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Recent studies have reported the simultaneous observation of Magnetosonic (MS) waves and Electromagnetic ion cyclotron (EMIC) waves. In this study, a detailed survey of the concurrent EMIC waves and MS waves is performed using the Van Allen Probes observations from 2012 to 2017. The results suggest that most of the concurrent EMIC waves and MS waves are H+ band EMIC waves and MS waves. The favorable geomagnetic conditions for the concurrent EMIC waves and MS waves are AE* = 500 nT inside the plasmapause and AE* = 100 nT outside the plasmapause. Although EMIC waves are generally stronger than MS waves, stronger MS waves than EMIC waves are more likely to occur during moderate and active geomagnetic times. Our study is consistent with the most recent research, and provides more detailed information that helps us to deepen our understanding of the concurrent EMIC waves and MS waves. Plain Language Summary Electromagnetic ions cyclotron (EMIC) waves and magnetosonic (MS) waves are two common wave mode in the Earth radiation belts and play important roles in radiation belt dynamics. Recent studies have reported observations of concurrent EMIC waves and MS waves and conducted a brief statistic survey. To deepen our understanding of the concurrent EMIC waves and MS waves, this study aims to comprehensively analyze the statistical occurrence pattern and associated global distribution of concurrent EMIC waves and MS waves. Our study demonstrated that most of the concurrent EMIC waves and MS waves are H+ band EMIC waves and MS waves, which reveals the deep relationship between MS waves and the concurrent H+ band EMIC waves. Our study also demonstrated that the concurrent EMIC waves are generally stronger than MS waves, while the wave amplitudes of MS waves can exceed EMIC waves during geomagnetically turbulent times. This result indicates that the combined scattering effect of the concurrent EMIC waves and MS waves could vary significantly under different geomagnetic conditions. [ABSTRACT FROM AUTHOR]

Precipitating auroral and radiation belt electrons are considered to play an important part in the natural forcing of the middle atmosphere with a possible impact on the climate system. Recent studies suggest that this forcing is underestimated in current chemistry-climate models. The HEPPA III intercomparison experiment is a collective effort to address this point. In this study, we apply electron ionization rates from three data-sets in four chemistry-climate models during a geomagnetically active period in April 2010. Results are evaluated by comparison with observations of nitric oxide (NO) in the mesosphere and lower thermosphere. Differences between the ionization rate data-sets have been assessed in a companion study. In the lower thermosphere, NO densities differ by up to one order of magnitude between models using the same ionization rate data-sets due to differences in the treatment of NO formation, model climatology, and model top height. However, a good agreement in the spatial and temporal variability of NO with observations lends confidence that the electron ionization is represented well above 80 km. In the mesosphere, the averages of model results from all chemistry-climate models differ consistently with the differences in the ionization-rate data-sets, but are within the spread of the observations, so no clear assessment on their comparative validity can be provided. However, observed enhanced amounts of NO in the mid-mesosphere below 70 km suggest a relevant contribution of the high-energy tail of the electron distribution to the hemispheric NO budget during and after the geomagnetic storm on April 6. [ABSTRACT FROM AUTHOR]

Precipitating auroral and radiation belt electrons are considered an important part of the natural forcing of the climate system. Recent studies suggest that this forcing is underestimated in current chemistryclimate models. The High Energy Particle Precipitation in the Atmosphere III intercomparison experiment is a collective effort to address this point. Here, eight different estimates of medium energy electron (MEE) (>30 keV) ionization rates are assessed during a geomagnetic active period in April 2010. The objective is to understand the potential uncertainty related to the MEE energy input. The ionization rates are all based on the Medium Energy Proton and Electron Detector (MEPED) on board the NOAA/POES and EUMETSAT/MetOp spacecraft series. However, different data handling, ionization rate calculations, and background atmospheres result in a wide range of mesospheric electron ionization rates. Although the eight data sets agree well in terms of the temporal variability, they differ by about an order of magnitude in ionization rate strength both during geomagnetic quiet and disturbed periods. The largest spread is found in the aftermath of enhanced geomagnetic activity. Furthermore, governed by different energy limits, the atmospheric penetration depth varies, and some differences related to latitudinal coverage are also evident. The mesospheric NO densities simulated with the Whole Atmospheric Community Climate Model driven by highest and lowest ionization rates differ by more than a factor of eight. In a follow-up study, the atmospheric responses are simulated in four chemistry-climate models (CCM) and compared to satellite observations, considering both the CCM structure and the ionization forcing. [ABSTRACT FROM AUTHOR]

