. The properties were retrieved from AVHRR measurements recorded by the afternoon (post meridiem – PM) satellites of the National Oceanic and Atmospheric Administration (NOAA) Polar Operational Environmental Satellite (POES) missions. The cloud properties in version 3 are of improved quality compared with the precursor dataset version 2, providing better global quality scores for cloud detection, cloud phase and ice water path based on validation results against A-Train sensors. Furthermore, the parameter set was extended by a suite of broadband radiative flux properties. They were calculated by combining the retrieved cloud properties with thermodynamic profiles from reanalysis and surface properties. The flux properties comprise upwelling and downwelling and shortwave and longwave broadband fluxes at the surface (bottom of atmosphere – BOA) and top of atmosphere (TOA). All fluxes were determined at the AVHRR pixel level for all-sky and clear-sky conditions, which will particularly facilitate the assessment of the cloud radiative effect at the BOA and TOA in future studies. Validation of the BOA downwelling fluxes against the Baseline Surface Radiation Network (BSRN) shows a very good agreement. This is supported by comparisons of multi-annual mean maps with NASA's Clouds and the Earth's Radiant Energy System (CERES) products for all fluxes at the BOA and TOA. The Cloud_cci AVHRR-PM version 3 (Cloud_cci AVHRR-PMv3) dataset allows for a large variety of climate applications that build on cloud properties, radiative flux properties and/or the link between them. For the presented Cloud_cci AVHRR-PMv3 dataset a digital object identifier has been issued: https://doi.org/10.5676/DWD/ESA_Cloud_cci/AVHRR-PM/V003 (Stengel et al., 2019).

Direct Observation of Subrelativistic Electron Precipitation Potentially Driven by EMIC Waves

AbstractElectromagnetic ion cyclotron (EMIC) waves are known to typically cause electron losses into Earth's upper atmosphere at >~1 MeV, while the minimum energy of electrons subject to efficient EMIC‐driven precipitation loss is unresolved. This letter reports electron precipitation from subrelativistic energies of ~250 keV up to ~1 MeV observed by the Focused Investigations of Relativistic Electron Burst Intensity, Range and Dynamics (FIREBIRD‐II) CubeSats, while two Polar Operational Environmental Satellites (POES) observed proton precipitation nearby. Van Allen Probe A detected EMIC waves (~0.7–2.0 nT) over the similar L shell extent of electron precipitation observed by FIREBIRD‐II, albeit with a ~1.6 magnetic local time (MLT) difference. Although plasmaspheric hiss and magnetosonic waves were also observed, quasi‐linear calculations indicate that EMIC waves were the most efficient in driving the electron precipitation. Quasi‐linear theory predicts efficient precipitation at >0.8–1 MeV (due to H‐band EMIC waves), suggesting that other mechanisms are required to explain the observed subrelativistic electron precipitation.

Extravehicular Electron Measurement Based on an Intravehicular Pixel Detector

AbstractWe demonstrate indirect measurement of the electron flux outside space vehicles designed to carry crew, using a pixel detector situated inside the space craft. While the vehicle walls shield the detector from directly seeing the electrons, we show that bremsstrahlung generated in the shielding is visible and furthermore that fluorescence induced by this bremsstrahlung in components of the detector generates a sharply peaked characteristic signature. We show that measurements from the Exploration Test Flight 1 of the Orion Multi‐Purpose Crew Vehicle and from the International Space Station correlate very well with AE9‐based modeled electron flux predictions and with measurements from other instruments on the International Space Station and on the Polar Operational Environmental Satellites. We also present detailed Monte Carlo simulations performed with FLUKA, confirming the mechanism generating our electron signal.

