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Electromagnetic ion cyclotron (EMIC) waves are believed to play an important role in the dynamics of the inner magnetosphere, including the ring current, the radiation belts and potentially, the cold plasma. In this work, we investigate their occurrence in the magnetosphere and the geomagnetic and solar wind conditions which lead to their excitation. We use an automated detection algorithm of EMIC waves observed by Van Allen Probes over the entire mission duration between 2012 and 2019. Consistent with earlier studies, we find that the H+ band occurrence maximizes in the dayside magnetosphere during enhancements of solar wind dynamic pressure. Both the H+ and He+ band are also generated along the duskside magnetosphere during disturbed geomagnetic conditions. In addition, to H+ and He+ bands commonly surveyed, we investigate the occurrence of H+ waves above and below 0.5 H+ gyrofrequency, as well as wave occurrence in the N+ and O++ bands. Most H+ waves are observed in the band below 0.5fH+. We find several events in the N+ band, indicative of their very low occurrence. The O++ band is observed during disturbed geomagnetic conditions and high solar wind dynamic pressure at low L‐shells. Its radial localization coincides with the O++ torus. This study provides a comprehensive picture of EMIC wave distribution and insight into ion composition in the inner magnetosphere during variable geomagnetic conditions. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are a type of plasma wave that occurs in the Earth's magnetosphere, typically in the frequency range of 0.1–5 Hz. These waves are generated by the interaction of magnetospheric ions and guided along the ambient magnetic field lines. EMIC waves are thought to significantly influence the dynamics of the Earth's most energetic particle populations, such as the ring current and the outer radiation belt, by inducing particle precipitation into the atmosphere. They can appear in multiple spectral bands, which are crucial for determining the energy of resonant particle interactions and identifying the presence of minor ions like He++, O++, and N+ in the magnetosphere. This study analyzes the distribution of EMIC wave activity using several years of data from the Van Allen Probes spacecraft, offering a detailed overview of the EMIC wave environment and providing insights into ion composition under varying geomagnetic conditions, which is essential for global magnetospheric modeling. Key Points: Most H+ waves are observed in the band below 0.5fH+O++ band is observed during disturbed conditions, radially coincident with cold O++ ion density enhancements previously observed by DE 1N+ band occurrence is low, less than 0.01% [ABSTRACT FROM AUTHOR]

The fast Van Allen radiation belt electron dynamics during geomagnetic storms have not yet been fully explained, in part due to limitations of standard satellite missions in both orbit and the number of spacecraft. Here we overcome these limitations using measurements from the Global Positioning System (GPS) constellation during an acceleration event on 26 August 2018. We show that the acceleration of relativistic electrons occurs in two distinct bursts, each dominated by a different acceleration mechanism. The first burst enhances the radiation belt electrons by four orders of magnitude in 2 hr and is consistent with ULF‐wave radial diffusion. The second burst is likely caused by the local acceleration and delivers an order‐of‐magnitude increase in 20 min. This work demonstrates how distributed, operational measurements can be used to resolve phenomena not observable with previous capabilities, and that rapid energization of the radiation belt can occur much faster than previously reported. Plain Language Summary: In this paper, we present a detailed analysis of terrestrially‐trapped electron space radiation during the August 2018 geomagnetic storm. This event is characterized by a very fast enhancement in the trapped electron population that increases particle counts by more than a factor of a thousand over only 6 hr. Such fast dynamics cannot be resolved by typical survey missions due to their long orbital periods. We instead use measurements from 20 satellites in the Global Positioning System (GPS) constellation, which allows us to perform an analysis of the space radiation dynamics on much shorter timescales. These GPS data reveal that the fast enhancement during the August 2018 storm occurred in two distinct bursts. By introducing a novel technique for GPS particle data analysis, we also determine that each of the bursts is governed by different physical processes that act on different timescales. The revealed fast dynamics of near‐Earth trapped radiation point toward a need to reevaluate the classic paradigm that the changes in the radiation levels are slow and can be revealed by surveys with a low number of spacecraft. Indeed, we foresee a critical role for constellation measurements, such as from GPS, in the future of radiation belt science. Key Points: Constellation measurements of the Van Allen radiation belt electrons can be used to reveal fast nonadiabatic changes at sub‐orbit timescalesElectron acceleration during the August 2018 storm consists of two distinct acceleration bursts governed by different physical processesULF‐wave radial diffusion and local acceleration can significantly alter radiation belt electron content on timescales of minutes to hours [ABSTRACT FROM AUTHOR]