After the launch of Van Allen Probes, the three‐belt structures of ultra‐relativistic electrons are discovered. In this study, we investigate the three‐belt structures of sub‐MeV electrons, which may form under different mechanism compared with those of ultra‐relativistic electrons and are worth in‐depth analysis. Based on the differential flux data from Magnetic Electron Ion Spectrometer onboard Radiation Belt Storm Probes‐B satellite, we find 54 events, in which two comparable peaks of sub‐MeV electron fluxes and a slot appear where there should be the outer radiation belt. Through the statistical analysis, the three‐belt structures of sub‐MeV electrons are found to be closely related to symmetric H‐component (SYM‐H) and Auroral Electrojet (AE) indices. The 2‐day SYM‐H minimum and AE maximum before the event have a linear trend with the remnant belt and the "second slot" locations. The L‐values of the remnant belt and the "second slot" of different energy electrons decrease as energy increases in general and show interesting characteristics during their temporal evolution. Moreover, the lifetime of the remnant belt of different energy electrons increases as energy increases. We find similarities and differences between sub‐MeV and ultra‐relativistic electrons three‐belt events, which provides a new perspective in three‐belt structure study. Plain Language Summary: An important result of the Van Allen Probes mission was that the ultra‐relativistic electron (>2MeV) radiation belts, containing an inner belt, a slot region and an outer belt, often show a three‐belt structure. That is, two belts and a slot region appear where there should be the outer radiation belt. The dominate mechanism to form the structure is still under debate. Recently, such structure for sub‐MeV electrons was also reported in a case study. Since the physical properties of these two types of electrons are different, the characteristics and mechanism to form the structure may be different. In this work, we show that common occurrence and general characteristics of sub‐MeV electron three‐belt events. The location, formation, and temporal evolution of the structure are influenced by electron energy and magnetospheric conditions reflected by geomagnetic indices. We have also determined the relatively short lifetime of the intermediate belt (remnant belt) and explained why three‐belt events for sub‐MeV electrons are less visible. Key Points: We report for the first time the main statistical characteristics of sub‐MeV three‐belt events using Van Allen Probe dataThe occurrence and the location of the remnant belt for the sub‐MeV electrons are affected by electron energy and geomagnetic activitiesThe lifetime of sub‐MeV electrons in the remnant belt is much smaller, explaining why these three‐belt events are less visible [ABSTRACT FROM AUTHOR]

The access of solar energetic protons into the inner magnetosphere on September 7–8, 2017 is investigated by following reversed proton trajectories to compute the proton cutoff energy using the Dartmouth geomagnetic cutoff code (Kress et al., 2010, https://doi.org/10.1029/2009sw000488). The cutoff energies for protons coming from the west and east direction, the minimum and maximum cutoff energy respectively, are calculated every 5 min along the orbit of Van Allen Probes using TS07 and the Lyon‐Fedder‐Mobarry (LFM) MHD magnetic field model. The result shows that the cutoff energy increases significantly as the radial distance decreases, and that the cutoff energy decreases with the building up of the ring current during magnetic storms. Solar wind dynamic pressure also affects cutoff suppression (Kress et al., 2004, https://doi.org/10.1029/2003gl018599). The LFM‐RCM model shows stronger suppression of cutoff energy than TS07 during strong solar wind driving conditions. The simulation result is compared with proton flux measurements, showing consistent variation of the cutoff location during the September 7–8, 2017 geomagnetic storm. Key Points: The Dartmouth geomagnetic cutoff code is updated with TS07 and LFM MHD magnetic field snapshotsThe proton cutoff energy is calculated during the September 7–8, 2017 SEP eventComparison with data shows better correlation using LFM than TS04 and TS07 [ABSTRACT FROM AUTHOR]

We present the results of a multi‐point and multi‐instrument study of electromagnetic ion cyclotron (EMIC) waves and related energetic proton precipitation during a substorm. We analyze the data from Arase (ERG) and Van Allen Probes (VAPs) A and B spacecraft for an event of 16 and 17 UT on December 1, 2018. VAP‐A detected an almost dispersionless injection of energetic protons related to the substorm onset in the night sector. Then the proton injection was detected by VAP‐B and further by Arase, as a dispersive enhancement of energetic proton flux. The proton flux enhancement at every spacecraft coincided with the EMIC wave enhancement or appearance. This data show the excitation of EMIC waves first inside an expanding substorm wedge and then by a drifting cloud of injected protons. Low‐orbiting NOAA/POES and MetOp satellites observed precipitation of energetic protons nearly conjugate with the EMIC wave observations in the magnetosphere. The proton pitch‐angle diffusion coefficient and the strong diffusion regime index were calculated based on the observed wave, plasma, and magnetic field parameters. The diffusion coefficient reaches a maximum at energies corresponding well to the energy range of the observed proton precipitation. The diffusion coefficient values indicated the strong diffusion regime, in agreement with the equality of the trapped and precipitating proton flux at the low‐Earth orbit. The growth rate calculations based on the plasma and magnetic field data from both VAP and Arase spacecraft indicated that the detected EMIC waves could be generated in the region of their observation or in its close vicinity. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are believed to play a significant role in the dynamics of energetic protons and relativistic electrons in the Earth's magnetosphere. The properties of these waves are being intensively studied. We consider the conditions of the EMIC wave generation and the dynamics of the wave source during a substorm event using a unique configuration of three spacecraft (Arase and two Van Allen Probes). All spacecraft were at approximately the same distance from the Earth, forming a chain across the evening local time sector. Analyzing parameters of the wave generation obtained from in situ measured proton distribution function, we came to the conclusion that the waves could be generated within the substorm area, sometimes close to, but not necessary at the spacecraft location. As the substorm expands in longitude, the EMIC wave source exhibits a longitudinal drift. When substorm expansion stops, the wave generation region expands due to the magnetic drift of protons injected during the substorm. The observed wave properties show that the waves are able to precipitate energetic protons into the atmosphere. This is confirmed by observations of low orbiting satellites measuring proton precipitating fluxes. Key Points: Westward propagation of the EMIC wave generation region is due to both the substorm expansion and azimuthal drift of injected protonsStrong pitch‐angle diffusion regime is confirmed by observations of proton fluxes at low altitude and the diffusion coefficient calculationThe diffusion coefficient maximum corresponds well to the energy range of the observed proton precipitation [ABSTRACT FROM AUTHOR]