Cross Calibration of >16 MeV Proton Measurements From NOAA POES and EUMETSAT MetOp Satellites

AbstractA long‐term data set of energetic proton omnidirectional integral fluxes with an 8‐s resolution from National Oceanic and Atmospheric Administration (NOAA) Polar Operational Environmental Satellites (POES) and EUMETSAT MetOp satellites from 1978 to 2014 has been investigated comprehensively to capture the data bifurcations, calibration inconsistency, and some erroneous data. There are two categories of the data bifurcations. The upper and lower branches of both bifurcations can be clearly separated by magnetic local time or by magnetic latitude. The calibration inconsistency of POES and MetOp proton omnidirectional integral fluxes (>16 MeV) is solved by fitting the data from each pair of two satellites using a third‐degree polynomial function. The fitted curve is almost independent of Lm, B/B0, and time in the valid range. By these fitted functions, the >16 MeV proton omnidirectional integral fluxes from other POES and MetOp satellites are recalibrated to the NOAA‐15 measurements that are adopted as the reference standard. Analyses of the recalibrated data at the geomagnetic equator for about 3.5 solar cycles indicate that the maximal and minimal yearly values of >16 MeV proton omnidirectional integral fluxes are in 1987 and in 2002, respectively. By comparisons with the AP8 model predictions, the ratios of the observations in 1987 (in 2002) to the AP8 MIN (AP8 MAX) predictions are mainly within 0.5‐3 (0.1‐2), showing a dependence on Lm and B/B0.

PreMevE: New Predictive Model for Megaelectron‐Volt Electrons Inside Earth's Outer Radiation Belt

AbstractThis work designs a new model called PreMevE to predict storm time distributions of relativistic electrons within Earth's outer radiation belt. This model takes advantage of the cross‐energy, cross‐L‐shell, and cross‐pitch angle coherence associated with wave‐electron resonant interactions, ingests observations from belt boundaries—mainly by a National Oceanic and Atmospheric Administration Polar Operational Environmental Satellite in low‐Earth orbit, and provides high‐fidelity nowcast (multiple‐hour prediction) and forecast (>~1 day) of MeV electron fluxes over L‐shells between 2.8 and 7 through linear prediction filters. PreMevE can not only reliably anticipate incoming enhancements of MeV electrons during storms with at least 1‐day forewarning time but also accurately specify the evolving event‐specific electron spatial distributions afterward. The performance of PreMevE is assessed against long‐term in situ data from one Van Allen Probe and a Los Alamos National Laboratory geosynchronous satellite. This new model enhances our preparedness for severe MeV electron events in the future and further adds new science utility to existing and next‐generation low‐Earth orbit space infrastructure.

On the Close Correspondence Between Storm Time ULF Wave Power and the POES VLF Chorus Wave Amplitude Proxy

AbstractGround‐based Pc5 ultralow frequency (ULF) wave power in multiple ground‐based meridians is compared to the very low frequency (VLF) wave amplitude proxy, derived from Polar Operational Environmental Satellite (POES) precipitation, for the 33 storms studied by Li et al. (2015, https://doi.org/10.1002/2015GL065342). The results reveal common L‐shell and time profiles for the ULF waves and VLF proxy for every single storm, especially at L ≤ 6, and identical discrimination between efficient and inefficient radiation belt electron acceleration. The observations imply either ULF waves play a role in driving precipitation, which is falsely interpreted as VLF wave power in the proxy, ULF waves drive VLF waves (the reverse being energetically unfeasible), or both have a common driver with nearly identical L‐shell and time dependence. Global ground‐based ULF wave power coherence implies that a small number of meridians can be used to estimate storm time radial diffusion coefficients. However, the strong correspondence between ULF wave power and VLF wave proxy complicates causative assessments of electron acceleration.

