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Using the data from Van Allen Probes, we statistically study the acceleration conditions of ultrarelativistic electrons (Ek > 3 MeV) in the Earth's outer radiation belt. Based on the different high energy cutoffs of ultrarelativistic electrons, the acceleration events of ultrarelativistic electrons are divided into the four groups of "3.4 MeV events," "5.2 MeV events," "6.3 MeV events," and "7.7 MeV events." According to different solar wind and substorm conditions during the recovery stage, we classify solar wind into three categories: no high‐speed flows, short‐duration high‐speed flows, and long‐duration high‐speed flows. Substorms are divided into continuous substorm activity and non‐continuous substorm activity. Our statistical results show that (a) The different solar wind speeds can cause enhancements of ultrarelativistic electrons with different energies. The high solar wind speed is more important in the acceleration of ultrarelativistic electrons with higher energies (7.7 MeV) than lower energies (3.4, 5.2, and 6.3 MeV). (b) The different substorm activities during the recovery stage can cause enhancements of ultrarelativistic electrons with different energies. Continuous intense substorms in the early recovery stage can contribute to the rapid recovery and enhancements of ultrarelativistic electrons. The timing, intensity, and duration of substorms during the recovery stage are important to the acceleration of ultrarelativistic electrons. (c) Relativistic (1 MeV) electrons are the direct "source" of ultrarelativistic electrons (3.4, 5.2, and 6.3 MeV) in the outer radiation belt. These can help us further understand the electron dynamics of the Earth's outer radiation belt. Key Points: High solar wind speed is more important in the acceleration of ultrarelativistic electrons with higher energies than lower energiesThe timing, intensity, and duration of substorms during the recovery stage are important to the acceleration of ultrarelativistic electronsRelativistic electrons (Ek ∼ 1 MeV) are the direct "source" of ultrarelativistic electrons with lower energies in the outer radiation belt [ABSTRACT FROM AUTHOR]

Using the Van Allen Probes data, we statistically study the evolutions of the seed and relativistic electrons in the Earth's outer radiation belt during the storms. Based on the different storm intensities and durations of the main phase, the storm events are divided into "moderate and short (MS) events," "moderate and long (ML) events," "strong and short (SS) events," and "strong and long (SL) events." The results from a superposed epoch analysis show that (a) For most MS and SS events, the seed electron fluxes can exceed pre‐storm levels, and enhanced fluxes mainly occur around the storm peak. (b) For the ML and SL events, the seed electron fluxes greatly exceed pre‐storm levels by more than an order of magnitude, and enhanced fluxes mainly occur during the main phase. (c) For most MS events, MeV electron fluxes are difficult to return to or exceed pre‐storm levels. While MeV electron fluxes at some L* shells for some SS events can exceed pre‐storm levels. (d) For most ML or SL events, MeV electron fluxes at L* ≥4.0 or ≥3.5 can exceed pre‐storm levels, and enhanced fluxes mainly start to occur from the main phase. These results suggest that the duration of the main phase and storm intensity play important roles in the evolutions of the seed and MeV electrons in the Earth's outer radiation belt during the storms. These are important for our further understanding of the electron dynamics of the outer radiation belt. Key Points: For long main phase events, enhanced fluxes of the seed electrons during the main phase greatly exceed pre‐storm levelsFor long main phase events, MeV electron fluxes at L* ≥4.0 or ≥3.5 mainly start to increase from the storm main phaseThe duration of the main phase and storm intensity play important roles in the evolutions of the outer radiation belt electrons [ABSTRACT FROM AUTHOR]

Using the data from Van Allen Probe A and B, we investigate rapid enhancements of relativistic electrons in the Earth's outer radiation belt caused by the intense substorms (AEmax > ∼900 nT) for 29 events from January 2013 to April 2015. These intense substorms may occur during the storm main phase or recovery phase. Based on the different substorm evolution characteristics, the intense substorms are divided into non‐continuous intense substorm activities and continuous intense substorm activities. In this study, we set a criterion for rapid enhancements when the electron phase space densities for μ = 1,096, 2,290, and 3,311 MeV/G increased by more than two times in 9 hr. In the time interval of 9 hr, the local acceleration by chorus waves is the dominant process for accelerating the seed populations (hundred kiloelectron volts) up to MeV energies. Our statistical results show that enhanced chorus waves and seed electrons during the intense substorms are observed in the outer radiation belt. Continuous intense substorm activities can more rapidly (<9 hr) and efficiently accelerate relativistic electrons in the outer radiation belt than non‐continuous intense substorm activities. During the intense substorms, the electric field acceleration could contribute to rapid enhancements of relativistic electrons in the outer radiation belt. Our statistical study suggests that the intense substorms during geomagnetic storms have a significant effect on the rapid variations of relativistic electron dynamics. Key Points: The statistical study of rapid enhancements of MeV electrons in the outer radiation belt caused by the intense substorms is presentedLocal acceleration during continuous intense substorms can accelerate electrons to relativistic energies more rapidly and efficientlyThe electric field acceleration could contribute to rapid enhancements of MeV electrons in the outer radiation belt [ABSTRACT FROM AUTHOR]