Three new methods for estimating a ratio of the ultralow frequency (ULF; 1–100 mHz) wave equatorial electric field amplitude in the Earth's magnetosphere to ground magnetic field amplitudes for field line resonances (FLR) are described. These methods use ratios of the time series extrema, ratios of the envelope waveform and the ratio of the spectral amplitude at the FLR frequency. These methods were applied to four ULF resonance intervals; three detected by the Van Allen Probe A spacecraft and one detected by the POLAR spacecraft. The intervals were conjoined with the CARISMA and IMAGE ground magnetometer arrays. The spectral ratio results for the Van Allen Probe intervals were approximately twice to three times the ratios estimated from the two time series based methods. The POLAR interval showed similar values across all three methods. The differences are attributed to broadband frequency signals that modify the time series amplitudes, while the spectral method avoids these off‐resonant frequencies. Based on the results of this study, a spectral based method for calculating the ratio at the FLR frequency is best. Plain Language Summary: A reliable method to remote sense the amplitude of electric fields of natural frequencies in space using ground magnetic fields would remove difficulties in obtaining sufficient spatial information of fields that are thought to energize electrons to high energies. One recently proposed method maps ground magnetic field variations, b, to electric field variations, e, in space via the ratio, e/b. However, there are different ways to calculate e/b and the uncertainties of this metric have not been reported. This paper presents a study of three methods used to calculate e/b and the uncertainty limits; a ratio of the peaks/troughs of the fields, ratios of each point of the envelope waveform of the fields, and a ratio of the spectral amplitude of the frequency where resonance occurs. These methods were applied to electric field data obtained from two satellites with corresponding magnetic field data taken from ground magnetic field detectors at the footprint of the resonant magnetic field line. It was found that the frequency‐based method gave two to three times larger values and smaller uncertainty values compared with the other methods. Based on the results of this study, the frequency‐based method is preferred for estimating e/b. Key Points: Three methods for estimating the ratio of ultralow frequency electric fields in space to magnetic fields on the ground, e/b, are presentedRatio values for 106 field line resonant events were calculate from spacecraft and ground data over September 25, 2012 to February 25, 2014The uncertainties in e/b for using the spectral method were consistently smaller than the other methods [ABSTRACT FROM AUTHOR]

Van Allen Probes observations of ion spectra often show a sustained gap within a very narrow energy range throughout the full orbit. To understand their formation mechanism, we statistically investigate the characteristics of the narrow gaps for oxygen ions and find that they are most frequently observed near the noon sector with a peak occurrence rate of over 30%. The magnetic moment (µ) of the oxygen ions in the gap shows a strong dependence on magnetic local time (MLT), with higher and lower µ values in the morning and afternoon sectors, respectively. Moreover, we find through superposed epoch analysis that the gap formation also depends on geomagnetic conditions. Those gaps formed at lower magnetic moments (µ < 3,000 keV/G) are associated with stable convection electric fields, which enable magnetospheric ions to follow a steady drift pattern that facilitates the gap formation by corotational drift resonance. On the other hand, gaps with higher µ values are statistically preceded by a gradual increase of geomagnetic activity. We suggest that ions within the gap were originally located inside the Alfven layer following closed drift paths, before they were transitioned into open drift paths as the convection electric field was enhanced. The sunward drift of these ions, with very low fluxes, forms a drainage void in the dayside magnetosphere manifested as the sustained gap in the oxygen spectrum. This scenario is supported by particle-tracing simulations, which reproduce most of the observed characteristics and therefore provide new insights into inner magnetospheric dynamics. [ABSTRACT FROM AUTHOR]

The impact of radial diffusion in storm time radiation belt dynamics is well-debated. In this study we quantify the changes and variability in radial diffusion coefficients during geomagnetic storms. A statistical analysis of Van Allen Probes data (2012-2019) is conducted to obtain measurements of the magnetic and electric power spectral densities for Ultra Low Frequency (ULF) waves, and corresponding radial diffusion coefficients. The results show global wave power enhancements occur during the storm main phase, and continue into the recovery phase. Local time asymmetries show sources of wave power are both external solar wind driving and internal sources from coupling with ring current ions and substorms. Wave power enhancements are also observed at low L values (L < 4). The accessibility of wave power to low L is attributed to a depression of the Alfvén continuum. The increased wave power drives enhancements in both the magnetic and electric field diffusion coefficients by more than an order of magnitude. Significant variability in diffusion coefficients is observed, with values ranging over several orders of magnitude. A comparison to the Kp parameterized empirical model of Ozeke et al. (2014) is conducted and indicates important differences during storm times. Although the electric field diffusion coefficient is relatively well described by the empirical model, the magnetic field diffusion coefficient is approximately ~10 times larger than predicted. We discuss how differences could be attributed to data set limitations and assumptions. Alternative storm-time radial diffusion coefficients are provided as a function of L* and storm phase. [ABSTRACT FROM AUTHOR]