Determination of the Equatorial Electron Differential Flux From Observations at Low Earth Orbit

AbstractVariations in the high‐energy relativistic electron flux of the radiation belts depend on transport, acceleration, and loss processes, and importantly on the lower‐energy seed population. However, data on the seed population is limited to a few satellite missions. Here we present a new method that utilizes data from the Medium Energy Proton/Electron Detector on board the low‐altitude Polar Operational Environmental Satellites to retrieve the seed population at a pitch angle of 90°. The integral flux values measured by Medium Energy Proton/Electron Detector relate to a low equatorial pitch angle and were converted to omnidirectional flux using parameters obtained from fitting one or two functions to pitch angle distributions given by three and a half years of Van Allen Probes data. Two methods to convert from integral to differential flux are explored. One utilizes integral and differential flux energy distributions from the AE9 model, the second employs an iterative fitting approach based on a Reverse Monte Carlo (RMC) method. The omnidirectional differential flux was converted to an equatorial pitch angle of 90°, again using statistical pitch angle distributions from Van Allen Probe data. We validate the resulting 90° flux for 100‐ to 600‐keV electrons against measurements from the Van Allen Probes and show an average agreement within a factor of 4 for L* > 3.7. The resulting data set offers a high time resolution, across multiple magnetic local time planes, and may be used to formulate event‐specific low‐energy boundary conditions for radiation belt models.

HIRS Outgoing Longwave Radiation—Daily Climate Data Record: Application toward Identifying Tropical Subseasonal Variability

This study describes the development of a new globally gridded climate data record (CDR) for daily outgoing longwave radiation (OLR) using the High-Resolution Infrared Radiation Sounder (HIRS) sensor. The new product, hereafter referred to as HIRS OLR, has several differences and advantages over the widely-used daily OLR dataset derived from the Advanced Very High-Resolution Radiometer (AVHRR) sensor on the same NOAA Polar Operational Environmental Satellites (POES), hereafter AVHRR OLR. As a CDR, HIRS OLR has been intersatellite-calibrated to provide the most homogeneous record possible. AVHRR OLR only used the daytime and nighttime overpasses from a single satellite at a time, which creates some challenges for resolving the large diurnal cycle of OLR. HIRS OLR leverages all available overpasses and then calibrates geostationary estimates of OLR to represent that cycle more faithfully. HIRS also has more spectral channels, including those for measuring water vapor, which provides a more accurate measure of OLR. This difference is particularly relevant for large-scale convective systems such as the El Niño–Southern Oscillation and the Madden–Julian Oscillation, whereby the HIRS OLR can better identify the subtropical variability between the tropical convection and the extratropical teleconnections.

EMIC Wave Events During the Four GEM QARBM Challenge Intervals

AbstractThis paper presents observations of electromagnetic ion cyclotron (EMIC) waves from multiple data sources during the four Geospace Environment Modeling challenge events in 2013 selected by the Geospace Environment Modeling Quantitative Assessment of Radiation Belt Modeling focus group: 17 and 18 March (stormtime enhancement), 31 May to 2 June (stormtime dropout), 19 and 20 September (nonstorm enhancement), and 23–25 September (nonstorm dropout). Observations include EMIC wave data from the Van Allen Probes, Geostationary Operational Environmental Satellite, and Time History of Events and Macroscale Interactions during Substorms spacecraft in the near‐equatorial magnetosphere and from several arrays of ground‐based search coil magnetometers worldwide, as well as localized ring current proton precipitation data from low‐altitude Polar Operational Environmental Satellite spacecraft. Each of these data sets provides only limited spatial coverage, but their combination shows consistent occurrence patterns and reveals some events that would not be identified as significant using near‐equatorial spacecraft alone. Relativistic and ultrarelativistic electron flux observations, phase space density data, and pitch angle distributions based on data from the Relativistic Electron‐Proton Telescope and Magnetic Electron Ion Spectrometer instruments on the Van Allen Probes during these events show two cases during which EMIC waves are likely to have played an important role in causing major flux dropouts of ultrarelativistic electrons, particularly near L* ~4.0. In three other cases, identifiable smaller and more short‐lived dropouts appeared, and in five other cases, these waves evidently had little or no effect.