Energetic particle precipitation into the atmosphere has been identified as a key loss process for electrons in the Earth's outer radiation belt region. However, direct measurements of the electron flux precipitating into the atmosphere are challenging from high altitude low inclination spacecraft, such as the Van Allen Probes, due to the small angular size of the loss cone along the orbital path of these high‐altitude spacecraft. Here we use data from the Polar Orbiting Environmental Satellites (POES)/Space Environment Monitor in low‐Earth orbit to assess the relationship between the trapped and precipitating electron flux in integral energy channels >30, >100, and >300 keV. Our results highlight that there is a strong non‐linear relationship between the flux of trapped and precipitating electrons, with the ratio of precipitating to trapped flux only becoming significantly enhanced once the trapped flux reaches a critical level. This transition from low to high levels of precipitation is consistent with the theory proposed by Kennel and Petschek (K‐P) (1966) https://doi.org/10.1029/JZ071i001p00001, whereby intense chorus waves are excited and trigger pitch angle diffusion, resulting in energetic particle precipitation and limiting the trapped flux. Using electron flux data from POES and chorus wave data from the Van Allen Probes, we further test the observations against predictions from the K‐P theory. A particle tracing model is also utilized to illustrate a direct link between the drift paths of injected electrons, the occurrence of chorus waves and the spatial distribution of strong electron precipitation into the atmosphere at different energies. Key Points: Trapped and precipitating electron fluxes become equal once the trapped flux hits a flux limit, indicating strong diffusion is occurringEvents with both high trapped and precipitating flux occur more often in the dawn sector, matching the drift paths of injected electronsIntense chorus waves occur mostly near dawn, suggesting injected electrons excite them and limit the flux via a Kennel‐Petschek‐like process [ABSTRACT FROM AUTHOR]

Understanding intense electron precipitation is crucial for characterizing radiation belt loss and assessing related impacts on the atmosphere. We investigate the evolution of electron flux during an ensemble of 70 geomagnetic storms, focusing on equatorial and low‐Earth orbit observations of trapped and precipitating ∼30–100 keV energy electrons. We reveal that the most intense electron precipitation is associated with equatorial flux capping through self‐limiting processes, for example, as described theoretically by Kennel and Petschek (1966, https://doi.org/10.1029/jz071i001p00001). Our results indicate that the most intense electron precipitation is caused by electron injections associated with self‐limiting processes. Dawn side injections are observed to have fluxes that exceed the Kennel‐Petschek limit, consistent with the excitation of strong chorus waves and resulting in intense precipitation and return of the trapped flux to the Kennel‐Petschek limit. Our results clearly demonstrate the important role of self‐limiting processes in affecting the dynamics of newly injected electrons and driving intense electron precipitation. Plain Language Summary: In this study, we investigate the behavior of electrons in Earth's magnetosphere and how they impact the upper atmosphere during geomagnetic storms. We focus on electron precipitation events with energies that are often associated with pulsating aurora. Our findings show that during the most intense periods of electron precipitation, the electron flux in the equatorial region is associated with a natural process that caps it at an upper limit. We also reveal that electron injections occurring on the dawn side of the magnetosphere trigger these processes, leading to the most significant electron precipitation events occurring in this region. Our results highlight the importance of these natural self‐limiting processes in shaping the behavior of the radiation belts, and the potential impact of the related precipitation of these particles into the upper atmosphere. Understanding these processes is crucial for studying space weather and its potential effects on our planet's atmosphere and climate. Key Points: We investigate the evolution of electron flux and pitch angle distributions (PADs) during intense precipitation eventsLow Earth orbit PADs show strong pitch angle diffusion associated with intense precipitation at dawn side magnetic local timesBoth are associated with eventual flux capping at the Kennel‐Petschek limit after electron injections [ABSTRACT FROM AUTHOR]

We use Van Allen Probes electron data during 70 geomagnetic storms to examine the response of equatorial pitch angle distributions (PADs) at L* = 4.0–4.5 to a theoretical upper limit on stably trapped particle fluxes. Of the energies examined, 54 and 108 keV electron PADs isotropize to a previously assumed level within 6 hr of reaching the limit, near‐identically across all 70 storms, consistent with rapid pitch angle scattering due to chorus wave interactions. In around 30% of events, 54 keV electrons completely exceed the KP limit, before being quickly subdued. 470 and 749 keV PADs show clear indications of an upper limit, though less aligned with the calculated limit used here. The consistency of an absolute upper limit shown across all events demonstrates the importance of this phenomena in both the limiting effect on electron flux and consistently influencing electron PAD evolution during geomagnetic storms. These results also highlight the need for further investigation, particularly related to the limiting of higher energy electrons. Plain Language Summary: High energy electrons in the space environment around Earth, known as the outer Van Allen radiation belt, can cause severe damage to satellites in orbit. Simultaneously measuring many electrons traveling in different directions and at different energies can help us understand which processes are present and influencing radiation belt dynamics in more detail. Our results show evidence of a process which limits electrons in the heart of the outer radiation belt to a maximum intensity level. The directional measurements provide further insight into the processes causing these energy‐dependent changes, showing signs of already known mechanisms such as interaction with particular magnetic waves. Key Points: During ∼30% of 70 geomagnetic storms, 54 keV electrons exceed the upper flux limit described by Kennel and Petschek theoryEquatorial electron pitch angle distributions (PADs) up to 749 keV isotropize to a consistent level upon reaching an upper limitSkewed, highly modal flux distributions from 70 geomagnetic storms emphasize this result in all energies, notably including 470 and 749 keV [ABSTRACT FROM AUTHOR]

