Strong enhancements of outer Van Allen belt electrons have been shown to have a clear dependence on solar wind speed and on the duration of southward interplanetary magnetic field. However, individual case study analyses also have demonstrated that many geomagnetic storms produce little in the way of outer belt enhancements and, in fact, may produce substantial losses of relativistic electrons. In this study, focused upon a key period in August–September 2014, we use GOES geostationary orbit electron flux data and Van Allen Probes particle and fields data to study the process of radiation belt electron acceleration. One particular interval, 13–22 September, initiated by a short‐lived geomagnetic storm and characterized by a long period of primarily northward interplanetary magnetic field (IMF), showed strong depletion of relativistic electrons (including an unprecedented observation of long‐lasting depletion at geostationary orbit) while an immediately preceding, and another immediately subsequent, storm showed strong radiation belt enhancement. We demonstrate with these data that two distinct electron populations resulting from magnetospheric substorm activity are crucial elements in the ultimate acceleration of highly relativistic electrons in the outer belt: the source population (tens of keV) that give rise to VLF wave growth and the seed population (hundreds of keV) that are, in turn, accelerated through VLF wave interactions to much higher energies. ULF waves may also play a role by either inhibiting or enhancing this process through radial diffusion effects. If any components of the inner magnetospheric accelerator happen to be absent, the relativistic radiation belt enhancement fails to materialize.

We present a new framework to study the time evolution and dynamics of the outer Van Allen belt electron fluxes. The framework is entirely based on the large‐scale solar wind storm drivers and their substructures. The Van Allen Probe observations, revealing the electron flux behavior throughout the outer belt, are combined with continuous, long‐term (over 1.5 solar cycles) geosynchronous orbit data set from GOES and solar wind measurements A superposed epoch analysis, where we normalize the timescales for each substructure (sheath, ejecta, and interface region) allows us to avoid smearing effects and to distinguish the electron flux evolution during various driver structures. We show that the radiation belt response is not random: The electron flux variations are determined by the combined effect of the structured solar wind driver and prestorm electron flux levels. In particular, we find that loss mechanisms dominate during stream interface regions, coronal mass ejection (CME) ejecta, and sheaths while enhancements occur during fast streams trailing the stream interface or the CME.

Between 4 and 10 September 2017, multiple solar eruptions occurred from active region AR12673. NOAA's and NASA's well‐instrumented spacecraft observed the evolution of these geoeffective events from their solar origins, through the interplanetary medium, to their geospace impacts. The 6 September X9.3 flare was the largest to date for the nearly concluded solar cycle 24 and, in fact, the brightest recorded since an X17 flare in September 2005, which occurred during the declining phase of solar cycle 23. Rapid ionization of the sunlit upper atmosphere occurred, disrupting high‐frequency communications in the Caribbean region while emergency managers were scrambling to provide critical recovery services caused by the region's devastating hurricanes. The 10 September west limb eruption resulted in the first solar energetic particle event since 2012 with sufficient flux and energy to yield a ground level enhancement. Spacecraft at L1, including DSCOVR, sampled the associated interplanetary coronal mass ejections minutes before their collision with Earth's magnetosphere. Strong compression and erosion of the dayside magnetosphere occurred, placing geosynchronous satellites in the magnetosheath. Subsequent geomagnetic storms produced magnificent auroral displays and elevated hazards to power systems. Through the lens of NOAA's space weather R‐S‐G storm scales, this event period increased hazards for systems susceptible to elevated “radio blackout” (R3‐strong), “solar radiation storm” (S3‐strong), and “geomagnetic storm” (G4‐severe) conditions. The purpose of this paper is to provide an overview of the September 2017 space weather event, and a summary of its consequences, including forecaster, post‐event analyst, and communication operator perspectives.