A Statistical Study of Spatial Variation of Relativistic Electron Precipitation Energy Spectra With Polar Operational Environmental Satellites

AbstractThe mechanisms that drive relativistic electron precipitation (REP) from the radiation belts can be better understood with a better knowledge of the particle energies involved. National Oceanic and Atmospheric Administration Polar Operational Environmental Satellites, being a network of multiple satellites, can provide multiple point spectral data over a long time period, including the Van Allen Probe's era. The number of energy channels is limited, but the particle detectors on Polar Operational Environmental Satellites have a narrow field of view allowing an investigation of bounce loss cone particles. We use the ratio of count rates in the E3 (>300 keV) and the P6 (>700 keV) channels as a parameter to define spectral hardness. Using this parameter, the spatial variation of spectral hardness of REP events was investigated. It was found that very soft events were mostly found in the dusk‐midnight‐early morning magnetic local time sectors and L∼ 5–7 while the hardest events were located in the postnoon sector peaking at L∼ 4–5. The hardest events peaked at lower L shells, and less than 20% were coincident with low‐energy (30–80 keV) proton precipitation. Further, around 70% of nightside REP coincident with proton precipitation was associated with stretched magnetic field lines indicating that curvature scattering may have been an important driver. Around 62% of nightside REP coincident with proton precipitation associated with relaxed magnetic field lines, suggesting a mechanism other than magnetic field curvature scattering, was highly energetic.

Formation of 30 KeV Proton Isotropic Boundaries During Geomagnetic Storms

AbstractWe study the origin of the 30 keV proton isotropic boundary (IB) in the nightside auroral zone during geomagnetic storms, particularly, to address the recent results that the adiabaticity parameter K (ratio of the magnetic field line curvature radius to the particle gyroradius at the equator) on the IB field line can be much larger comparing to its theoretical estimate K ∼ 8 for the field line curvature (FLC) scattering mechanism. During nine storms in 2011–2013, we investigate ∼2,000 IBs observed by low‐altitude Polar Operational Environmental Satellites (POES) satellites and apply the TS05 magnetospheric model to estimate the K value in the equatorial part of the IB field line. The statistical distribution of the estimated K parameter, while being rather broad, is centered on K = 9–13. For smaller subset of ∼250 IBs, the concurrent magnetic field measurements on board Time History of Events and Macroscale Interaction During Substorms probes in the equatorial magnetotail were used to correct the estimated K‐values accounting for the TS05 deviations from the real magnetic configuration. After correction, the K distribution becomes narrower, being still centered on K = 9–12. Different estimates give percentages of events with K < 13, which can be attributed to IBs formed by FLC scattering, between 60% and 80%. Finally, we have not found any dependence of the K distribution on magnetic local time and IB latitude, except for events with IB located at extremely low latitudes (<59°). These findings imply that the FLC scattering is a dominant mechanism of IB formation operating in a variety of magnetospheric conditions.

Does a Local B‐Minimum Appear in the Tail Current Sheet During a Substorm Growth Phase?

AbstractMagnetic configurations with dBz/dr > 0 in the midtail current sheet are potentially unstable to various instabilities associated with the explosive substorm onset. Their existence is hard to confirm with observations of magnetospheric spacecraft. Here we use remote sensing by low‐altitude spacecraft that measured the loss cone filling rate during electron‐rich solar particle event, providing information about magnetic properties of the tail current sheet. We found a latitudinally localized anisotropic 30 keV electron loss cone region embedded inside an extended region of isotropic solar electron precipitation. It was persistently observed for more than 0.5 h during isolated growth phase event by six Polar Operational Environmental Satellites spacecraft, which crossed the premidnight auroral oval. The embedded anisotropic region was observed 1° poleward of the outer radiation belt boundary over 4–5 h wide magnetic local time sector, suggesting a persistent ridge‐type Bz2/j maximum in the equatorial plasma sheet at distances 15–20 RE. We discuss infrequent observations of such events taking into account recent results of global magnetohydrodynamic simulations.