We utilize 17 years of combined Van Allen Probes and Arase data to statistically analyze the response of the inner magnetosphere to the orientation of the interplanetary magnetic field (IMF) By component. Past studies have demonstrated that the IMF By component introduces a similarly oriented By component into the magnetosphere. However, these studies have tended to focus on field lines in the magnetotail only reaching as close to the Earth as the geosynchronous orbit. By exploiting data from these inner magnetospheric spacecraft, we have been able to investigate the response at radial distances of <7RE. When subtracting the background magnetic field values, provided by the T01 and IGRF magnetic field models, we find that the IMF By component does affect the configuration of the magnetic field lines in the inner magnetosphere. This control is observed throughout the inner magnetosphere, across both hemispheres, all radial distances, and all magnetic local time sectors. The ratio of IMF By to the observed By residual, also known as the "penetration efficiency," is found to be ~0.33. The IMF Bz component is found to increase, or inhibit, this control depending upon its orientation. [ABSTRACT FROM AUTHOR]

We investigate the rare events of sudden appearances of relativistic electrons (>700 keV), which are normally confined to the Van Allen belts, in the slot region. The frequency of occurrence of these events is on average 1-2 per year. To cope with the scarcity of events, in this study, we examine 21 years of trapped relativistic electron fluxes available from the POES and MetOp Space Environment Monitor (SEM-2). Our statistical analysis shows that these events can occur even during moderate geomagnetic activity. Occurrence of these events correlates with high-speed solar winds or interplanetary coronal mass ejections depending on the phase of the solar cycle. A strong correlation of these events is found with ultra-low frequency (ULF) wave activity, which can be used to predict these events with more than 75% accuracy. [ABSTRACT FROM AUTHOR]

Radiation belt electrons have a complicated relationship with geomagnetic activity. We select electron measurements from 7 years of DEMETER and 6 years of Van Allen Probes data during geomagnetic storms to conduct statistical analysis focusing on the correlation between electron flux and Dst index. We report, for the first time, an upper limit of electron fluxes observed by both satellites throughout the inner and outer belts across a wide energy range from ~100s keV to multi-MeV. The upper flux limit is determined at different L's and energies, for example, 1.9 X 107/cm²-s-sr-MeV at 470 keV at L = 1.5 and 3.6 X 105/cm²-s-sr-MeV at 3.4 MeV at L = 4 (Van Allen Probes). We present the energy spectra of the electron flux upper limit at different L shells and find the measured upper flux limit to be at least three times higher than the predicted flux from the AE8/AE9 models, although the spectral shape is remarkably similar. We show that the average flux with an applied time lag is better correlated with the Dst index and that the time lag optimizing the correlation coefficient is larger at lower L and at higher energies. These findings present the underlying challenges to model the dynamic variation of relativistic electrons in the inner magnetosphere and are important information for space weather considerations. [ABSTRACT FROM AUTHOR]

The flux of energetic electrons in the outer radiation belt shows a high variability. The interactions of electrons with very low frequency (VLF) chorus waves play a significant role in controlling the flux variation of these particles. Quantifying the effects of these interactions is crucially important for accurately modeling the global dynamics of the outer radiation belt and to provide a comprehensive description of electron flux variations over a wide energy range (from the source population of 30 keV electrons up to the relativistic core population of the outer radiation belt). Here, we use a synthetic chorus wave model based on a combined database compiled from the Van Allen Probes and Cluster spacecraft VLF measurements to develop a comprehensive parametric model of electron lifetimes as a function of L‐shell, electron energy, and geomagnetic activity. The wave model takes into account the wave amplitude dependence on geomagnetic latitude, wave normal angle distribution, and variations of wave frequency with latitude. We provide general analytical formulas to estimate electron lifetimes as a function of L‐shell (for L = 3.0 to L = 6.5), electron energy (from 30 keV to 2 MeV), and geomagnetic activity parameterized by the AE index. The present model lifetimes are compared to previous studies and analytical results and also show a good agreement with measured lifetimes of 30 to 300 keV electrons at geosynchronous orbit. Plain Language Summary: The space surrounding our planet is full of charged particles trapped in donut‐shaped belts called the Van Allen radiation belts that encircle the Earth. These charged particles can cause significant malfunctions and unexpected failures to spacecraft electronics. The intensity of the radiation belts vary as these energetic particles interact with very low frequency (VLF) waves such as chorus waves. Therefore, it is crucially important to accurately quantify the wave‐particle interactions for accurately modeling and forecasting the global dynamics of the Van Allen radiation belts. Predicting the magnitude and duration of potentially hazardous conditions could help satellite operators to switch off nonessential satellite electronic systems to reduce malfunctions and unexpected failures only during the most dangerous periods. In this study, we use a chorus wave model based on multi‐satellite wave measurements to calculate the lifetimes of these charged particles in the Earth's Van Allen radiation belts. We then compare our results with previous studies, analytical results, and measured data. Key Points: A comprehensive parametric model of electron lifetime in the outer radiation belt has been developedModel lifetimes are compared with analytical results and measured dataThe model provides accurate estimates of electron lifetimes over a wide range of energies as a function of geomagnetic activity [ABSTRACT FROM AUTHOR]