On 7 January 2014, a solar storm erupted, which eventually compressed the Earth's magnetosphere leading to the generation of chorus waves. These waves enhanced local wave‐particle interactions and led to the precipitation of electrons from 10 s eV to 100 s keV. This paper shows observations of a low energy cutoff in the precipitation spectrum from Van Allen Probe B Helium Oxygen Proton Electron measurements. This low energy cutoff is well replicated by the predicted loss calculated from pitch angle diffusion coefficients from wave and plasma observations on Probe B. To our knowledge, this is the first time a single spacecraft has been used to demonstrate an accurate theoretical prediction for chorus wave‐induced precipitation and its low energy cutoff. The specific properties of the precipitating soft electron spectrum have implications for ionospheric activity, with the lowest energies mainly contributing to thermospheric and ionospheric upwelling, which influences satellite drag and ionospheric outflow. Plain Language Summary: On 7 January 2014, a large storm erupted from the Sun. This storm encountered the Earth and compressed the magnetosphere a few days later. The compression of the magnetosphere led to the creation of chorus waves, a wave‐type known to interact only with electrons with specific energies. In this case, the waves interacted with electrons in the magnetosphere's outer radiation belt. They caused the loss of electrons from 10 s eV to 100 s keV into the ionosphere and upper atmosphere. This paper uses theory to determine which energies we expect will interact with the observed chorus wave. We use the Helium Oxygen Proton Electron instrument from the Van Allen Probes to see if our predictions are correct. We care about these processes because the loss of these electrons can affect ionospheric activity. Key Points: Upper band chorus waves can have a minimum resonant energy in the 10 s eV energy rangeChanges in the minimum resonant energy can change the cut off for what lower energy particles will be lostThe lower energy cut off can be observed in the Van Allen Probes Helium Oxygen Proton Electron data [ABSTRACT FROM AUTHOR]

Chorus waves play a key role in outer Van Allen electron belt dynamics through cyclotron resonance. Here, we use Van Allen Probes data to reveal a new and distinct population of intense chorus waves excited in the heart of the radiation belt during the main phase of geomagnetic storms. The power of the waves is typically ~ 2–3 orders of magnitude greater than pre-storm levels, and are generated when fluxes of ~ 10–100 keV electrons approach or exceed the Kennel–Petschek limit. These intense chorus waves rapidly scatter electrons into the loss cone, capping the electron flux to a value close to the limit predicted by Kennel and Petschek over 50 years ago. Our results are crucial for understanding the limits to radiation belt fluxes, with accurate models likely requiring the inclusion of this chorus wave-driven flux-limiting process, that is independent of the acceleration mechanism or source responsible for enhancing the flux. [ABSTRACT FROM AUTHOR]

The flux of relativistic electrons in the terrestrial Van Allen radiation belt can vary by orders of magnitude during a geomagnetic storm. The response is typically assumed to be controlled by an often delicate balance between acceleration and loss processes. Here we analyze all 133 magnetic storms from the NASA Van Allen Probes era. We show how the belts demonstrate a repeatable response which limits not only the worst case flux, but also controls and delivers a limit to the severity of the spectral hardness, consistent with the theory of Kennel and Petschek first developed over 50 years ago. When this theory is extended to relativistic energies, the observed electron flux is seen to reach a naturally limited worst case with a clear energy dependence. Here we show how the Kennel‐Petschek theory can explain the evolution and hardening of the electron spectrum during geomagnetic storms. It also introduces an energy‐dependent maximum flux limit. This limit is never reached during the Van Allen Probes era at energies above 2.6 MeV, meanwhile, it is reached within hours at lower energies (∼100 keV) in almost every storm. No current radiation belt models include this effect; without it, they cannot accurately predict the consequent radiation dynamics. Overall, our results demonstrate how this creates a remarkable absolute natural limit to both the spectral hardness and the severity of extreme electron space radiation. Key Points: Van Allen Probes' electron data reveals dynamical hardening of energy spectra associated with an energy‐dependent maximum flux levelsAfter hardening, the worst case relativistic electron radiation levels were capped at the Kennel‐Petschek limit for energies <∼2.6 MeVAlthough the energy spectrum for the higher energy electrons also hardened, their maximum observed fluxes remained below the limit [ABSTRACT FROM AUTHOR]