Enabled by the comprehensive measurements from the Magnetic Electron Ion Spectrometer (MagEIS), Helium Oxygen Proton Electron mass spectrometer (HOPE), and Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instruments onboard Van Allen Probes in the heart of the radiation belt, the relative contributions of ions with different energies and species to the ring current energy density and their dependence on the phases of geomagnetic storms are quantified. The results show that lower energy (<50 keV) protons enhance much more often and also decay much faster than higher‐energy protons. During the storm main phase, ions with energies <50 keV contribute more significantly to the ring current than those with higher energies; while the higher‐energy protons dominate during the recovery phase and quiet times. The enhancements of higher‐energy proton fluxes as well as energy content generally occur later than those of lower energy protons, which could be due to the inward radial diffusion. For the 29 March 2013 storm we investigated in detail that the contribution from O⁺ is ~25% of the ring current energy content during the main phase and the majority of that comes from <50 keV O⁺. This indicates that even during moderate geomagnetic storms the ionosphere is still an important contributor to the ring current ions. Using the Dessler‐Parker‐Sckopke relation, the contributions of ring current particles to the magnetic field depression during this geomagnetic storm are also calculated. The results show that the measured ring current ions contribute about half of the Dst depression.

A correction method to remove proton contamination from the electron channels of the Polar‐orbiting Operational Environmental Satellites Medium Energy Proton/Electron Detector (MEPED) is described. Proton contamination estimates are based on measurements in five of the MEPED proton spectral channels. A constrained inversion of the MEPED proton channel response function matrix is used to calculate proton differential flux spectra. In this inversion, the proton energy distribution is described by a weighted combination of exponential, power law, and Maxwellian distributions. Proton contamination in the MEPED electron spectral channels is derived by applying the electron channel proton sensitivities to the proton fluxes from the best fit proton spectra. Once the electron channel measurements are corrected for proton contamination, an inversion of the electron channel response function matrix is used to calculate electron differential flux spectra. A side benefit of the method is that it yields an estimate for the integrated electron flux in the energy range from 300 keV to 2.5 MeV with a center energy at ~800 keV. The final product is a differential spectrum of electron flux covering the energy range from about 10 keV to 2.5 MeV that is devoid of proton contamination except during large solar proton events. Comparisons of corrected MEPED differential fluxes to the Detection of Electromagnetic Emissions Transmitted from Earthquake Regions Instrument for Detecting Particles show that MEPED fluxes are greater than what is expected from altitude‐induced particle population changes; this is attributed at least partially to measurement differences in pitch angle range.

Electromagnetic ion cyclotron (EMIC) waves can drive precipitation of tens of keV protons and relativistic electrons, and are a potential candidate for causing radiation belt flux dropouts. In this study, we quantitatively analyze three cases of EMIC‐driven precipitation, which occurred near the dusk sector observed by multiple Low‐Earth‐Orbiting (LEO) Polar Operational Environmental Satellites/Meteorological Operational satellite programme (POES/MetOp) satellites. During EMIC wave activity, the proton precipitation occurred from few tens of keV up to hundreds of keV, while the electron precipitation was mainly at relativistic energies. We compare observations of electron precipitation with calculations using quasi‐linear theory. For all cases, we consider the effects of other magnetospheric waves observed simultaneously with EMIC waves, namely, plasmaspheric hiss and magnetosonic waves, and find that the electron precipitation at MeV energies was predominantly caused by EMIC‐driven pitch angle scattering. Interestingly, each precipitation event observed by a LEO satellite extended over a limited L shell region (ΔL ~ 0.3 on average), suggesting that the pitch angle scattering caused by EMIC waves occurs only when favorable conditions are met, likely in a localized region. Furthermore, we take advantage of the LEO constellation to explore the occurrence of precipitation at different L shells and magnetic local time sectors, simultaneously with EMIC wave observations near the equator (detected by Van Allen Probes) or at the ground (measured by magnetometers). Our analysis shows that although EMIC waves drove precipitation only in a narrow ΔL, electron precipitation was triggered at various locations as identified by POES/MetOp over a rather broad region (up to ~4.4 hr MLT and ~1.4 L shells) with similar patterns between satellites.