High‐energy trapped particles in the radiation belts constitute potential threats to the functionality of satellites as they enter into those regions. In the inner radiation belt, the characteristics of high‐energy (>20 MeV) protons variations during geomagnetic activity times have been studied by implementing 4‐year (2013–2016) observations of the Van Allen probes. An empirical formula has been used to remove the satellite orbit effect, by which proton fluxes have been normalized to the geomagnetic equator. Case studies show that the region of L < 1.7 is relatively stable, while L > 1.7 is more dynamic and the most significant variation of proton fluxes occurs at L = 2.0. The 4‐year survey at L = 2.0 indicates that for every geomagnetic storm, sharp descent in proton fluxes is accompanied by the corresponding depression of SYM‐H index, with a one‐to‐one correspondence, regardless of the storm intensity. Proton flux dropouts are synchronous with SYM‐H reduction with similar short timescales. Our observational results reveal that high‐energy protons in the inner radiation belt are very dynamic, especially for the outer zone of the inner belt, which is beyond our previous knowledge. Plain Language Summary: The inner radiation belt, extending from 1,000 km to 2 Re above the Earth, is filled with a lot of high‐energy protons. These protons may affect the safety of the satellite as they are traveling into this region. Therefore, understanding the behavior of the proton belt is very important regarding the functionality of satellites. Early studies have found that protons in the inner radiation belt are stable over very long timescales. A few studies were focused on variations of inner zone protons during the huge geomagnetic storms. Van Allen Probes provide a great opportunity to explore whether geomagnetic activities affect the inner zone. New and fascinating phenomenon has been found that proton fluxes have prompt responses and one‐to‐one correspondence to geomagnetic disturbances, regardless of the storm intensity. This has not been reported before. Our results reveal that the high‐energy protons in the inner radiation belt are very dynamic, especially for the outer zone of the inner belt, which is contrast to the common knowledge. Key Points: Proton fluxes are normalized to geomagnetic equator for eliminating satellite orbit effectHigh‐energy protons in the inner radiation belt are very dynamic, especially for the outer zone of the inner belt, not stable as previous knowledgeProton flux has prompt responses to geomagnetic disturbances with one‐to‐one correspondence, regardless of geomagnetic storm intensity [ABSTRACT FROM AUTHOR]

With 64‐month magnetic data from Van Allen Probes, we have studied not only the global distribution, wave normal angle (θ), and ellipticity (ε) of electromagnetic ion cyclotron (EMIC) waves, but also the dependence of their occurrence rates and magnetic amplitudes on the AE* index (the mean value of AE index over previous 1 hr). Our results show that H+ band waves are preferentially detected at 5 ≤ L ≤ 6.5, in the noon sector. They typically have small θ (<30°) and weakly left‐hand polarization but become more oblique and linearly polarized at larger magnetic latitudes or L‐shells. With the increase of AE* index, their occurrence rate significantly increases in the noon sector, and their source region extends to dusk sector. He+ band waves usually occur in the predawn and morning sectors at 3 ≤ L ≤ 4.5. They generally have moderate θ (30 °  − 40°) and left‐hand polarization and also become more oblique and linearly polarized at larger latitudes or L‐shells. There is a clear enhancement of occurrence rate and amplitude during active geomagnetic periods, especially in the dusk and evening sectors. O+ band waves mainly occur at 3 ≤ L ≤ 4 in the predawn sector. They have either very small θ (<20°) or very large θ (>50°), and typically linear or weakly right‐hand polarization. During active periods, they mostly occur at the midnight sector and L < 3.5. As a valuable supplement to previous statistical studies, our result provides not only a more compresentive EMIC wave model for evaluating their effects on the radiation belt, but also detailed observational constraints on generation mechanisms of EMIC waves. Key Points: A statistical study of EMIC waves is performed using long‐term Van Allen Probes data in the inner magnetosphereWe have presented the distributions of occurrence rate, wave normal angle, and ellipticity for H+, He+, and O+ band wavesThe dependences of the occurrence rate and magnetic amplitude on the AE* index are also studied [ABSTRACT FROM AUTHOR]