We present a statistical study on the properties of equatorial electron pitch angle distributions (PADs) in the Earth's outer radiation belt, for the first time based on particle measurements from the entire Van Allen Probes mission. A detailed selection criteria is used to identify intervals when flux measurements at energies from 0.2 to 3.4 MeV are available across a wide range of pitch angles close to the geomagnetic equatorial plane. To better characterize the shape of each pitch angle distribution, the flux data is fitted to functions of the equatorial pitch angle based on Legendre polynomials. Using this technique, we show that the shape of the PADs strongly depends on the particle's energy, location and geomagnetic activity. These results are used to identify the dominant physical processes responsible for creating PADs with different shapes. The results presented here mainly focus on the occurrence statistics and properties of butterfly PADs. Significantly, we find clear evidence that butterfly PADs have a peak flux that preferentially occurs at two distinct and discrete equatorial pitch angles, either 35° ± 5° or 65° ± 5°. Energy, L‐shell and magnetic local time variations indicate that the flux peaks at equatorial pitch angles ∼35° appear consistent with magnetopause shadowing, whilst those peaking at ∼65° may be associated with wave‐particle interactions. We also show that the flux at low and high pitch angles, for both butterfly and nonbutterfly PADs, remains remarkably well correlated even as the flux intensity varies by four orders of magnitude. Plain Language Summary: We present a statistical study on the properties and occurrence of equatorial electron pitch angle distributions (PADs) in the Earth's outer radiation belt based on particle measurements from the entire Van Allen Probes mission. The flux data is fitted to functions of the equatorial pitch angle. Using the PAD fits, the characteristics of PADs that have a local flux minimum near equatorial pitch angles of 90°, known as butterfly PADs, are determined. We find clear evidence that these butterfly PADs have a peak flux that preferentially occurs at two distinct and discrete equatorial pitch angles, either 35° ± 5° or 65° ± 5°. Energy, L‐shell and magnetic local time variations indicate that the butterfly PADs with flux peaks at equatorial pitch angles of ∼35° appear consistent with outer radiation belt electron loss through the magnetopause, whilst those peaking at ∼65° may be associated with wave‐particle interactions. We also show that the flux at low and high pitch angles, for both butterfly and nonbutterfly PADs, remains remarkably well correlated even as the flux intensity varies by four orders of magnitude. Key Points: Equatorial butterfly pitch angle distributions have preferred flux peaks at ∼35° or ∼65°, each occurring across different L and MLT rangesThe depth of butterfly distribution dips tends to get smaller as the flux increases, inconsistent with growing off‐equatorial peaksFrom L*∼3 to ∼5, the flux at 90° is highly correlated with the flux at lower pitch angles, even as the flux varies by four orders of magnitude [ABSTRACT FROM AUTHOR]

Properly characterizing fast relativistic electron losses in the terrestrial Van Allen belts remains a significant challenge for accurately simulating their dynamics. In particular, magnetopause shadowing losses can deplete the radiation belt within hours or even minutes, but can have long‐lasting impacts on the subsequent belt dynamics. By statistically analyzing seven years of data from the entire Van Allen Probes mission in the context of the last closed drift shell, here we show how these losses are much more organized and predictable than previously thought. Once magnetic storm electron dynamics are properly analyzed in terms of the location of the last closed drift shell, not only is the loss shown to be repeatable but its energy‐dependent spatio‐temporal evolution is also revealed to follow a very similar pattern from storm to storm. Employing an energy‐dependent ULF wave radial diffusion model, we further show for the first time how the similar and repeatable fractional loss of the pre‐storm electron population in each storm can be reproduced and explained. Empirical characterization of this loss may open a pathway toward improved radiation belt specification and forecast models. This is especially important since underestimates of loss can also create unrealistic sources in models, creating phantom electron radiation and leading to the prediction of an overly harsh radiation environment. Key Points: Magnetopause compression, characterized by inward motion of the last closed drift shell, results in rapid loss of radiation belt electronsThe associated loss develops in a similar and repeatable manner depleting the radiation belt by the same fraction in every event studiedThe L∗ and energy dependence of the loss is consistent with magnetopause shadowing losses enhanced by outward ULF wave radial diffusion [ABSTRACT FROM AUTHOR]