Internal Charge Estimates for Satellites in Low Earth Orbit and Space Environment Attribution

Space assets are continuously bathed by charged particles making their components susceptible to the effects of spacecraft charging. While their orbits are not embedded within the radiation belts, low earth orbiting assets with high inclinations do pass through the horns of these belts during each polar crossing, transiting through potentially dangerous charged particle populations many times per day. Occasionally, these low altitude horns include significant populations of energetic ~1 MeV electrons, which can penetrate typical spacecraft shielding and accumulate within dielectric materials and on ungrounded conductors, a process known as internal charging. The National Oceanic and Atmospheric Administration Polar Operational Environmental Satellites (POES) have experienced on-orbit anomalies in the boost voltage regulator (BVR) that is suspected to be associated with the accumulation and discharge of ~800 keV electrons. We have used observations from the Medium Energy Proton and Electron Detector (MEPED) instrument and a first principles model of charge accumulation [11] to develop estimates of electron internal charge (IC) accumulation over the lifetime of each POES and Metop spacecraft for a range of typical discharge time constants. With the advantage afforded by a larger database of anomalies, we are able to show that these BVR anomalies are generally not attributable in a simple way to the accumulation and subsequent discharge from >800 keV nor a higher fluence of lower energy >300 keV electrons. To the best of our knowledge, this paper presents the first long-term estimates of IC for spacecraft in highly inclined low earth orbits.

Detailed characteristics of radiation belt electrons revealed by CSSWE/REPTile measurements: Geomagnetic activity response and precipitation observation

AbstractEarth's outer radiation belt electrons are highly dynamic. We study the detailed characteristics of relativistic electrons in the outer belt using measurements from the Colorado Student Space Weather Experiment (CSSWE) mission, a low Earth orbit (LEO) CubeSat, which traverses the radiation belt four times in one orbit (~1.5 h) and has the advantage of measuring the dynamic activities of the electrons including their rapid precipitation. We focus on the measured electron response to geomagnetic activity for different energies to show that there are abundant sub‐MeV electrons in the inner belt and slot region. These electrons are further enhanced during active times, while there is a lack of >1.63 MeV electrons in these regions. We also show that the variation of measured electron flux at LEO is strongly dependent on the local magnetic field strength, which is far from a dipole approximation. Moreover, a specific precipitation band, which happened on 19 January 2013, is investigated based on the conjunctive measurement of CSSWE, the Balloon Array for Radiation belt Relativistic Electron Losses, and one of the Polar Operational Environmental Satellites. In this precipitation band event, the net loss of the 0.58–1.63 MeV electrons (L = 3.5–6) is estimated to account for 6.8% of the total electron content.

The magnetic local time distribution of energetic electrons in the radiation belt region

AbstractUsing 14 years of electron flux data from the National Oceanic and Atmospheric Administration Polar Operational Environmental Satellites, a statistical study of the magnetic local time (MLT) distribution of the electron population is performed across a range of activity levels, defined by AE, AE*, Kp, solar wind velocity (Vsw), and VswBz. Three electron energies (>30, >100, and >300 keV) are considered. Dawn‐dusk flux asymmetries larger than order of magnitude were observed for >30 and >100 keV electrons. For >300 keV electrons, dawn‐dusk asymmetries were primarily due to a decrease in the average duskside flux beyond L* ∼ 4.5 that arose with increasing activity. For the >30 keV population, substorm injections enhance the dawnside flux, which may not reach the duskside as the electrons can be on open drift paths and lost to the magnetopause. The asymmetries in the >300 keV population are attributed to the combination of magnetopause shadowing and >300 keV electron injections by large electric fields. We suggest that 3‐D radiation belt models could set the minimum energy boundary (Emin) to 30 keV or above at L* ∼ 6 during periods of low activity. However, for more moderate conditions, Emin should be larger than 100 keV and, for very extreme activities, ∼300 keV. Our observations show the extent that in situ electron flux readings may vary during active periods due to the MLT of the satellite and highlight the importance of 4‐D radiation belt models to fully understand radiation belt processes.