We explore the impact on the outer radiation belt of significant time‐integrated ap events of continuously elevated geomagnetic activity, making use of Van Allen Probes measurements in 2013–2017. We show that most high peaks of 1.8‐MeV electron flux occur during the 10 days immediately following such events, with stronger events leading to higher flux. Events larger than 800 nT·hr are always followed by a high peak of flux in the heart of the outer radiation belt. They lead to 10‐day‐averaged 1.8‐MeV electron fluxes of the order of 106 e/cm2/sr/s/MeV in this region and 8×104 e/cm2/sr/s/MeV near geostationary orbit. Accordingly, they can be considered as reliable precursors of high peaks of 2‐MeV electron flux throughout the outer belt. We demonstrate that the flux peaks following such significant events can be parameterized by different geomagnetic indices at different radial locations in 2013–2017. Comparisons between the corresponding modeled flux peaks and observations show a reasonable agreement for both the timing, duration, and level of the peaks with 2015 data from the Van Allen Probes and also with 2002–2012 data from Global Positioning System satellites and 2003–2016 data from Geostationary Operational Environmental Satellites, provided that dropouts are taken into account via an appropriate threshold on solar wind dynamic pressure. Plain Language Summary: Considering significant events of prolonged and elevated geomagnetic activity measured at middle‐latitude stations, we examine their effects on megaelectron volt electron fluxes that represent an important hazard for satellites throughout the outer radiation belt. Using measurements performed by the Van Allen Probes in 2013–2017, we find that high peaks of 1.8‐MeV electron flux appear in the immediate aftermath of all events in the heart of the outer radiation belt and last about 10 days. Such events can therefore be considered as reliable precursors of high flux peaks. The statistical dependence of flux peaks on different measures of precursor event strength and radial distance is obtained. The reliability of such a description over the long term is tested by comparisons with earlier measurements from other satellites in 2002–2012, further taking dropouts into account. These results should be useful for better understanding the effects of geomagnetic disturbances on megaelectron volt electrons and to improve forecasts of the timing and level of such peaks of 2‐MeV electron flux. Key Points: Significant time‐integrated ap geomagnetic events produce high peaks of 2‐MeV electron flux throughout the outer radiation beltThe 2‐MeV electron flux peaks produced by Int(ap) events are parameterized by event strength and radial distance based on Van Allen Probes dataThis model of 2‐MeV electron flux is consistent with earlier GPS and GOES measurements when dropouts are taken into account [ABSTRACT FROM AUTHOR]

In this study, access of solar energetic protons to the inner magnetosphere on 11 September 2017 is investigated by computing the reverse particle trajectories with the Dartmouth geomagnetic cutoff code (Kress et al., 2010). The maximum and minimum cutoff rigidity at each point along the orbit of Van Allen Probe A is numerically computed by extending the code to calculate cutoff rigidity for particles coming from arbitrary direction. Pulse height analyzed (PHA) data have the advantage of providing individual particle energies and effectively excluding background high‐energy proton contamination. This technique is adopted to study the cutoff locations for solar protons with different energy. The results demonstrate that cutoff latitude is lower for solar protons with higher energy, consistent with low‐altitude vertical cutoffs. Both the observations and numerical results show that proton access into the inner magnetosphere depends strongly on angle between particle arrival direction and magnetic west. The numerical result is approximately consistent with the observation that the energy of almost all solar protons stays above the minimum cutoff rigidity. Key Points: Pulse height analyzed data from the Van Allen Probes REPT instrument is compared with model solar proton geomagnetic cutoffsThe Dartmouth cutoff code is extended to calculate cutoff energy for particles arriving from arbitrary directions at the spinning spacecraftGeosynchronous transfer orbit measurements and calculated cutoff extend models previously applied to low‐altitude and geosynchronous cutoffs [ABSTRACT FROM AUTHOR]

We describe a new, more accurate procedure for estimating and removing inner zone background contamination from Van Allen Probes Magnetic Electron Ion Spectrometer (MagEIS) radiation belt measurements. This new procedure is based on the underlying assumption that the primary source of background contamination in the electron measurements at L shells less than three, energetic inner belt protons, is relatively stable. Since a magnetic spectrometer can readily distinguish between foreground electrons and background signals, we are able to exploit the proton stability to construct a model of the background contamination in each MagEIS detector by only considering times when the measurements are known to be background dominated. We demonstrate, for relativistic electron measurements in the inner zone, that the new technique is a significant improvement upon the routine background corrections that are used in the standard MagEIS data processing, which can "overcorrect" and therefore remove real (but small) electron fluxes. As an example, we show that the previously reported 1‐MeV injection into the inner zone that occurred in June of 2015 was distributed more broadly in L and persisted in the inner zone longer than suggested by previous estimates. Such differences can have important implications for both scientific studies and spacecraft engineering applications that make use of MagEIS electron data in the inner zone at relativistic energies. We compare these new results with prior work and present more recent observations that also show a 1‐MeV electron injection into the inner zone following the September 2017 interplanetary shock passage. Plain Language Summary: All measurements suffer from error, which can arise from a variety of sources, including from the instrument itself ("noise"), as well as measured signals that are not the intended observation ("background"). When measurement errors can be quantified and accounted for, measurement accuracy, precision, and uncertainty can all be improved. This often leads to new discoveries in science. This work describes a new technique for quantifying and mitigating measurement error due to backgrounds in the Earth's Van Allen radiation belts. This allows us to measure the inner radiation belt with an increased level of accuracy and precision, enabling new scientific understanding. Specifically, we find that the inner radiation belt is longer lived and of greater intensity than suggested by previous work. Such findings are important scientifically, as they provide ground truth for radiation belt models and allow us to test various theories regarding the growth and decay of the inner radiation belt. This work is also important from a practical standpoint, as it helps improve statistical models that are used for spacecraft design, to determine how best to shield spacecraft and sensitive electronics from the damaging effects of the inner radiation belt. Key Points: A new background correction algorithm for relativistic inner zone electrons is developedWe find important differences versus the standard algorithm, with several new/clarified features revealedData from the new algorithm should be used for quantitative inner zone studies at energies >0.7 MeV [ABSTRACT FROM AUTHOR]