The flux of energetic electrons in the terrestrial Van Allen Belts varies by orders of magnitude during a magnetic storm. Here, we show how the dynamics of these electrons are clearly separated by energy into two distinct populations which are governed by different storm time behavior. We reveal how self‐limiting processes, described theoretically by Kennel and Petschek (1966), https://doi.org/10.1029/JZ071i001p00001, govern the energy‐dependent dynamics of the electrons. For terrestrial magnetic storms, accelerated electrons with energies below ∼850 keV quickly reach a maximum energy‐dependent differential flux level which is almost the same in every storm. Higher energy electrons typically do not reach a limiting flux and instead are governed by the differential impacts of competing acceleration and loss mechanisms. In general, electron fluxes saturate at the Kennel‐Petschek limit first at lower energies, impacting higher energies later in the storm. For the most intense storms, the maximum energy at which the fluxes are capped can also increase. Plain Language Summary: Very high energy electrons in near‐Earth space constitute a space radiation threat to spacecraft. All of this space radiation is traveling close to the speed of light. However, as we show here, nature remarkably imposes a limit to the number of these space radiation particles which can be generated once the acceleration is strong enough. At lower energies, where the numbers of particles are typically larger, we show that there is an absolute limit to the severity of the space radiation. That is, there is a natural limit to how bad this radiation can get. At higher energies, however, while there is still a theoretical limit it is almost never reached. Our results are based on the interpretation of a 50‐year‐old theory developed by Kennel and Petschek (1966), https://doi.org/10.1029/JZ071i001p00001 but which has been largely untested until now. None of the modern radiation belt models incorporate these effects, and our results provide an important input for both improved radiation specification and for assessing worst case space radiation threats. As we show here, depending on energy, it can be the best of times, and the worst of times, for the extremes of the level of space radiation reached during a space storm. Key Points: Superposed epoch analysis of 70 geomagnetic storms reveals drastically different behavior of radiation belt electrons depending on energyElectrons with energies below ∼850 keV are revealed to be strongly affected by self‐limiting Kennel‐Petschek processes in almost every stormGenerally, the flux of higher energy particles is not capped and only becomes self‐limited for the most intense and long‐lasting storms [ABSTRACT FROM AUTHOR]

This study examines the rapid losses and acceleration of trapped relativistic and ultrarelativistic electron populations in the Van Allen radiation belt during the September 7-9, 2017 geomagnetic storm. By analyzing the dynamics of the last closed drift shell (LCDS), and the electron flux and phase space density (PSD), we show that the electron dropouts are consistent with magnetopause shadowing and outward radial diffusion to the compressed LCDS. During the recovery phase, an in-bound pass of Van Allen Probe A shows an apparent local peak in PSD, which does not exist in reality. A careful analysis of the multipoint measurements by the Van Allen Probes reveals instead how the apparent PSD peak arises from aliasing monotonic PSD profiles which are rapidly increasing due to acceleration from very fast inwards radial diffusion. In the absence of such multisatellite conjunctions during fast acceleration events, such peaks might otherwise be associated with local acceleration processes. Plain Language Summary This study presents a thorough analysis of terrestrially trapped electron space radiation during the September 2017 geomagnetic storm. By analyzing the measurements of the trapped electron population, we show that the predominant loss of the relativistic and ultrarelativistic electrons depleted from the radiation belt at the beginning of the storm arises from outwards loss into the solar wind and not downwards loss into the atmosphere. We also reveal, for the first time, that the signatures of the acceleration processes which refill the belts, after such losses can occur on much faster timescales than previously thought. Moreover, signatures attributed to the actions of high-frequency plasma waves are actually caused by a different physical phenomenon known as the radial diffusion. The new knowledge of the very fast rate of change of the amount of electron space radiation points to an urgent need to evaluate the processes, which control belt dynamics. As we show here, this can be faster than the orbital period of monitoring satellites. Overall, we show how the limited satellite spatiotemporal coverage may mask and confuse the signatures of the physical processes responsible. [ABSTRACT FROM AUTHOR]