Using the data from Van Allen Probes, we investigate in detail the seed populations in the outer radiation belt during the storm main phase. Based on the different flux variations of the seed electrons before the storm and around the storm peak, these storm events are divided into "flux decay events," "flux enhancement events" and "strong flux enhancement events." For flux decay events, a shorter storm main phase (≤12 hr) and smaller southward interplanetary magnetic field (IMF) and solar wind electric field (−15 nT < Bzmin < −5 nT, 2 mV/m < Eymax < 7 mV/m) are found. For flux enhancement events, large southward Bzmin and Eymax (−29 nT < Bzmin < −10 nT, 5 mV/m < Eymax < 11 mV/m) for the shorter main phase or smaller southward Bzmin and Eymax (−23 nT < Bzmin < −5 nT, 3 mV/m < Eymax < 11 mV/m) for the longer main phase (>12 hr) are observed. For strong flux enhancement events, larger southward Bzmin and Eymax (−32 nT < Bzmin < −13 nT, 5 mV/m < Eymax < 22 mV/m) for the shorter main phase or smaller southward Bzmin and Eymax (−26 nT < Bzmin < −9 nT, 3 mV/m < Eymax < 16 mV/m) for the longer main phase are observed. These suggest that the flux variations of the seed electrons during the storm main phase are highly influenced by the duration of the storm main phase, southward IMF, and solar wind electric field. Key Points: For flux decay events, the duration of the storm main phase is shorter, southward Bzmin and Eymax are smallerFor flux enhancement events, southward Bzmin and Eymax for the events with shorter main phase are larger than those with longer main phaseFor strong flux enhancement events, southward Bzmin and Eymax are larger than others under the same duration of the storm main phase [ABSTRACT FROM AUTHOR]

The spatial and temporal distributions of ionospheric electromagnetic ion cyclotron (EMIC) waves during magnetic storms from 2014 to 2018 were studied using Swarm observations. Ionospheric EMIC waves preferably occurred during storms and their recovery phases at subauroral regions within an average magnetic latitude of 40–55°. There were more wave events during more intense and longer period storms. However, the correlation between event number and storm duration was not good. This might be due to the effect of heavy oxygen ions on EMIC wave generation and the loss of ring current high‐energy ions by the EMIC waves. There are obvious magnetic local time (MLT) differences in the peak occurrence frequency of EMIC waves during storm phases. The enhanced solar wind dynamic pressure was favorable for duskside EMIC waves. With an increased substorm activity, the wave occurrence rate peak shifted from the morning side to the dusk‐premidnight sector. During the recovery phase of a storm, EMIC waves in the 12–24 MLT sector appeared preferably in the earlier part than those in the 00–12 MLT sector. This shift in local time is related to the eastward rotation of the plasmaspheric plume toward morning during the late storm recovery phase and its overlap with the ring current. Highest occurrence frequency of the storm time EMIC waves could be found in the South Atlantic Anomaly region, which might be related to the drift shell splitting and the wave propagation effect in the weak magnetic field region.Key Points: Global observations of ionospheric electromagnetic ion cyclotron (EMIC) wave events find more events in connection with stronger and longer‐lasting magnetic storms than during weakerDuring the early part of a storm recovery phase EMIC waves occur preferably in the 12–24 MLT sector while during the later part around 00–12 MLTHighest EMIC occurrence rates are found at longitudes around the South Atlantic Anomaly region [ABSTRACT FROM AUTHOR]

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

The geospace inner magnetosphere, within about 10 Earth radii, contains various plasma populations with energy from a few electron volts to megaelectron volts and plays important roles in regulating the energy density of the magnetosphere, the magnetic field configuration, and wave dynamics. As an integrated part of the magnetosphere, the inner magnetosphere region also ties to other regions and can change the global geospace circulation. Therefore, understanding both internal and external cross‐energy/population interactions can help further our knowledge of the inner magnetosphere dynamics and nonlinear feedback processes. In view of this, in the past 5 years (2014–2018), the Geospace Environment Modeling (GEM) Focus Group "inner magnetosphere cross‐energy/population interactions (IMCEPI)" has gathered and boosted community‐wide interactions among observation, simulation, and modeling studies. This commentary reports some major accomplishments of the interactive inner magnetosphere community that were advanced by the IMCEPI Focus Group discussions and layouts remaining challenges that need to be carried on. Key Points: Advancements on first‐principle ring current models, new empirical models on IM fields/waves/plasma and application of innovative techniquesAdvanced knowledge of IM characteristics, e.g., compositions, fields, coupling with ionosphere/tail region, and wave particle interactionsChallenges remain in numerical representation of IM and its linkage with other related areas; validation needed across various IM models [ABSTRACT FROM AUTHOR]