The evolution of the radiation belts in L‐shell (L), energy (E), and equatorial pitch angle (α0) is analyzed during the calm 11‐day interval (4–15 March) following the 1 March 2013 storm. Magnetic Electron and Ion Spectrometer (MagEIS) observations from Van Allen Probes are interpreted alongside 1D and 3D Fokker‐Planck simulations combined with consistent event‐driven scattering modeling from whistler mode hiss waves. Three (L, E, α0) regions persist through 11 days of hiss wave scattering; the pitch angle‐dependent inner belt core (L ~ <2.2 and E < 700 keV), pitch angle homogeneous outer belt low‐energy core (L > ~5 and E~ < 100 keV), and a distinct pocket of electrons (L ~ [4.5, 5.5] and E ~ [0.7, 2] MeV). The pitch angle homogeneous outer belt is explained by the diffusion coefficients that are roughly constant for α0 ~ <60°, E > 100 keV, 3.5 < L < Lpp ~ 6. Thus, observed unidirectional flux decays can be used to estimate local pitch angle diffusion rates in that region. Top‐hat distributions are computed and observed at L ~ 3–3.5 and E = 100–300 keV. Plain Language Summary: We study the evolution of the radiation belts during quiet geomagnetic times from satellite observations and numerical codes. We reach a global understanding of the trapped electrons variation with time, space, energy, and pitch angle (the angle of the velocity vector with the magnetic field). We exhibit three stable regions, which are less sensitive to scattering from hiss waves, while, on the other hand, hiss causes flux decay over 12 days that forms the slot region between the inner and outer belt. The existing theory explains why the outer belt electron decay is independent of pitch angle but dependent upon energy. This implies that satellite observations can reveal local pitch angle diffusion rates, themselves intimately connected with the wave properties. Thus, a connection is made between observed wave properties and observed/computed scattered electron flux, consistent with theory. Regions where the flux is pitch angle dependent are isolated in the low‐energy slot region where we show that the real shape is a smoothed version of the ideal top‐hat distribution computed from theory. The impact of this work is improved understanding of the belt evolution for space weather prediction, with a proposed event‐driven method that accurately (within ×2) predicts the electron flux decay after storms. Key Points: Global computations of the (L, E, α0) structure of the evolving radiation belt during quiet times agree well with observationsThe inner belt decay is pitch angle dependent, while the outer belt is much more homogeneous with two distinct (L, E) regionsThe homogeneity of the pitch angle diffusion coefficient due to hiss waves explains the uniform outer belt decay and why 1D and 3D simulations agree [ABSTRACT FROM AUTHOR]

The diminishing strength of the Earth's magnetic dipole over recent millennia is accompanied by the increasing prominence of the geomagnetic South Atlantic Anomaly (SAA), which spreads over the South Atlantic Ocean and South America. The longevity of this feature at millennial timescales is elusive because of the scarcity of continuous geomagnetic data for the region. Here, we report a unique geomagnetic record for the last ~1500 y that combines the data of two welldated stalagmites from Pau d'Alho cave, located close to the presentday minimum of the anomaly in central South America. Magnetic directions and relative paleointensity data for both stalagmites are generally consistent and agree with historical data from the last 500 y. Before 1500 CE, the data adhere to the geomagnetic model ARCH3K.1, which is derived solely from archeomagnetic data. Our observations indicate rapid directional variations (>0.1°/y) from approximately 860 to 960 CE and approximately 1450 to 1750 CE. A similar pattern of rapid directional variation observed from South Africa precedes the South American record by 224 ± 50 y. These results confirm that fast geomagnetic field variations linked to the SAA are a recurrent feature in the region. We develop synthetic models of reversed magnetic flux patches at the core-mantle boundary and calculate their expression at the Earth's surface. The models that qualitatively resemble the observational data involve westward (and southward) migration of midlatitude patches, combined with their expansion and intensification. [ABSTRACT FROM AUTHOR]

The geomagnetic field has been decaying at a rate of ~5% per century from at least 1840, with indirect observations suggesting a decay since 1600 or even earlier. This has led to the assertion that the geomagnetic field may be undergoing a reversal or an excursion. We have derived a model of the geomagnetic field spanning 30-50 ka, constructed to study the behavior of the two most recent excursions: the Laschamp and Mono Lake, centered at 41 and 34 ka, respectively. Here, we show that neither excursion demonstrates field evolution similar to current changes in the geomagnetic field. At earlier times, centered at 49 and 46 ka, the field is comparable to today's field, with an intensity structure similar to today's South Atlantic Anomaly (SAA); however, neither of these SAA-like fields develop into an excursion or reversal. This suggests that the current weakened field will also recover without an extreme event such as an excursion or reversal. The SAA-like field structure at 46 ka appears to be coeval with published increases in geomagnetically modulated beryllium and chlorine nuclide production, despite the global dipole field not weakening significantly in our model during this time. This agreement suggests a greater complexity in the relationship between cosmogenic nuclide production and the geomagnetic field than is commonly assumed. [ABSTRACT FROM AUTHOR]