Models of monochromatic Pc5 (2–7 mHz) ultralow frequency (ULF) wave interactions with high energy (greater than ∼1 MeV) electrons predict drift resonant interactions that can cause rapid radial transport and acceleration. There are few reports of electron drift resonance at energies greater than ∼1 MeV, in contrast to lower energies; moreover, all previous reports occur in the aftermath of interplanetary shocks. These two facts are difficult to reconcile with theory and numerical simulations predicting that greater than ∼1 MeV drift resonances should occur more often and in a wider variety of driving conditions. In this study, we show that a combination of observational sampling biases and nominal radial phase space density gradients is one explanation for this discrepancy between theory and observations. In particular, we examine electron dynamics in two case studies with very similar satellite coverage, solar wind conditions, and Pc5 wave properties, yet with different radial phase space density profiles. Using global wave and particle observations, we show that the events have vastly different particle responses despite having similar wave properties. Placing these results in context with past studies, we further show that nominal radial PSD gradients near geostationary orbit can mask the expected drift resonance particle response and explain (1) the small number of past greater than ∼1 MeV drift resonance reports and (2) the restriction of these reports to interplanetary shock events. We argue that future observational studies characterizing radial transport via drift resonance should examine global particle dynamics, including observations of the radial phase space density profile. Plain Language Summary: The Earth's radiation belts are dynamic, with variations in radiation intensity depending on many factors. During certain conditions radiation in this region can damage satellite electronics. A variety of plasma waves can interact with this radiation and affect its overall intensity; thus, a characterization of these waves and their related interactions is needed to predict radiation belt dynamics. In this study, we examine a resonant interaction between large‐scale plasma waves—wavelengths of 10,000 km or more—and high‐energy electrons near geostationary orbit (GEO). We find that one of the predicted observational signatures for the resonant interaction can be masked by background radiation conditions if observations are only collected near GEO. We also find that past reports of these resonances favor a particular set of solar wind driving conditions because of this masking effect and that these resonances may in fact occur in a broader range of conditions and spatial regions. We propose that future studies seeking to characterize how these resonances affect radiation belt dynamics should use observations from a broader spatial region whenever possible and/or incorporate background radiation conditions into their analysis. Key Points: Pc5 wave events with similar properties/satellite locations yet different radial PSD gradients yield vastly different particle responsesRadial PSD gradients mask expected drift resonance particle response near ∼1 MeV, reducing likelihood of detection near geostationary orbitA small number of ∼1 MeV electron drift resonance reports and restriction to shock events can be explained by nominal radial PSD gradients [ABSTRACT FROM AUTHOR]

We present simulations of the outer radiation belt electron flux during the March 2015 and 2013 storms using a radial diffusion model. Despite differences in disturbance short‐time intensity between the two storms, the response of the ultra‐relativistic electrons in the outer radiation belt was remarkably similar, both showing a sudden drop in the electron flux followed by a rapid enhancement in the outer belt flux to levels over an order of magnitude higher than those observed during the pre‐storm interval. Simulations of the ultra‐relativistic electron flux during the March 2015 storm show that outward radial diffusion can explain the flux dropout down to L*~4. However, in order to reproduce, the observed flux dropout at L* < 4 requires the addition of a loss process characterized by an electron lifetime of around 1 hr operating below L*~3.5 during the flux dropout interval. Nonetheless, during the pre‐storm and recovery phase of both storms, the radial diffusion simulation reproduces the observed flux dynamics. For the March 2013 storm, the flux dropout across all L‐shells is reproduced by outward radial diffusion activity alone. However, during the flux enhancement interval at relativistic energies, there is evidence of a growing local peak in the electron phase space density at L*~3.8, consistent with local acceleration such as by very low frequency chorus waves. Overall, the simulation results for both storms can accurately reproduce the observed electron flux only when event specific radial diffusion coefficients are used, instead of the empirical diffusion coefficients derived from ultra‐low frequency wave statistics. Key Points: The March 2013 outer radiation belt flux dropout is consistent with fast outward ULF wave radial diffusion to a compressed magnetopauseOutward radial diffusion at high L combined with a loss process occurring on L < 3.5 is required to explain the March 2015 flux dropoutEvent specific radial diffusion coefficients should be used to simulate outer belt flux dynamics especially during the storm main phase [ABSTRACT FROM AUTHOR]

We present the temporal evolution of electron Phase Space Density (PSD) in the outer radiation belt during the intense March 2015 geomagnetic storm. Comparing observed PSD profiles as a function of L* at fixed first, M, and second, K, adiabatic invariants with those produced by simulations is critical for determining the physical processes responsible for the outer radiation belt dynamics. Here we show that the bulk of the accelerated and enhanced outer radiation belt population consists of electrons with K < 0.17 G1/2Re. For these electrons, the observed PSD versus L* profiles during the recovery phase of the storm have a positive radial gradient. We compare the observed temporal evolution of the PSD profiles during the recovery phase with those produced by radial diffusion simulations driven by observed Ultralow Frequency wave power as measured on the ground. Our results indicate that the dominant flux enhancement, inside L* < 5, in the heart of the outer radiation belt during the March 2015 geomagnetic storm is consistent with that produced by fast inward radial diffusion of electrons from a dynamic outer boundary driven by enhanced Ultralow Frequency wave power. Key Points: The March 2015 outer radiation belt flux enhancement was dominated by a population of accelerated low‐K electrons with K < 0.17 G1/2ReDuring the flux recovery phase no growing PSD peaks occurred inside L*≲5, suggesting local heating was not the dominant acceleration processThe observed flux enhancement inside L* < 5 is reproduced by our ULF wave radial diffusion simulation without including local acceleration [ABSTRACT FROM AUTHOR]