This paper presents a statistical study of the equatorward boundary of small-scale field-aligned currents (SSFACs) and investigates the relation between this boundary and the plasmapause (PP). The PP data used for validation were derived from in situ electron density observations of NASA's Van Allen Probes. We confirmed the findings of a previous study by the same authors obtained from the observations of the CHAMP satellite SSFAC and the NASA IMAGE satellite PP detections, namely that the two boundaries respond similarly to changes in geomagnetic activity, and they are closely located in the near midnight MLT sector, suggesting a dynamic linkage. Dayside PP correlates with the delayed time history of the SSFAC boundary. We interpreted this behaviour as a direct consequence of co-rotation: the new PP, formed on the night side, propagates to the dayside by rotating with Earth. This finding paves the way toward an efficient PP monitoring tool based on an SSFAC index derived from vector magnetic field observations at low-Earth orbit. [ABSTRACT FROM AUTHOR]

Radiation in space was the first discovery of the space age. Earth's radiation belts consist of energetic particles that are trapped by the geomagnetic field and encircle the planet. The electron radiation belts usually form a two-zone structure with a stable inner zone and a highly variable outer zone, which forms and disappears owing to wave-particle interactions on the timescale of a day, and is strongly influenced by the very-low-frequency plasma waves. Recent observations revealed a third radiation zone at ultrarelativistic energies, with the additional medium narrow belt (long-lived ring) persisting for approximately 4 weeks. This new ring resulted from a combination of electron losses to the interplanetary medium and scattering by electromagnetic ion cyclotron waves to the Earth's atmosphere. Here we show that ultrarelativistic electrons can stay trapped in the outer zone and remain unaffected by the very-low-frequency plasma waves for a very long time owing to a lack of scattering into the atmosphere. The absence of scattering is explained as a result of ultrarelativistic particles being too energetic to resonantly interact with waves at low latitudes. This study shows that a different set of physical processes determines the evolution of ultrarelativistic electrons. [ABSTRACT FROM AUTHOR]

The Van Allen radiation belts contain ultrarelativistic electrons trapped in Earth's magnetic field. Since their discovery in 1958, a fundamental unanswered question has been how electrons can be accelerated to such high energies. Two classes of processes have been proposed: transport and acceleration of electrons from a source population located outside the radiation belts (radial acceleration) or acceleration of lower-energy electrons to relativistic energies in situ in the heart of the radiation belts (local acceleration). We report measurements from NASA's Van Allen Radiation Belt Storm Probes that clearly distinguish between the two types of acceleration. The observed radial profiles of phase space density are characteristic of local acceleration in the heart of the radiation belts and are inconsistent with a predominantly radial acceleration process. [ABSTRACT FROM AUTHOR]

Abstract: The evolution of the particle background at an altitude of ~540 km during the time interval between 1996 and 2007 is studied using the particle monitor of the High Energy X-ray Timing Experiment on board NASA''s Rossi X-ray Timing Explorer. A special emphasis of this study is the location and strength of the South Atlantic Anomaly (SAA). The size and strength of the SAA are anti-correlated with the 10.7 cm radio flux of the Sun, which leads the SAA strength by ~1 year reflecting variations in solar heating of the upper atmosphere. The location of the SAA is also found to drift westwards with an average drift rate of about 0.3°/yr following the drift of the geomagnetic-field configuration. Superimposed to this drift rate are irregularities, where the SAA suddenly moves eastwards and where furthermore the speed of the drift changes. The most prominent of these irregularities is found in the second quarter of 2003 and another event took place in 1999. We suggest that these events are previously unrecognized manifestations of the geomagnetic jerks of the Earth''s magnetic field. [Copyright &y& Elsevier]

Abstract: Discovery of the Van Allen radiation belts by instrumentation flown on Explorer 1 in 1958 was the first major discovery of the Space Age. A view of the belts as distinct inner and outer zones of energetic particles with different sources was modified by observations made during the Cycle 22 maximum in solar activity in 1989–1991, the first approaching the activity level of the International Geophysical Year of 1957–1958. The dynamic variability of outer zone electrons was measured by the NASA–Air Force Combined Radiation Release and Effects Satellite launched in July 1990. This variability is caused by distinct types of heliospheric structure which vary with the solar cycle. The largest fluxes averaged over a solar rotation occur during the declining phase from solar maximum, when high-speed streams and co-rotating interaction regions (CIRs) dominate the inner heliosphere, leading to recurrent storms. Intense episodic events driven by high-speed interplanetary shocks launched by coronal mass ejections (CMEs) prevail around solar maximum when CMEs occur most frequently. Only about half of moderate storms, defined by intensity of the ring current, lead to an overall flux increase, emphasizing the need to quantify loss as well as source processes; both increase when the magnetosphere is strongly driven. Three distinct types of acceleration are described in this review: prompt and diffusive radial transport, which increases energy while conserving the first invariant, and local acceleration by waves, which change the first invariant. The latter also produce pitch angle diffusion and loss, as does outward radial transport, especially when the magnetosphere is compressed. The effect of a dynamic magnetosphere boundary on radiation belt electrons is described in the context of MHD-test particle simulations driven by measured solar wind input. [Copyright &y& Elsevier]