Using the total radiation belt electron content calculated from Van Allen Probe phase space density, the time-dependent and global response of the outer radiation belt during storms is statistically studied. Using phase space density reduces the impacts of adiabatic changes in the main phase, allowing a separation of adiabatic and nonadiabatic effects and revealing a clear modality and repeatable sequence of events in storm time radiation belt electron dynamics. This sequence exhibits an important first adiabatic invariant (μ)-dependent behavior in the seed (150 MeV/G), relativistic (1,000 MeV/G), and ultrarelativistic (4,000 MeV/G) populations. The outer radiation belt statistically shows an initial phase dominated by loss followed by a second phase of rapid acceleration, while the seed population shows little loss and immediate enhancement. The time sequence of the transition to the acceleration is also strongly μ dependent and occurs at low μ first, appearing to be repeatable from storm to storm. [ABSTRACT FROM AUTHOR]

Abstract: We present observations of very fast radiation belt loss as resolved using high time resolution electron flux data from the constellation of Global Positioning System (GPS) satellites. The time scale of these losses is revealed to be as short as ∼0.5–2 hr during intense magnetic storms, with some storms demonstrating almost total loss on these time scales and which we characterize as radiation belt extinction. The intense March 2013 and March 2015 storms both show such fast extinction, with a rapid recovery, while the September 2014 storm shows fast extinction but no recovery for around 2 weeks. By contrast, the moderate September 2012 storm which generated a three radiation belt morphology shows more gradual loss. We compute the last closed drift shell (LCDS) for each of these four storms and show a very strong correspondence between the LCDS and the loss patterns of trapped electrons in each storm. Most significantly, the location of the LCDS closely mirrors the high time resolution losses observed in GPS flux. The fast losses occur on a time scale shorter than the Van Allen Probes orbital period, are explained by proximity to the LCDS, and progress inward, consistent with outward transport to the LCDS by fast ultralow frequency wave radial diffusion. Expressing the location of the LCDS in L*, and not model magnetopause standoff distance in units of RE, clearly reveals magnetopause shadowing as the cause of the fast loss observed by the GPS satellites. [ABSTRACT FROM AUTHOR]

Geospace magnetic storms, driven by the solar wind, are associated with increases or decreases in the fluxes of relativistic electrons in the outer radiation belt. We examine the response of relativistic electrons to four intense magnetic storms, during which the minimum of the Dst index ranged from -105 to -387 nT, and compare these with concurrent observations of ultra-low-frequency (ULF) waves from the trans-Scandinavian IMAGE magnetometer network and stations from multiple magnetometer arrays available through the worldwide SuperMAG collaboration. The latitudinal and global distribution of Pc5 wave power is examined to determine how deep into the magnetosphere these waves penetrate. We then investigate the role of Pc5 wave activity deep in the magnetosphere in enhancements of radiation belt electrons population observed in the recovery phase of the magnetic storms. We show that, during magnetic storms characterized by increased post-storm electron fluxes as compared to their pre-storm values, the earthward shift of peak and inner boundary of the outer electron radiation belt follows the Pc5 wave activity, reaching L shells as low as 3-4. In contrast, the one magnetic storm characterized by irreversible loss of electrons was related to limited Pc5 wave activity that was not intensified at low L shells. These observations demonstrate that enhanced Pc5 ULF wave activity penetrating deep into the magnetosphere during the main and recovery phase of magnetic storms can, for the cases examined, distinguish storms that resulted in increases in relativistic electron fluxes in the outer radiation belts from those that did not. [ABSTRACT FROM AUTHOR]

Space-based measurements of Earth's magnetic field are required to understand the plasma processes responsible for energising particles in the Van Allen radiation belts and influencing space weather. This paper describes a prototype fluxgate magnetometer instrument developed for the proposed Canadian Space Agency's (CSA) Outer Radiation Belt Injection, Transport, Acceleration and Loss Satellite (ORBITALS) mission and which has applications in other space and suborbital applications. The magnetometer is designed to survive and operate in the harsh environment of Earth's radiation belts and measure low-frequency magnetic waves, the magnetic signatures of current systems, and the static background magnetic field. The new instrument offers improved science data compared to its predecessors through two key design changes: direct digitisation of the sensor and digital feedback from two cascaded pulse-width modulators combined with analog temperature compensation. These provide an increase in measurement bandwidth up to 450Hz with the potential to extend to at least 1500Hz. The instrument can resolve 8 pT on a 65 000 nT field with a magnetic noise of less than 10 pT/ √Hz at 1Hz. This performance is comparable with other recent digital fluxgates for space applications, most of which use some form of sigma-delta (ΣΔ ) modulation for feedback and omit analog temperature compensation. The prototype instrument was successfully tested and calibrated at the Natural Resources Canada Geomagnetics Laboratory. [ABSTRACT FROM AUTHOR]

Enhancements of electron fluxes in the outer radiation belt have been closely linked to increases in solar wind speed and density as well as to prolonged intervals of southward interplanetary magnetic field. Periodic oscillations in the Earth's magnetic field with frequencies in the range of a few millihertz (ultralow frequency or ultralow frequency waves) may be an intermediary through which these solar wind drivers influence radiation belt dynamics due to their potential for resonant interactions with energetic electrons causing the radial migration of resonant electrons. Using data from more than 180 ground magnetometers contributing to the worldwide SuperMAG collaboration, we explore possible relationships between relativistic electron flux variations and the spatial and temporal profiles of ultralow frequency wave power contained in the Pc5 frequency band (2–7 mHz). During 19 geomagnetic storms marked by relativistic (1.5 MeV < E < 6 MeV) electron flux enhancements and 19 storms that led to prolonged electron flux depletions, Pc5 wave power is found penetrating to L shells as low as 2–3. The enhancement of Pc5 wave power starts almost simultaneously with the storm onset. The depth of wave activity penetration was found associated with the strength of geomagnetic activity (Spearman's ρ = 0.54), which is also related to the location of electron flux maximum observed in the recovery phase. Pc5 wave activity persists longer (for up to ≈62 hr) for those storms that produced relativistic electrons. We also investigate the combination of interplanetary conditions necessary to differentiate the response of relativistic electron fluxes to geomagnetic storms. A coupling function that captures the increased reconnection rate at the dayside magnetopause affecting magnetospheric processes which may produce Pc5 wave power offers an additional key to further understanding the outer belt dynamics. Key Points: We separate the global ULF Pc5 wave activity during geomagnetic storms that produce increases and decreases in outer belt electron fluxesULF waves penetrate to low L for both enhancements and depletions but persist for a prolonged period for outer belt enhancement events.Wave penetration depth (L < 2) correlates with storm intensity, similarly to the penetration of the outer electron belt within the slot region [ABSTRACT FROM AUTHOR]

Electron flux measurements are an important diagnostic for interactions between ultralow‐frequency (ULF) waves and relativistic (∼1 MeV) electrons. Since measurements are collected by particle detectors with finite energy channel width, they are affected by a phase mixing process that can obscure these interactions. We demonstrate that ultrahigh‐resolution electron measurements from the Magnetic Electron Ion Spectrometer on the Van Allen Probes mission—obtained using a data product that improves the energy resolution by roughly an order of magnitude—are crucial for understanding ULF wave‐particle interactions. In particular, the ultrahigh‐resolution measurements reveal a range of complex dynamics that cannot be resolved by standard measurements. Furthermore, the standard measurements provide estimates for the ULF flux modulation amplitude, period, and phase that may not be representative of true flux modulations, potentially leading to ambiguous conclusions concerning electron dynamics. Plain Language Summary: Plasma waves with periods of a few minutes interact with high‐energy electrons in the near‐Earth space environment. Understanding how the waves affect the electrons is important, as these electrons can potentially damage orbiting satellites. However, these interactions are obscured by a process known as phase mixing where electrons with different energies behave differently yet are mixed together in measurements that have finite energy resolution. We show how a unique data set obtained from National Aeronautics and Space Administration's Van Allen Probes mission can dramatically increase the energy resolution, revealing a range of interactions that are obscured by measurements with lower energy resolution. Key Points: Previous electron measurements could not resolve many energy‐dependent ULF wave interactions with >∼1 MeV electrons due to phase mixingMagEIS histogram channels resolve many interactions: dE/E of 2‐3% compared to standard 20‐30% that is already considered high resolutionWithout ultrahigh‐resolution measurements, ambiguous conclusions may be reached concerning >∼1 MeV electron dynamics (e.g., drift resonance) [ABSTRACT FROM AUTHOR]